PERLFAQ4(1perl)
🎀 NAME 🎀
Bestie, meet perlfaq4 — aka Data Manipulation 💾. This is the perlfaq4 page from the Perl Programmers Reference Guide (the 1perl manual), and honestly? It's your new best friend. ✨
🌸 VERSION 🌸
version 5.20250619 🦋
💖 DESCRIPTION 💖
Okay so basically this section of the FAQ is where we answer every question you could ever have about manipulating numbers, dates, strings, arrays, hashes, and other miscellaneous data issues. Periodt. 💅
🔢 Data: Numbers 🔢
😭 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? 😭
For the long, unhinged, gory explanation, go read David Goldberg's "What Every Computer Scientist Should Know About Floating-Point Arithmetic" at http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf. 📚
Here's the actual tea though: internally, your computer represents floating-point numbers in binary. Digital (as in powers of two) computers literally cannot store all numbers exactly, so some real numbers lose precision in the process. 💀 This is a problem with how computers store numbers and it affects every computer language, not just Perl. No cap.
perlnumber shows you the gory details of number representations and conversions if you're curious.
Wanna cap the number of decimal places in your numbers? Use the printf or sprintf function. Peep "Floating-point Arithmetic" in perlop for more.
printf "%.2f", 10/3;
my $number = sprintf "%.2f", 10/3;🤨 Why is int() broken? 🤨
Bestie, your int() is most probably working just fine. It's the numbers that aren't quite what you think they are. 🙈
First, go read the answer to "Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?" up there ☝️.
Like, for example, this:
print int(0.6/0.2-2), "\n";will, on most computers, print 0, not 1 — because even something as innocent as 0.6 and 0.2 cannot be presented exactly by floating-point numbers. What you're thinking of as 'three' in that expression is really more like 2.9999999999999995559. 😭
🐙 Why isn't my octal data interpreted correctly? 🐙
(contributed by brian d foy 💖)
You're probably trying to convert a string to a number, and Perl only converts that as a decimal number. When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest of the digits are in base 10:
my $string = '0644';
print $string + 0; # prints 644
print $string + 44; # prints 688, certainly not octal!This problem usually involves one of the Perl built-ins that has the same name as a Unix command that uses octal numbers as arguments on the command line. In this example, chmod on the command line knows its first argument is octal because that's literally what it does:
%prompt> chmod 644 fileSo if you want to use the same literal digits (644) in Perl, you have to tell Perl to treat them as octal numbers — either by prefixing the digits with a 0 or by using oct:
chmod( 0644, $filename ); # right, has leading zero
chmod( oct(644), $filename ); # also correctThe problem really shows up when you take your numbers from something that Perl thinks is a string, like a command-line argument in @ARGV:
chmod( $ARGV[0], $filename ); # wrong, even if "0644"
chmod( oct($ARGV[0]), $filename ); # correct, treat string as octalPro tip 👑: you can always double-check the value you're using by printing it in octal notation to make sure it matches what you think it should be. Print it in octal and decimal:
printf "0%o %d", $number, $number;🎯 Does Perl have a round() function? What about ceil() and floor()? Trig functions? 🎯
Remember that int() merely truncates toward 0. ✂️ For rounding to a certain number of digits, sprintf() or printf() is usually the easiest route:
printf("%.3f", 3.1415926535); # prints 3.142The POSIX module (part of the standard Perl distribution) implements ceil(), floor(), and a number of other mathematical and trigonometric functions:
use POSIX;
my $ceil = ceil(3.5); # 4
my $floor = floor(3.5); # 3In perls 5.000 to 5.003, trigonometry was done in the Math::Complex module. With 5.004, the Math::Trig module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the Math::Complex module, and some functions can break out from the real axis into the complex plane — for example, the inverse sine of 2. 🌈
Rounding in financial applications can have serious implications 😬, and the rounding method used should be specified precisely. In those cases, bestie, it probably pays not to trust whichever system of rounding Perl is using, and instead to implement the rounding function you need yourself.
To see why, notice how you'll still have an issue on half-way-point alternation:
for (my $i = -5; $i <= 5; $i += 0.5) { printf "%.0f ",$i }
-5 -4 -4 -4 -3 -2 -2 -2 -1 -0 0 0 1 2 2 2 3 4 4 4 5Don't blame Perl — it's the same as in C. IEEE says we have to do this. 💅 Perl numbers whose absolute values are integers under 2**31 (on 32-bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed.
🔄 How do I convert between numeric representations/bases/radixes? 🔄
As always with Perl, there is more than one way to do it 💅. Below are a few examples of approaches to making common conversions between number representations. This is intended to be representational rather than exhaustive, iykyk.
Some of the examples later in perlfaq4 use the Bit::Vector module from CPAN. The reason you might choose Bit::Vector over the perl built-in functions is that it works with numbers of ANY size, that it is optimized for speed on some operations, and that for at least some programmers the notation might be familiar. 😌
🟣 How do I convert hexadecimal into decimal
Using perl's built in conversion of 0x notation:
my $dec = 0xDEADBEEF;Using the hex function:
my $dec = hex("DEADBEEF");Using pack:
my $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));Using the CPAN module Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
my $dec = $vec->to_Dec();🟣 How do I convert from decimal to hexadecimal
Using sprintf:
my $hex = sprintf("%X", 3735928559); # upper case A-F
my $hex = sprintf("%x", 3735928559); # lower case a-fUsing unpack:
my $hex = unpack("H*", pack("N", 3735928559));Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(32, -559038737);
my $hex = $vec->to_Hex();And Bit::Vector supports odd bit counts:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(33, 3735928559);
$vec->Resize(32); # suppress leading 0 if unwanted
my $hex = $vec->to_Hex();🟣 How do I convert from octal to decimal
Using Perl's built in conversion of numbers with leading zeros:
my $dec = 033653337357; # note the leading 0!Using the oct function:
my $dec = oct("33653337357");Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new(32);
$vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
my $dec = $vec->to_Dec();🟣 How do I convert from decimal to octal
Using sprintf:
my $oct = sprintf("%o", 3735928559);Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(32, -559038737);
my $oct = reverse join('', $vec->Chunk_List_Read(3));🟣 How do I convert from binary to decimal
Perl 5.6 lets you write binary numbers directly with the 0b notation:
my $number = 0b10110110;Using oct:
my $input = "10110110";
my $decimal = oct( "0b$input" );Using pack and ord:
my $decimal = ord(pack('B8', '10110110'));Using pack and unpack for larger strings:
my $int = unpack("N", pack("B32",
substr("0" x 32 . "11110101011011011111011101111", -32)));
my $dec = sprintf("%d", $int);
# substr() is used to left-pad a 32-character string with zeros.Using Bit::Vector:
my $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
my $dec = $vec->to_Dec();🟣 How do I convert from decimal to binary
Using sprintf (perl 5.6+):
my $bin = sprintf("%b", 3735928559);Using unpack:
my $bin = unpack("B*", pack("N", 3735928559));Using Bit::Vector:
use Bit::Vector;
my $vec = Bit::Vector->new_Dec(32, -559038737);
my $bin = $vec->to_Bin();The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader. 😘
💅 Why doesn't & work the way I want it to? 💅
Perl's & bitwise operator works on both numbers and strings, which sometimes produces surprising results when you expected a number but actually received a string. You probably expected perl to automatically convert the operands to numbers like the mathematical operators would. Nope! Instead, perl treats string operands as bitvectors. 😵
Consider the bitwise difference between the number 3 and the bitvector represented by "3". A number has the bit pattern for its magnitude. The number 3 is 0b11 (a 2 and a 1). The bitvector has the bit pattern that is the ordinal value for each octet, and that value is unrelated to any numeric value that the digit represents. The character "3" is the bitvector 0b0011_0011.
These operations have different results even though you might think they look like the same "number":
11 & 3; # 0b0000_1011 & 0b0000_0011
# -> 0b0000_0011 (number 3)
"11" & "3"; # 0b0011_0001_0011_0001 & 0b0011_0011
# -> 0b0011_0001 (ASCII char "1")Note that if any operand has a numeric value, perl uses numeric semantics (although you should not count on this):
my($i, $j) = ( 11, 3 ); # $i & $j # 11 & 3 -> 3
my($i, $j) = ("11", 3 ); # $i & $j # 11 & 3 -> 3
my($i, $j) = ("11", "3"); # $i & $j # "11" & "3" -> 1Remember that a perl scalar can have both string and numeric values at the same time. 👯 A value that starts as a string and has never encountered a numeric operation has no numeric value yet. Perl does this to save time and work, so it doesn't have to decide a numeric value for a scalar it might never use as a number. In that case, string semantics wins. But, if there is a numeric value already, numeric semantics win. Force perl to compute the numeric value by adding 0:
my($i, $j) = ("11", "3"); $j += 0; # $i & $j # "11" & 3 -> 3However, this is not a documented feature — or, as perlop says, it "is not well defined". 😬 One way to ensure numeric semantics is to explicitly convert both of values to numbers:
(0+$i) & (0+$j)To fix this whole mess, Perl v5.22 said "we're separating the string vibe from the number vibe, period." The bitwise feature brought in four brand-new operators that do string semantics only: &., |., ^., and ~.. Meanwhile the OGs — &, |, ^, and ~ — got locked to numeric semantics only.
Turn the feature on explicitly like this:
use feature qw(bitwise);Or, if you're on v5.28 or later, just require a minimum perl with use:
use v5.28; # bitwise feature for freeHow do I multiply matrices? 🧮✨
Use the Math::Matrix or Math::MatrixReal modules (they're on CPAN, bestie) or the PDL extension (also on CPAN). Pick your fighter 💅
How do I perform an operation on a series of integers? 🔢
To call a function on every element of an array and collect what comes back, use:
my @results = map { my_func($_) } @array;Like, for example:
my @triple = map { 3 * $_ } @single;If you wanna call a function on each element but you literally do not care about the results:
foreach my $iterator (@array) {
some_func($iterator);
}And to hit every integer in a (small) range:
my @results = map { some_func($_) } (5 .. 25);⚠️ But heads up queen, in this form the .. operator builds an actual list of every integer in the range, which can eat a LOT of memory if the range is huge. No cap.
The problem does not happen when you use .. inside a for loop though, because there the range operator gets optimized to just iterate over the range instead of building the whole list. So:
my @results = ();
for my $i (5 .. 500_005) {
push(@results, some_func($i));
}or even
push(@results, some_func($_)) for 5 .. 500_005;neither of these will make an intermediate list of 500,000 integers. Slay 💖
How can I output Roman numerals? 🏛️
Get the <http://www.cpan.org/modules/by-module/Roman> module. That's it, that's the answer.
Why aren't my random numbers random? 🎲😭
If you're on a Perl before 5.004, you gotta call srand once at the start of your program to seed the random number generator:
BEGIN { srand() if $] < 5.004 }5.004 and later call srand automatically at the beginning, so you don't have to. And do not call srand more than once — that makes your numbers less random, not more. It's giving self-sabotage, fr fr 💀
Computers are good at being predictable and bad at being random (despite all the appearances caused by bugs in your programs :-). The random article in the "Far More Than You Ever Wanted To Know" collection at <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz>, courtesy of Tom Phoenix, goes deeper on this. And John von Neumann said: "Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin."
Perl relies on the underlying system for how rand and srand are implemented; on some systems, the generated numbers just aren't random enough (especially on Windows: see <http://www.perlmonks.org/?node_id=803632>). Several CPAN modules in the Math namespace implement better pseudorandom generators; see for example Math::Random::MT ("Mersenne Twister", fast). More algorithms for random numbers are described in "Numerical Recipes in C" at <http://www.nr.com/>
There are also several modules for sampling system sources of random numbers, which are useful for security tokens and cryptography. For example, Crypt::URandom. See the "CPAN Author's Guide to Random Data for Security" at <https://security.metacpan.org/docs/guides/random-data-for-security.html>
How do I get a random number between X and Y? 🎯
To get a random number between two values, you use the rand() built-in to get a random number between 0 and 1, and then you shift that into the range you actually want.
rand($x) returns a number such that 0 <= rand($x) < $x. So what you want perl to figure out is a random number in the range from 0 to the difference between your X and Y.
Meaning: to get a number between 10 and 15 inclusive, you want a random number between 0 and 5 that you can then add to 10:
my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )So here's a cute little function that abstracts all that away. It picks a random integer between the two given integers (inclusive). Like, random_int_between(50,120):
sub random_int_between {
my($min, $max) = @_;
# Assumes that the two arguments are integers themselves!
return $min if $min == $max;
($min, $max) = ($max, $min) if $min > $max;
return $min + int rand(1 + $max - $min);
}✨ Data: Dates ✨
🗓️ How do I find the day or week of the year?
Okay so first up: the day of the year? It's just sitting in the list that the localtime function gives you back. And pro tip — if you don't hand localtime an argument, it uses the current time. So generous of it 💖
my $day_of_year = (localtime)[7];The POSIX module can also format a date as the day of the year, or the week of the year:
use POSIX qw/strftime/;
my $day_of_year = strftime "%j", localtime;
my $week_of_year = strftime "%W", localtime;And if you wanna snag the day of year for any date (not just now)? Use POSIX's mktime to get a time in epoch seconds, then feed that to localtime:
use POSIX qw/mktime strftime/;
my $day_of_year = strftime "%j",
localtime( mktime( 0, 0, 0, 18, 11, 87 ) );You can also pull up Time::Piece — it ships with Perl and gives you a localtime that returns an object (so fancy ✨):
use Time::Piece;
my $day_of_year = localtime->yday;
my $week_of_year = localtime->week;And the Date::Calc module has two functions for this too, bestie:
use Date::Calc;
my $day_of_year = Day_of_Year( 1987, 12, 18 );
my $week_of_year = Week_of_Year( 1987, 12, 18 );🥳 How do I find the current century or millennium?
These two little functions, that's it, no notes:
sub get_century {
return int((((localtime(shift || time))[5] + 1999))/100);
}
sub get_millennium {
return 1+int((((localtime(shift || time))[5] + 1899))/1000);
}⚠️ heads up bestie ⚠️ On some systems, the POSIX module'sstrftime()function has been extended in a non-standard way to use a%Cformat, which those systems sometimes claim is the "century". It isn't. Because on most such systems, that's only the first two digits of the four-digit year, and so it cannot be used to reliably determine the current century or millennium. Don't get played. 🙈
🆚 How can I compare two dates and find the difference?
(contributed by brian d foy)
You could just store all your dates as a number and then subtract. But life isn't always that simple, bestie 😭
The Time::Piece module, which comes with Perl, replaces localtime with a version that returns an object. It also overloads the comparison operators so you can compare them directly:
use Time::Piece;
my $date1 = localtime( $some_time );
my $date2 = localtime( $some_other_time );
if( $date1 < $date2 ) {
print "The date was in the past\n";
}You can also get differences with a subtraction, and that hands you back a Time::Seconds object:
my $date_diff = $date1 - $date2;
print "The difference is ", $date_diff->days, " days\n";And if you want to work with formatted dates, the Date::Manip, Date::Calc, or DateTime modules can help you out 💅
🔢 How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same format, you can split it up and pass the parts to timelocal in the standard Time::Local module. Otherwise? Go look into the Date::Calc, Date::Parse, and Date::Manip modules from CPAN 🦋
🌟 How can I find the Julian Day?
(contributed by brian d foy and Dave Cross)
You can use the Time::Piece module — part of the Standard Library — which can convert a date/time to a Julian Day:
$ perl -MTime::Piece -le 'print localtime->julian_day'
2455607.7959375Or the modified Julian Day:
$ perl -MTime::Piece -le 'print localtime->mjd'
55607.2961226851Or even the day of the year (which is what some people think of as a Julian day):
$ perl -MTime::Piece -le 'print localtime->yday'
45You can do all the same things with the DateTime module too:
$ perl -MDateTime -le'print DateTime->today->jd'
2453401.5
$ perl -MDateTime -le'print DateTime->today->mjd'
53401
$ perl -MDateTime -le'print DateTime->today->doy'
31There's also the Time::JulianDay module on CPAN. But please make sure you actually want to find a Julian day, because literally so many people have different ideas about what a Julian day even is (peep <http://www.hermetic.ch/cal_stud/jdn.htm> for example):
$ perl -MTime::JulianDay -le 'print local_julian_day( time )'
55608🌅 How do I find yesterday's date?
(contributed by brian d foy)
To do it correctly, you use one of the Date modules, because those work with calendars instead of times. The DateTime module makes it simple, and it gives you the same time of day, just the day before — even when daylight saving time changes happen:
use DateTime;
my $yesterday = DateTime->now->subtract( days => 1 );
print "Yesterday was $yesterday\n";You can also use the Date::Calc module with its Today_and_Now function:
use Date::Calc qw( Today_and_Now Add_Delta_DHMS );
my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 );
print "@date_time\n";Most people try to use the time rather than the calendar to figure out dates, but that assumes every day is twenty-four hours long. And for most people, there are two days a year when they aren't: the switch to and from summer time throws the whole thing off 😵💫 So like, the rest of these suggestions will be wrong sometimes. Sorry bestie, that's just the tea.
Starting with Perl 5.10, Time::Piece and Time::Seconds are part of the standard distribution, so you might think you could do something like this:
use Time::Piece;
use Time::Seconds;
my $yesterday = localtime() - ONE_DAY; # WRONG
print "Yesterday was $yesterday\n";The Time::Piece module exports a new localtime that returns an object, and Time::Seconds exports the ONE_DAY constant which is a set number of seconds. Which means it always gives you the time 24 hours ago — and that is not always yesterday. 💀 This can cause problems around the end of daylight saving time, when there's one day that is 25 hours long.
You have the same problem with Time::Local, which will give the wrong answer for those exact same special cases:
# contributed by Gunnar Hjalmarsson
use Time::Local;
my $today = timelocal 0, 0, 12, ( localtime )[3..5];
my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; # WRONG
printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;⏳ Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant?
(contributed by brian d foy)
Perl itself never had a Y2K problem — although that never stopped people from creating Y2K problems all on their own 🙈 See the documentation for localtime for its proper use.
Starting with Perl 5.12, localtime and gmtime can handle dates past 03:14:08 January 19, 2038, which is when a 32-bit based time would overflow. You still might get a warning on a 32-bit perl:
% perl5.12 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )'
Integer overflow in hexadecimal number at -e line 1.
Wed Nov 1 19:42:39 5576711And on a 64-bit perl, you can get even bigger dates for those really long-running projects 👑
% perl5.12 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )'
Thu Nov 2 00:42:39 5576711Though ngl, you're still out of luck if you need to keep track of decaying protons. 💀
🎀 Data: Strings 🎀
✅ How do I validate input?
(contributed by brian d foy)
There are so many ways to make sure values are what you expect or what you wanna accept. Besides the specific examples we cover in the perlfaq, you can also look at the modules with Assert and Validate in their names, plus other modules such as Regexp::Common.
Some modules have validation for particular types of input, like Business::ISBN, Business::CreditCard, Email::Valid, and Data::Validate::IP. Slay 💅
🔓 How do I unescape a string?
Okay so it depends — what do you even mean by "escape", bestie? URL escapes are dealt with over in perlfaq9. Shell escapes with the backslash (\) character get removed with:
s/\\(.)/$1/g;Just so you know: this will not expand\nor\tor any other special escapes. Nope. Not happening. 🚫
✂️ How do I remove consecutive pairs of characters?
(contributed by brian d foy)
You can use the substitution operator to find pairs of characters (or runs of characters) and replace them with a single instance. In this substitution, we find a character in (.). The memory parentheses store the matched character in the back-reference \g1, and we use that to require that the same thing immediately follows it. Then we replace that part of the string with the character in $1.
s/(.)\g1/$1/g;We can also use the transliteration operator, tr///. In this example, the search list side of our tr/// contains nothing — but the c option complements that, so it ends up containing everything. The replacement list also contains nothing, so the transliteration is almost a no-op since it won't do any replacements (or, more exactly, it replaces the character with itself). But the s option squashes duplicated and consecutive characters in the string, so a character never shows up next to itself:
my $str = 'Haarlem'; # in the Netherlands
$str =~ tr///cs; # Now Harlem, like in New York🪄 How do I expand function calls in a string?
(contributed by brian d foy)
This is documented in perlref, and even though it's genuinely not the easiest thing to read, it does work, I promise. In each of these examples, we call the function inside the braces used to dereference a reference. If we have more than one return value, we can construct and dereference an anonymous array. In this case, we're calling the function in list context:
print "The time values are @{ [localtime] }.\n";But if we want to call the function in scalar context, we gotta do a bit more work. You can really have any code you like inside those braces, so you simply have to end with the scalar reference — how you do that is up to you, and you can use code inside the braces. Note that using parens creates a list context, so we need scalar to force scalar context onto the function:
print "The time is ${\(scalar localtime)}.\n"
print "The time is ${ my $x = localtime; \$x }.\n";And if your function already returns a reference? Then you don't need to create the reference yourself, bestie:
sub timestamp { my $t = localtime; \$t }
print "The time is ${ timestamp() }.\n";The Interpolation module can also do a lot of magic for you ✨ You can specify a variable name — in this case E — to set up a tied hash that does the interpolation for you. It has several other methods to do this as well.
use Interpolation E => 'eval';
print "The time values are $E{localtime()}.\n";But honestly? In most cases, it's probably easier to just use string concatenation, which also forces scalar context 🫶
print "The time is " . localtime() . ".\n";🔍 How do I find matching/nesting anything?
Okay bestie, so if you wanna find something sitting between two single characters, a pattern like /x([^x]*)x/ hands you the bits in between in $1. Easy. For multiple ones you need something more like /alpha(.*?)omega/. And for nested patterns and/or balanced expressions? That's the so-called (?PARNO) construct (available since perl 5.10), bestie, it ate. The CPAN module Regexp::Common can help you build such regular expressions too (peep Regexp::Common::balanced and Regexp::Common::delimited specifically).
More complex cases will require you to write a parser, probably using a parsing module from CPAN, like Regexp::Grammars, Parse::RecDescent, Parse::Yapp, Text::Balanced, or Marpa::R2. No cap.
🔄 How do I reverse a string?
Use reverse() in scalar context, as documented in "reverse" in perlfunc. That's it, that's the whole thing 💅
my $reversed = reverse $string;📏 How do I expand tabs in a string?
You can lowkey do it yourself:
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;Or you can just use the Text::Tabs module (part of the standard Perl distribution) and let it handle the vibes:
use Text::Tabs;
my @expanded_lines = expand(@lines_with_tabs);📝 How do I reformat a paragraph?
Use Text::Wrap (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);Heads up though: the paragraphs you give to Text::Wrap should not contain embedded newlines. And Text::Wrap doesn't justify the lines (flush-right). It's giving "does the bare minimum and slays."
Or use the CPAN module Text::Autoformat. Formatting files can be easily done by making a shell alias, like so:
alias fmt="perl -i -MText::Autoformat -n0777 \
-e 'print autoformat $_, {all=>1}' $*"See the documentation for Text::Autoformat to appreciate its many capabilities. She's iconic, fr fr. ✨
✂️ How can I access or change N characters of a string?
You can access the first characters of a string with substr(). To get the first character, for example, start at position 0 and grab the string of length 1.
my $string = "Just another Perl Hacker";
my $first_char = substr( $string, 0, 1 ); # 'J'To change part of a string, you can use the optional fourth argument which is the replacement string.
substr( $string, 13, 4, "Perl 5.8.0" );You can also use substr() as an lvalue. Main character energy 🎀
substr( $string, 13, 4 ) = "Perl 5.8.0";5️⃣ How do I change the Nth occurrence of something?
Bestie, you have to keep track of N yourself. For example, let's say you want to change the fifth occurrence of "whoever" or "whomever" into "whosoever" or "whomsoever", case insensitively. These all assume that $_ contains the string to be altered.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
}ige;In the more general case, you can use the /g modifier in a while loop, keeping count of matches.
$WANT = 3;
$count = 0;
$_ = "One fish two fish red fish blue fish";
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
}
}That prints out: "The third fish is a red one." 🐟 You can also use a repetition count and repeated pattern like this:
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;🔢 How can I count the number of occurrences of a substring within a string?
There are a number of ways, with varying efficiency. If you want a count of a certain single character (X) within a string, you can use the tr/// function like so:
my $string = "ThisXlineXhasXsomeXx'sXinXit";
my $count = ($string =~ tr/X//);
print "There are $count X characters in the string";This is fine if you are just looking for a single character. However, if you are trying to count multiple character substrings within a larger string, tr/// won't work. What you can do is wrap a while() loop around a global pattern match. For example, let's count negative integers:
my $string = "-9 55 48 -2 23 -76 4 14 -44";
my $count = 0;
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";Another version uses a global match in list context, then assigns the result to a scalar, producing a count of the number of matches. Chef's kiss 🧚
my $count = () = $string =~ /-\d+/g;🔠 How do I capitalize all the words on one line?
(contributed by brian d foy)
Damian Conway's Text::Autoformat handles all of the thinking for you:
use Text::Autoformat;
my $s = "Going to the desert with a camel";
printf "%-10s %s\n", "original:", $s;
for my $style (qw( sentence title highlight )) {
printf "%-10s %s\n",
"$style:",
autoformat($s, { case => $style }) =~ s/\R+\z//r;
}Each style has a different idea of what it should do with small words:
original: Going to the desert with a camel
sentence: Going to the desert with a camel
title: Going To The Desert With A Camel
highlight: Going to the Desert with a CamelTrying this yourself in a simple regex has more than a few pitfalls, ngl. Perl "words" are groups of \w+, but that's not what you want to capitalize. Some of those characters aren't even letters. What if you used a word boundary, \b?
my $string = "A camel's journey";
$string =~ s/\b(\w)/\U$1/g; # A Camel'S JourneyHow is Perl supposed to know not to capitalize that "s"? 😭 You could work harder to look for preceding whitespace or the beginning of the string:
$string =~ s/( (?:^|\s) \w ) /\U$1/xg; # A Camel's JourneyOr maybe you match the entire group, lowercase it all ("L"), then uppercase the first letter ("\u"):
$string =~ s/([\w']+)/\u\L$1/g;Each of these have their annoying edge cases. Perl v5.22 added fancier Unicode word and sentence boundaries to fix this sort of problem. The \b{wb} word boundary ignores some things that it doesn't think start or end "human" words:
use v5.22;
my $string = "A camel's journey";
$string =~ s/\b{wb}(\w)/\U$1/g; # A Camel's JourneyThe details are in Unicode Technical Report #29 (<https://unicode.org/reports/tr29/>), and you should make sure that its idea of a word boundary corresponds to yours.
Now, what if you don't want to capitalize that "and" or "the"? Just use Text::Autoformat and get on with the next problem. :) 💖
🪓 How can I split a [character]-delimited string except when inside [character]?
Several modules can handle this sort of parsing--Text::Balanced, Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
Take the example case of trying to split a string that is comma-separated into its different fields. You can't use split(/,/) because you shouldn't split if the comma is inside quotes. For example, take a data line like this:
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of Mastering Regular Expressions, to handle these for us. He suggests (assuming your string is contained in $text):
my @new = ();
push(@new, $+) while $text =~ m{
"([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes
| ([^,]+),?
| ,
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, "like "this"". 👀
Alternatively, the Text::ParseWords module (part of the standard Perl distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);For parsing or generating CSV, though, using Text::CSV rather than implementing it yourself is highly recommended; you'll save yourself odd bugs popping up later by just using code which has already been tried and tested in production for years. Trust. 🫶
🧼 How do I strip blank space from the beginning/end of a string?
(contributed by brian d foy)
A substitution can do this for you. For a single line, you want to replace all the leading or trailing whitespace with nothing. You can do that with a pair of substitutions:
s/^\s+//;
s/\s+$//;You can also write that as a single substitution, although it turns out the combined statement is slower than the separate ones. That might not matter to you, though:
s/^\s+|\s+$//g;In this regular expression, the alternation matches either at the beginning or the end of the string since the anchors have a lower precedence than the alternation. With the /g flag, the substitution makes all possible matches, so it gets both. Remember, the trailing newline matches the \s+, and the $ anchor can match to the absolute end of the string, so the newline disappears too. Just add the newline to the output, which has the added benefit of preserving "blank" (consisting entirely of whitespace) lines which the ^\s+ would remove all by itself:
while( <> ) {
s/^\s+|\s+$//g;
print "$_\n";
}For a multi-line string, you can apply the regular expression to each logical line in the string by adding the /m flag (for "multi-line"). With the /m flag, the $ matches before an embedded newline, so it doesn't remove it. This pattern still removes the newline at the end of the string:
$string =~ s/^\s+|\s+$//gm;Remember that lines consisting entirely of whitespace will disappear, since the first part of the alternation can match the entire string and replace it with nothing. If you need to keep embedded blank lines, you have to do a little more work. Instead of matching any whitespace (since that includes a newline), just match the other whitespace:
$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;🛏️ How do I pad a string with blanks or pad a number with zeroes?
In the following examples, $pad_len is the length to which you wish to pad the string, $text or $num contains the string to be padded, and $pad_char contains the padding character. You can use a single character string constant instead of the $pad_char variable if you know what it is in advance. And in the same way you can use an integer in place of $pad_len if you know the pad length in advance.
The simplest method uses the sprintf function. It can pad on the left or right with blanks and on the left with zeroes and it will not truncate the result. The pack function can only pad strings on the right with blanks and it will truncate the result to a maximum length of $pad_len.
# Left padding a string with blanks (no truncation):
my $padded = sprintf("%${pad_len}s", $text);
my $padded = sprintf("%*s", $pad_len, $text); # same thing# Right padding a string with blanks (no truncation):
my $padded = sprintf("%-${pad_len}s", $text);
my $padded = sprintf("%-*s", $pad_len, $text); # same thing
# Left padding a number with 0 (no truncation):
my $padded = sprintf("%0${pad_len}d", $num);
my $padded = sprintf("%0*d", $pad_len, $num); # same thing
# Right padding a string with blanks using pack (will truncate):
my $padded = pack("A$pad_len",$text);okay so if blank-or-zero padding isn't your vibe and you want some other character doing the padding, there's a whole lil toolkit for that 💅. Every single one of these builds a pad string with the x operator and glues it onto $text. Heads up bestie: these methods do not truncate $text, periodt.
Left and right padding with any character, creating a new string (your original stays untouched, queen 👑):
my $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
my $padded = $text . $pad_char x ( $pad_len - length( $text ) );Left and right padding with any character, but this time we're modifying $text directly (no new variable, we're committing ✨):
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
$text .= $pad_char x ( $pad_len - length( $text ) );🔍 How do I extract selected columns from a string?
(contributed by brian d foy)
so if you already know which columns hold your data, substr is right there and it eats one single column for breakfast:
my $column = substr( $line, $start_column, $length );and you can absolutely use split if your columns are separated by whitespace or some other delimiter — as long as whitespace or that delimiter can't show up inside the data itself, iykyk:
my $line = ' fred barney betty ';
my @columns = split /\s+/, $line;
# ( '', 'fred', 'barney', 'betty' );
my $line = 'fred||barney||betty';
my @columns = split /\|/, $line;
# ( 'fred', '', 'barney', '', 'betty' );Now do not do this for comma-separated values, bestie, I'm begging 🙈 — that format is a bit more complicated. Use one of the modules that actually handles CSV, like Text::CSV, Text::CSV_XS, or Text::CSV_PP 💖.
And if you wanna break apart an entire line of fixed columns, that's unpack with the A (ASCII) format. Slap a number after the format specifier and that's your column width 🎀. More details live in the pack and unpack entries in perlfunc.
my @fields = unpack( "A8 A8 A8 A16 A4", $line );⚠️ tiny but crucial: spaces in the format argument tounpackdo not mean literal spaces. If your data is actually space-separated, you probably wantsplitinstead.
🔊 How do I find the soundex value of a string?
(contributed by brian d foy)
use the Text::Soundex module, simple as that 💫. And if you're going for fuzzy or close matching, you might also try the String::Approx, Text::Metaphone, and Text::DoubleMetaphone modules — pick your fighter 🌸.
💌 How can I expand variables in text strings?
(contributed by brian d foy)
okay so, real talk: if you can avoid this, avoid it. Or if you can use a templating system like Text::Template or Template Toolkit, do that instead, no cap. You might even get the whole job done with sprintf or printf:
my $string = sprintf 'Say hello to %s and %s', $foo, $bar;But, for that one-off simple case where you do not want to drag a whole templating system into the group chat, you can use a string with two Perl scalar variables in it. In this example we want $foo and $bar expanded to their values:
my $foo = 'Fred';
my $bar = 'Barney';
$string = 'Say hello to $foo and $bar';One way is the substitution operator with a double /e flag. The first /e evaluates $1 on the replacement side and turns it into $foo. The second /e starts with $foo and swaps in its value. So $foo becomes 'Fred', and that's what's left in the string 🪄:
$string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'💀 the/ewill also silently ignore violations of strict, replacing undefined variable names with the empty string. And because I'm using/e(twice even!), I inherit all the exact same security problems as string-formeval. If there's something odd in$foo— say something like"@{[ system "rm -rf /" ]}"— then bestie I could genuinely get myself in trouble. 🚨
To dodge the security problem entirely, you could pull values from a hash instead of evaluating variable names. With a single /e I can check whether the hash has the value, and if it doesn't, drop in a marker — here "???" — to signal that something got missed:
my $string = 'This has $foo and $bar';
my %Replacements = (
foo => 'Fred',
);
# $string =~ s/\$(\w+)/$Replacements{$1}/g;
$string =~ s/\$(\w+)/
exists $Replacements{$1} ? $Replacements{$1} : '???'
/eg;
print $string;🐍 Does Perl have anything like Ruby's #{} or Python's f string?
unlike those two, Perl lets you embed a variable naked in a double quoted string, e.g. "variable $variable" 😌. When there's no whitespace or other non-word character following the variable name, you can add braces (e.g. "foo ${foo}bar") to make sure it parses the way you meant.
An array can also be embedded directly in a string, and by default it gets expanded with spaces between the elements. That default LIST_SEPARATOR can be changed by assigning a different string to the special variable $", like local $" = ', ';.
Perl also supports references within a string, giving you the equivalent of what those other two languages can do.
"${\ ... }" embedded in a string works for most simple statements, like an object->method call. For more complex code, wrap it in a do block: "${\ do{...} }".
And when you want a list expanded per $", use "@{[ ... ]}" ✨:
use Time::Piece;
use Time::Seconds;
my $scalar = 'STRING';
my @array = ( 'zorro', 'a', 1, 'B', 3 );
# Print the current date and time and then Tomorrow
my $t = Time::Piece->new;
say "Now is: ${\ $t->cdate() }";
say "Tomorrow: ${\ do{ my $T=Time::Piece->new + ONE_DAY ; $T->fullday }}";
# some variables in strings
say "This is some scalar I have $scalar, this is an array @array.";
say "You can also write it like this ${scalar} @{array}.";
# Change the $LIST_SEPARATOR
local $" = ':';
say "Set \$\" to delimit with ':' and sort the Array @{[ sort @array ]}";you may also wanna look at the module Quote::Code, plus templating tools like Template::Toolkit and Mojo::Template 🌈.
See also: "How can I expand variables in text strings?" and "How do I expand function calls in a string?" in this FAQ.
😤 What's wrong with always quoting "$vars"?
the problem, bestie, is that those double-quotes force stringification — coercing numbers and references into strings — even when you don't want them to be strings 💀. Think of it like this: double-quote expansion is used to produce new strings. If you already have a string... why do you need more?? 🙈
If you get used to writing odd stuff like this:
print "$var"; # BAD
my $new = "$old"; # BAD
somefunc("$var"); # BADyou're gonna be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct:
print $var;
my $new = $old;
somefunc($var);otherwise, besides slowing you down, you'll break code the moment the thing in the scalar is neither a string nor a number, but a reference:
func(\@array);
sub func {
my $aref = shift;
my $oref = "$aref"; # WRONG
}you can also trip into subtle problems on those few operations in Perl that genuinely do care about the string-vs-number difference — like the magical ++ autoincrement operator or the syscall() function 👀.
and stringification also destroys arrays 😭:
my @lines = `command`;
print "@lines"; # WRONG - extra blanks
print @lines; # right📜 Why don't my <<HERE documents work?
Here documents live in perlop. Check these three things, in order, every time 🔍:
- There must be no space after the
<<part. - There (probably) should be a semicolon at the end of the opening token.
- You can't (easily) have any space in front of the tag.
- There needs to be at least a line separator after the end token.
so if you want to indent the text inside the here document, you can do this:
# all in one
(my $VAR = <<HERE_TARGET) =~ s/^\s+//gm;
your text
goes here
HERE_TARGETbut — important — the HERE_TARGET must still be flush against the margin. And if you want that indented too, you'll have to quote it in the indentation 💅:
(my $quote = <<' FINIS') =~ s/^\s+//gm;
...we will have peace, when you and all your works have
perished--and the works of your dark master to whom you
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
$quote =~ s/\s+--/\n--/;and here's a nice general-purpose fixer-upper function for indented here documents 🛠️. It expects to be called with a here document as its argument. It checks whether each line begins with a common substring, and if so, strips that substring off. Otherwise, it takes the amount of leading whitespace found on the first line and removes that much off each subsequent line:
sub fix {
local $_ = shift;
my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\g1\g2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
($white, $leader) = (/^(\s+)/, '');
}
s/^\s*?$leader(?:$white)?//gm;
return $_;
}this works with leading special strings, dynamically determined:
my $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
@@@ int
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
@@@ while ( op = (*op->op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOPor with a fixed amount of leading whitespace, with the remaining indentation correctly preserved 🎀:
my $poem = fix<<EVER_ON_AND_ON;
Now far ahead the Road has gone,
And I must follow, if I can,
Pursuing it with eager feet,
Until it joins some larger way
Where many paths and errands meet.
And whither then? I cannot say.
--Bilbo in /usr/src/perl/pp_ctl.c
EVER_ON_AND_ON✨ starting with Perl version 5.26, the language gained a much simpler and cleaner way to write indented here documents: the tilde (~) modifier. See "Indented Here-docs" in perlop for the details.
📚 Data: Arrays
🧺 What is the difference between a list and an array?
(contributed by brian d foy)
A list is a fixed collection of scalars, bestie. Locked in. An array is a variable that holds a variable collection of scalars, so it can flex and change 💅. And because an array can hand over its collection for list operations, list operations totally work on arrays too:
# slices
( 'dog', 'cat', 'bird' )[2,3];
@animals[2,3];
# iteration
foreach ( qw( dog cat bird ) ) { ... }
foreach ( @animals ) { ... }
my @three = grep { length == 3 } qw( dog cat bird );
my @three = grep { length == 3 } @animals;
# supply an argument list
wash_animals( qw( dog cat bird ) );
wash_animals( @animals );But array operations — the ones that change the scalars, rearrange them, or add/subtract some scalars — those only work on actual arrays. They can't work on a list, because a list is fixed, periodt. Array operations include "shift", "unshift", "push", "pop", and "splice".
An array can also change its length, which is lowkey iconic:
$#animals = 1; # truncate to two elements
$#animals = 10000; # pre-extend to 10,001 elementsYou can change an array element, but you can not change a list element. No cap:
$animals[0] = 'Rottweiler';
qw( dog cat bird )[0] = 'Rottweiler'; # syntax error!
foreach ( @animals ) {
s/^d/fr/; # works fine
}
foreach ( qw( dog cat bird ) ) {
s/^d/fr/; # Error! Modification of read only value!
}Okay but here's the plot twist 🍓: if the list element is itself a variable, it looks like you can change a list element. But nope — the list element is the variable, not the data. You're not changing the list element, you're changing something the list element refers to. The list element itself doesn't change: it's still the same variable, bestie.
You also gotta be careful about context, fr fr. You can assign an array to a scalar to get the number of elements in the array. But this only works for arrays:
my $count = @animals; # only works with arraysIf you try to do the same thing with what you think is a list, you get a whole different vibe. Even though it looks like you have a list on the righthand side, Perl actually sees a bunch of scalars separated by a comma:
my $scalar = ( 'dog', 'cat', 'bird' ); # $scalar gets birdSince you're assigning to a scalar, the righthand side is in scalar context. The comma operator (yes bestie, it's an operator!) in scalar context evaluates its lefthand side, throws away the result, then evaluates its righthand side and returns that result. So effectively, that list-lookalike assigns $scalar its rightmost value. So many people mess this up because they pick a list-lookalike whose last element is also the count they expect 💀:
my $scalar = ( 1, 2, 3 ); # $scalar gets 3, accidentallyWhat is the difference between $array[1] and @array[1]?
(contributed by brian d foy)
The difference is the sigil, that special little character in front of the array name ✨. The "$" sigil means "exactly one item", while the "@" sigil means "zero or more items". The "$" gets you a single scalar, while the "@" gets you a list.
The confusion arises because people incorrectly assume that the sigil denotes the variable type. It does not, bestie 🫶.
The $array[1] is a single-element access to the array. It's gonna return the item in index 1 (or undef if there's no item there). If you intend to get exactly one element from the array, this is the form you should use.
The @array[1] is an array slice, even though it has only one index. You can pull out multiple elements simultaneously by specifying additional indices as a list, like @array[1,4,3,0].
Using a slice on the lefthand side of the assignment supplies list context to the righthand side. This can lead to unexpected results 💀. For instance, if you want to read a single line from a filehandle, assigning to a scalar value is fine:
$array[1] = <STDIN>;However, in list context, the line input operator returns all of the lines as a list. The first line goes into @array[1] and the rest of the lines mysteriously disappear 🙈:
@array[1] = <STDIN>; # most likely not what you wantEither the "use warnings" pragma or the -w flag will warn you when you use an array slice with a single index.
How can I remove duplicate elements from a list or array?
(contributed by brian d foy)
Use a hash. When you think the words "unique" or "duplicated", think "hash keys". That's the whole vibe check 🔥.
If you don't care about the order of the elements, you could just create the hash then extract the keys. It's not important how you create that hash: just that you use "keys" to get the unique elements.
my %hash = map { $_, 1 } @array;
# or a hash slice: @hash{ @array } = ();
# or a foreach: $hash{$_} = 1 foreach ( @array );
my @unique = keys %hash;If you want to use a module, try the "uniq" function from List::Util. In list context it returns the unique elements, preserving their order in the list. In scalar context, it returns the number of unique elements.
use List::Util qw(uniq);
my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 8 ); # 1,2,3,4,5,6,8
my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 8 ); # 7You can also go through each element and skip the ones you've already seen before. Use a hash to keep track. The first time the loop sees an element, that element has no key in %seen. The "next" statement creates the key and immediately uses its value, which is "undef", so the loop continues to the "push" and increments the value for that key. The next time the loop sees that same element, its key exists in the hash and the value for that key is true (since it's not 0 or "undef"), so the next skips that iteration and the loop goes to the next element.
my @unique = ();
my %seen = ();
foreach my $elem ( @array ) {
next if $seen{ $elem }++;
push @unique, $elem;
}You can write this more briefly using a grep, which does the same thing. Slay 🌸.
my %seen = ();
my @unique = grep { ! $seen{ $_ }++ } @array;How can I tell whether a certain element is contained in a list or array?
(portions of this answer contributed by Anno Siegel and brian d foy)
Hearing the word "in" is a big clue that you probably should have used a hash, not a list or array, to store your data. Hashes are literally designed to answer this question quickly and efficiently. Arrays are not. Periodt.
That being said, there are several ways to approach this. If you're gonna make this query many times over arbitrary string values, the fastest way is probably to invert the original array and maintain a hash whose keys are the first array's values:
my @blues = qw/azure cerulean teal turquoise lapis-lazuli/;
my %is_blue = ();
for (@blues) { $is_blue{$_} = 1 }Now you can check whether $is_blue{$some_color}. It might have been a good idea to keep the blues all in a hash in the first place 👑
If the values are all small integers, you could use a simple indexed array. This kind of array will take up less space:
my @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
my @is_tiny_prime = ();
for (@primes) { $is_tiny_prime[$_] = 1 }
# or simply @is_tiny_prime[@primes] = (1) x @primes;Now you check whether $is_tiny_prime[$some_number].
If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead:
my @articles = ( 1..10, 150..2000, 2017 );
undef $read;
for (@articles) { vec($read,$_,1) = 1 }Now check whether "vec($read,$n,1)" is true for some $n.
These methods guarantee fast individual tests but require a re-organization of the original list or array. They only pay off if you have to test multiple values against the same array.
If you're testing only once, the standard module List::Util exports the function "any" for this purpose. It works by stopping once it finds the element. It's written in C for speed, and its Perl equivalent looks like this subroutine:
sub any (&@) {
my $code = shift;
foreach (@_) {
return 1 if $code->();
}
return 0;
}If speed is of little concern, the common idiom uses grep in scalar context (which returns the number of items that passed its condition) to traverse the entire list. This does have the benefit of telling you how many matches it found, though.
my $is_there = grep $_ eq $whatever, @array;If you want to actually extract the matching elements, simply use grep in list context.
my @matches = grep $_ eq $whatever, @array;How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
Use a hash. Here's code to do both and more. It assumes that each element is unique in a given array:
my (@union, @intersection, @difference);
my %count = ();
foreach my $element (@array1, @array2) { $count{$element}++ }
foreach my $element (keys %count) {
push @union, $element;
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}Note that this is the symmetric difference, that is, all elements in either A or in B but not in both. Think of it as an xor operation. That's the tea ☕.
How do I test whether two arrays or hashes are equal?
The following code works for single-level arrays. It uses a stringwise comparison, and does not distinguish defined versus undefined empty strings. Modify if you have other needs.
$are_equal = compare_arrays(\@frogs, \@toads);
sub compare_arrays {
my ($first, $second) = @_;
no warnings; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
}
return 1;
}For multilevel structures, you may wish to use an approach more like this one. It uses the CPAN module FreezeThaw:
use FreezeThaw qw(cmpStr);
my @a = my @b = ( "this", "that", [ "more", "stuff" ] );
printf "a and b contain %s arrays\n",
cmpStr(\@a, \@b) == 0
? "the same"
: "different";This approach also works for comparing hashes. Here we'll demonstrate two different answers:
use FreezeThaw qw(cmpStr cmpStrHard);
my %a = my %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
$a{EXTRA} = \%b;
$b{EXTRA} = \%a;
printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";
printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";The first one reports that both hashes contain the same data, while the second reports that they do not. Which you prefer is left as an exercise to the reader 🦋.
How do I find the first array element for which a condition is true? 🔍
okay bestie, so you want the first element in @array that passes the vibe check? The first() function from List::Util is the moment, and it ships with Perl 5.8. This example finds the first element that contains "Perl".
use List::Util qw(first);
my $element = first { /Perl/ } @array;If you cannot use List::Util, you can make your own loop to do the same thing. The second you find the element, you stop the loop with last.
my $found;
foreach ( @array ) {
if( /Perl/ ) { $found = $_; last }
}Want the array index instead? Use firstidx() from List::MoreUtils:
use List::MoreUtils qw(firstidx);
my $index = firstidx { /Perl/ } @array;Or write it yourself: iterate through the indices and check the array element at each index until you find one that satisfies the condition.
my( $found, $index ) = ( undef, -1 );
for( $i = 0; $i < @array; $i++ ) {
if( $array[$i] =~ /Perl/ ) {
$found = $array[$i];
$index = $i;
last;
}
}How do I handle linked lists? 🔗
(contributed by brian d foy)
Perl’s arrays do not have a fixed size, so you don’t need linked lists if you just want to add or remove items. You can use array operations such as push, pop, shift, unshift, or splice to do that.
Sometimes, however, linked lists can be useful in situations where you want to “shard” an array so you have many small arrays instead of a single big array. You can keep arrays longer than Perl’s largest array index, lock smaller arrays separately in threaded programs, reallocate less memory, or quickly insert elements in the middle of the chain.
Steve Lembark goes through the details in his YAPC::NA 2009 talk “Perly Linked Lists” ( <http://www.slideshare.net/lembark/perly-linked-lists> ), although you can just use his LinkedList::Single module.
How do I handle circular lists? 🎡
(contributed by brian d foy)
If you want to cycle through an array endlessly, you can increment the index modulo the number of elements in the array:
my @array = qw( a b c );
my $i = 0;
while( 1 ) {
print $array[ $i++ % @array ], "\n";
last if $i > 20;
}You can also use Tie::Cycle to use a scalar that always has the next element of the circular array:
use Tie::Cycle;
tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ];
print $cycle; # FFFFFF
print $cycle; # 000000
print $cycle; # FFFF00The Array::Iterator::Circular creates an iterator object for circular arrays:
use Array::Iterator::Circular;
my $color_iterator = Array::Iterator::Circular->new(
qw(red green blue orange)
);
foreach ( 1 .. 20 ) {
print $color_iterator->next, "\n";
}How do I shuffle an array randomly? 🎲
If you either have Perl 5.8.0 or later installed, or if you have Scalar-List-Utils 1.03 or later installed, you can say:
use List::Util 'shuffle';
@shuffled = shuffle(@list);If not, you can use a Fisher-Yates shuffle.
sub fisher_yates_shuffle {
my $deck = shift; # $deck is a reference to an array
return unless @$deck; # must not be empty!
my $i = @$deck;
while (--$i) {
my $j = int rand ($i+1);
@$deck[$i,$j] = @$deck[$j,$i];
}
}
# shuffle my mpeg collection
#
my @mpeg = <audio/*/*.mp3>;
fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
print @mpeg;Note that the above implementation shuffles an array in place, unlike the List::Util::shuffle() which takes a list and returns a new shuffled list.
You’ve probably seen shuffling algorithms that work using splice, randomly picking another element to swap the current element with
srand;
@new = ();
@old = 1 .. 10; # just a demo
while (@old) {
push(@new, splice(@old, rand @old, 1));
}This is bad because splice is already O(N), and since you do it N times, you just invented a quadratic algorithm; that is, O(N**2). This does not scale, although Perl is so efficient that you probably won’t notice this until you have rather largish arrays.
How do I process/modify each element of an array? 🛠️
Use for/foreach:
for (@lines) {
s/foo/bar/; # change that word
tr/XZ/ZX/; # swap those letters
}Here’s another; let’s compute spherical volumes:
my @volumes = @radii;
for (@volumes) { # @volumes has changed parts
$_ **= 3;
$_ *= (4/3) * 3.14159; # this will be constant folded
}which can also be done with map() which is made to transform one list into another:
my @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;If you want to do the same thing to modify the values of the hash, you can use the values function. As of Perl 5.6 the values are not copied, so if you modify $orbit (in this case), you modify the value.
for my $orbit ( values %orbits ) {
($orbit **= 3) *= (4/3) * 3.14159;
}Prior to perl 5.6 values returned copies of the values, so older perl code often contains constructions such as @orbits{keys %orbits} instead of values %orbits where the hash is to be modified.
How do I select a random element from an array? 🎯
Use the rand() function (see "rand" in perlfunc):
my $index = rand @array;
my $element = $array[$index];Or, simply:
my $element = $array[ rand @array ];How do I permute N elements of a list? 🔄
Use the List::Permutor module on CPAN. If the list is actually an array, try the Algorithm::Permute module (also on CPAN). It’s written in XS code and is very efficient:
use Algorithm::Permute;
my @array = 'a'..'d';
my $p_iterator = Algorithm::Permute->new ( \@array );
while (my @perm = $p_iterator->next) {
print "next permutation: (@perm)\n";
}For even faster execution, you could do:
use Algorithm::Permute;
my @array = 'a'..'d';
Algorithm::Permute::permute {
print "next permutation: (@array)\n";
} @array;Here’s a little program that generates all permutations of all the words on each line of input. The algorithm embodied in the permute() function is discussed in Volume 4 (still unpublished) of Knuth’s The Art of Computer Programming and will work on any list:
#!/usr/bin/perl -n
# Fischer-Krause ordered permutation generator
sub permute (&@) {
my $code = shift;
my @idx = 0..$#_;
while ( $code->(@_[@idx]) ) {
my $p = $#idx;
--$p while $idx[$p-1] > $idx[$p];
my $q = $p or return;
push @idx, reverse splice @idx, $p;
++$q while $idx[$p-1] > $idx[$q];
@idx[$p-1,$q]=@idx[$q,$p-1];
}
}
permute { print "@_\n" } split;The Algorithm::Loops module also provides the NextPermute and NextPermuteNum functions which efficiently find all unique permutations of an array, even if it contains duplicate values, modifying it in-place: if its elements are in reverse-sorted order then the array is reversed, making it sorted, and it returns false; otherwise the next permutation is returned.
NextPermute uses string order and NextPermuteNum numeric order, so you can enumerate all the permutations of 0..9 like this:
use Algorithm::Loops qw(NextPermuteNum);
my @list= 0..9;
do { print "@list\n" } while NextPermuteNum @list;How do I sort an array by (anything)? 🧮
Supply a comparison function to sort() (described in "sort" in perlfunc):
@list = sort { $a <=> $b } @list;The default sort function is cmp, string comparison, which would sort (1, 2, 10) into (1, 10, 2). <=>, used above, is the numerical comparison operator.
If you have a complicated function needed to pull out the part you want to sort on, then don’t do it inside the sort function. Pull it out first, because the sort BLOCK can be called many times for the same element. Here’s an example of how to pull out the first word after the first number on each item, and then sort those words case-insensitively.
my @idx;
for (@data) {
my $item;
($item) = /\d+\s*(\S+)/;
push @idx, uc($item);
}
my @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];which could also be written this way, using a trick that’s come to be known as the Schwartzian Transform:
my @sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;If you need to sort on several fields, the following paradigm is useful.
my @sorted = sort {
field1($a) <=> field1($b) ||
field2($a) cmp field2($b) ||
field3($a) cmp field3($b)
} @data;This can be conveniently combined with precalculation of keys as given above.
See the sort article in the “Far More Than You Ever Wanted To Know” collection in <http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz> for more about this approach.
See also the question later in perlfaq4 on sorting hashes.
How do I manipulate arrays of bits? 🧩
Use pack() and unpack(), or else vec() and the bitwise operations.
For example, you don’t have to store individual bits in an array (which would mean that you’re wasting a lot of space). To convert an array of bits to a string, use vec() to set the right bits. This sets $vec to have bit N set only if $ints[N] was set:
my @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... )
my $vec = '';
foreach( 0 .. $#ints ) {
vec($vec,$_,1) = 1 if $ints[$_];
}The string $vec only takes up as many bits as it needs. For instance, if you had 16 entries in @ints, $vec only needs two bytes to store them (not counting the scalar variable overhead).
Here’s how, given a vector in $vec, you can get those bits into your @ints array:
sub bitvec_to_list {
my $vec = shift;
my @ints;
# Find null-byte density then select best algorithm
if ($vec =~ tr/\0// / length $vec > 0.95) {
use integer;
my $i; # This method is faster with mostly null-bytes
while($vec =~ /[^\0]/g ) {
$i = -9 + 8 * pos $vec;
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
push @ints, $i if vec($vec, ++$i, 1);
}
}
else {
# This method is a fast general algorithm
use integer;
my $bits = unpack "b*", $vec;
push @ints, 0 if $bits =~ s/^(\d)// && $1;
push @ints, pos $bits while($bits =~ /1/g);
}
return \@ints;
}okay so like — the sparser your bit vector is, the faster this whole thing goes. it's giving optimization. (big thanks to Tim Bunce and Winfried Koenig for this one, icons fr 💅)
and bestie, you can shrink that while loop so much with this glow-up from Benjamin Goldberg:
while($vec =~ /[^\0]+/g ) {
push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
}orrr you could just use the CPAN module Bit::Vector and let someone else do the math:
my $vector = Bit::Vector->new($num_of_bits);
$vector->Index_List_Store(@ints);
my @ints = $vector->Index_List_Read();Bit::Vector serves up efficient methods for bit vectors, sets of small integers, and "big int" math. the range?? unmatched. 💜
here's a way more extra illustration using vec():
# vec demo
my $vector = "\xff\x0f\xef\xfe";
print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
unpack("N", $vector), "\n";
my $is_set = vec($vector, 23, 1);
print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
pvec($vector);
set_vec(1,1,1);
set_vec(3,1,1);
set_vec(23,1,1);
set_vec(3,1,3);
set_vec(3,2,3);
set_vec(3,4,3);
set_vec(3,4,7);
set_vec(3,8,3);
set_vec(3,8,7);
set_vec(0,32,17);
set_vec(1,32,17);
sub set_vec {
my ($offset, $width, $value) = @_;
my $vector = '';
vec($vector, $offset, $width) = $value;
print "offset=$offset width=$width value=$value\n";
pvec($vector);
}
sub pvec {
my $vector = shift;
my $bits = unpack("b*", $vector);
my $i = 0;
my $BASE = 8;
print "vector length in bytes: ", length($vector), "\n";
@bytes = unpack("A8" x length($vector), $bits);
print "bits are: @bytes\n\n";
}🌸 Why does defined() return true on empty arrays and hashes? 🌸
the short story, bestie: you should probably only use defined on scalars or functions, not on aggregates (that's arrays and hashes, iykyk). for the full tea, go peek at "defined" in perlfunc in the 5.004 release or later of Perl. 💖
👑 Data: Hashes (Associative Arrays) 👑
✨ How do I process an entire hash? ✨
(contributed by brian d foy)
so there's a couple of ways to run through a whole hash, and honestly pick your vibe. you can grab a list of keys and then walk each key one by one, or you can pull off one key-value pair at a time.
to go through all the keys, use the keys function. it extracts every key out of the hash and hands them back to you as a list, and then you can grab the value through whatever key you're currently processing:
foreach my $key ( keys %hash ) {
my $value = $hash{$key};
...
}once you've got that list of keys, you can mess with the list before you ever touch the hash elements. like, you can sort the keys so everything comes out in lexical order:
foreach my $key ( sort keys %hash ) {
my $value = $hash{$key};
...
}or maybe you only wanna process some of them, no big deal. if you only care about the keys that start with "text:", you can select just those with grep:
foreach my $key ( grep /^text:/, keys %hash ) {
my $value = $hash{$key};
...
}now if the hash is huge, you might not wanna be out here creating a giant list of keys — that's so much memory, not the move. to save some, you can grab one key-value pair at a time using each(), which returns a pair you haven't seen yet:
while( my( $key, $value ) = each( %hash ) ) {
...
}⚠️ theeachoperator returns pairs in apparently random order, so if ordering matters to you, you gotta stick with thekeysmethod. no negotiating on that one.
also each() can be a lil bit tricky, ngl. you can't add or delete keys of the hash while you're using it — not without possibly skipping or re-processing some pairs after Perl internally rehashes all of the elements. 😭 and on top of that, a hash has only one iterator, so if you mix keys, values, or each on the same hash, you risk resetting the iterator and messing up your entire processing situation. for more details go read the "each" entry in perlfunc.
🌈 How do I merge two hashes? 🌈
(contributed by brian d foy)
before you decide to merge two hashes, you gotta decide two whole things: what happens if both hashes contain the same keys, and whether you wanna leave the original hashes exactly as they were. main character decisions only. 💅
if you do wanna preserve the originals, copy one hash (%hash1) into a new hash (%new_hash), then add the keys from the other hash (%hash2) into that new hash. checking whether the key already exists in %new_hash is your moment to decide what to do about duplicates:
my %new_hash = %hash1; # make a copy; leave %hash1 alone
foreach my $key2 ( keys %hash2 ) {
if( exists $new_hash{$key2} ) {
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else {
$new_hash{$key2} = $hash2{$key2};
}
}and if you don't wanna create a new hash at all? you can still use this same looping technique, just swap %new_hash out for %hash1:
foreach my $key2 ( keys %hash2 ) {
if( exists $hash1{$key2} ) {
warn "Key [$key2] is in both hashes!";
# handle the duplicate (perhaps only warning)
...
next;
}
else {
$hash1{$key2} = $hash2{$key2};
}
}but listen — if you genuinely do not care that one hash is out here overwriting keys and values from the other, you can just use a hash slice to slap one hash onto another. in that case, values from %hash2 replace values from %hash1 whenever they share a key:
@hash1{ keys %hash2 } = values %hash2;🦋 What happens if I add or remove keys from a hash while iterating over it? 🦋
(contributed by brian d foy)
the easy answer is "Don't do that!" like, genuinely. 🫶
that said — if you iterate through the hash with each(), you can delete the key most recently returned without worrying about it. but if you delete or add other keys, the iterator may skip them or double up on them, because perl may rearrange the hash table underneath you. see the entry for each() in perlfunc for the details.
💜 How do I look up a hash element by value? 💜
create a reverse hash:
my %by_value = reverse %by_key;
my $key = $by_value{$value};but bestie, that's not particularly efficient, let's be honest. it'd be more space-efficient to use:
while (my ($key, $value) = each %by_key) {
$by_value{$value} = $key;
}now — if your hash could have repeated values, the methods above will only find one of the associated keys. that may or may not worry you, and honestly only you can decide if that's a vibe. but if it does worry you, you can always reverse the hash into a hash of arrays instead:
while (my ($key, $value) = each %by_key) {
push @{$key_list_by_value{$value}}, $key;
}🎀 How can I know how many entries are in a hash? 🎀
(contributed by brian d foy)
this is super similar to "How do I process an entire hash?", also in perlfaq4, but a bit simpler in the common cases.
you can use the keys() built-in function in scalar context to find out how many entries you've got in a hash:
my $key_count = keys %hash; # must be scalar context!but if you wanna know how many entries have a defined value, that's a whole different thing. you gotta check each value, and a grep is super handy here:
my $defined_value_count = grep { defined } values %hash;and you can use that exact same structure to count entries any way you like. want the count of keys with vowels in them? just test for that instead:
my $vowel_count = grep { /[aeiou]/ } keys %hash;grep in scalar context returns the count. and if you want the actual list of matching items instead, just use it in list context, periodt:
my @defined_values = grep { defined } values %hash;⚠️ heads up: thekeys()function also resets the iterator, which means you may see strange results if you use this between uses of other hash operators such aseach(). be careful out here. 💀
⭐ How do I sort a hash (optionally by value instead of key)? ⭐
(contributed by brian d foy)
to sort a hash, start with the keys. in this example we hand the list of keys to the sort function, which then compares them as strings. the output list has the keys in string order. and once we have the keys, we can go through them to build a report that lists the keys in string order:
my @keys = sort { $a cmp $b } keys %hash;
foreach my $key ( @keys ) {
printf "%-20s %6d\n", $key, $hash{$key};
}we could get way more fancy in the sort() block though. instead of comparing the keys themselves, we can compute a value with them and use that value as the comparison.
for instance, to make our report order case-insensitive, we use fc to safely lowercase the keys before comparing them:
use v5.16;
my @keys = sort { fc($a) cmp fc($b) } keys %hash;earlier versions of this answer used lc, but that could give unexpected results with some Unicode strings. see "fc" in perlfunc for the details. the Unicode::UCD module does the same thing for earlier perls. 🌈
note: if the computation is expensive or the hash has many elements, you may wanna look at the Schwartzian Transform to cache the computation results. 🧚
if we wanna sort by the hash value instead, we use the hash key to look it up. we still get out a list of keys, but this time they're ordered by their value:
my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;and from there we can go even more complex. if the hash values are the same, we can provide a secondary sort on the hash key:
use v5.16;
my @keys = sort {
$hash{$a} <=> $hash{$b}
or
fc($a) cmp fc($b)
} keys %hash;💖 How can I always keep my hash sorted? 💖
okay bestie, so you want your hash sorted?? You can look into using the DB_File module and tie() using the $DB_BTREE hash bindings as documented in "In Memory Databases" in DB_File. The Tie::IxHash module from CPAN might also be instructive. Although this does keep your hash sorted, you might not like the slowdown you suffer from the tie interface. Are you sure you need to do this? :)
🎀 What's the difference between delete and undef with hashes? 🎀
Hashes contain pairs of scalars: the first is the key, the second is the value. The key will be coerced to a string, although the value can be any kind of scalar: string, number, or reference. If a key $key is present in %hash, exists($hash{$key}) will return true. The value for a given key can be undef, in which case $hash{$key} will be undef while "exists $hash{$key}" will return true. This corresponds to ($key, "undef") being in the hash.
Pictures help... Here's the %hash table:
keys values
+------+------+
| a | 3 |
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+And these conditions hold
$hash{'a'} is true
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is true
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is trueIf you now say
undef $hash{'a'}your table now reads:
keys values
+------+------+
| a | undef|
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+and these conditions now hold; changes in caps:
$hash{'a'} is FALSE
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is FALSE
exists $hash{'a'} is true (Perl 5 only)
grep ($_ eq 'a', keys %hash) is trueNotice the last two: you have an undef value, but a defined key! the way that works is so iconic fr fr 💅
Now, consider this:
delete $hash{'a'}your table now reads:
keys values
+------+------+
| x | 7 |
| d | 0 |
| e | 2 |
+------+------+and these conditions now hold; changes in caps:
$hash{'a'} is false
$hash{'d'} is false
defined $hash{'d'} is true
defined $hash{'a'} is false
exists $hash{'a'} is FALSE (Perl 5 only)
grep ($_ eq 'a', keys %hash) is FALSESee, the whole entry is gone! 💀
🦋 Why don't my tied hashes make the defined/exists distinction? 🦋
This depends on the tied hash's implementation of EXISTS(). For example, there isn't the concept of undef with hashes that are tied to DBM* files. It also means that exists() and defined() do the same thing with a DBM* file, and what they end up doing is not what they do with ordinary hashes. no cap.
🌸 How do I reset an each() operation part-way through? 🌸
(contributed by brian d foy)
You can use the "keys" or "values" functions to reset "each". To simply reset the iterator used by "each" without doing anything else, use one of them in void context:
keys %hash; # resets iterator, nothing else.
values %hash; # resets iterator, nothing else.See the documentation for "each" in perlfunc. iykyk
✨ How can I get the unique keys from two hashes? ✨
First you extract the keys from the hashes into lists, then solve the "removing duplicates" problem described above. For example:
my %seen = ();
for my $element (keys(%foo), keys(%bar)) {
$seen{$element}++;
}
my @uniq = keys %seen;Or more succinctly:
my @uniq = keys %{{%foo,%bar}};Or if you really want to save space:
my %seen = ();
while (defined ($key = each %foo)) {
$seen{$key}++;
}
while (defined ($key = each %bar)) {
$seen{$key}++;
}
my @uniq = keys %seen;💜 How can I store a multidimensional array in a DBM file? 💜
Either stringify the structure yourself (no fun), or else get the MLDBM (which uses Data::Dumper) module from CPAN and layer it on top of either DB_File or GDBM_File. You might also try DBM::Deep, but it can be a bit slow. ngl
🎀 How can I make my hash remember the order I put elements into it? 🎀
Use the Tie::IxHash from CPAN.
use Tie::IxHash;
tie my %myhash, 'Tie::IxHash';
for (my $i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
my @keys = keys %myhash;
# @keys = (0,1,2,3,...)obsessed with this module actually 🫶
💅 Why does passing a subroutine an undefined element in a hash create it? 💅
(contributed by brian d foy)
Are you using a really old version of Perl?
Normally, accessing a hash key's value for a nonexistent key will not create the key.
my %hash = ();
my $value = $hash{ 'foo' };
print "This won't print\n" if exists $hash{ 'foo' };Passing $hash{ 'foo' } to a subroutine used to be a special case, though. Since you could assign directly to $_[0], Perl had to be ready to make that assignment so it created the hash key ahead of time:
my_sub( $hash{ 'foo' } );
print "This will print before 5.004\n" if exists $hash{ 'foo' };
sub my_sub {
# $_[0] = 'bar'; # create hash key in case you do this
1;
}Since Perl 5.004, however, this situation is a special case and Perl creates the hash key only when you make the assignment:
my_sub( $hash{ 'foo' } );
print "This will print, even after 5.004\n" if exists $hash{ 'foo' };
sub my_sub {
$_[0] = 'bar';
}However, if you want the old behavior (and think carefully about that because it's a weird side effect), you can pass a hash slice instead. Perl 5.004 didn't make this a special case:
my_sub( @hash{ qw/foo/ } );🌈 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? 🌈
Usually a hash ref, perhaps like this:
$record = {
NAME => "Jason",
EMPNO => 132,
TITLE => "deputy peon",
AGE => 23,
SALARY => 37_000,
PALS => [ "Norbert", "Rhys", "Phineas"],
};References are documented in perlref and perlreftut. Examples of complex data structures are given in perldsc and perllol. Examples of structures and object-oriented classes are in perlootut.
🫶 How can I use a reference as a hash key? 🫶
(contributed by brian d foy and Ben Morrow)
Hash keys are strings, so you can't really use a reference as the key. When you try to do that, perl turns the reference into its stringified form (for instance, HASH(0xDEADBEEF)). From there you can't get back the reference from the stringified form, at least without doing some extra work on your own. it's giving workaround energy 😭
Remember that the entry in the hash will still be there even if the referenced variable goes out of scope, and that it is entirely possible for Perl to subsequently allocate a different variable at the same address. This will mean a new variable might accidentally be associated with the value for an old.
If you have Perl 5.10 or later, and you just want to store a value against the reference for lookup later, you can use the core Hash::Util::Fieldhash module. This will also handle renaming the keys if you use multiple threads (which causes all variables to be reallocated at new addresses, changing their stringification), and garbage-collecting the entries when the referenced variable goes out of scope.
If you actually need to be able to get a real reference back from each hash entry, you can use the Tie::RefHash module, which does the required work for you. slay ✨
🧚 How can I check if a key exists in a multilevel hash? 🧚
(contributed by brian d foy)
The trick to this problem is avoiding accidental autovivification. If you want to check three keys deep, you might naïvely try this:
my %hash;
if( exists $hash{key1}{key2}{key3} ) {
...;
}Even though you started with a completely empty hash, after that call to "exists" you've created the structure you needed to check for "key3":
%hash = (
'key1' => {
'key2' => {}
}
);That's autovivification. You can get around this in a few ways. The easiest way is to just turn it off. The lexical "autovivification" pragma is available on CPAN. Now you don't add to the hash:
{
no autovivification;
my %hash;
if( exists $hash{key1}{key2}{key3} ) {
...;
}
}The Data::Diver module on CPAN can do it for you too. Its "Dive" subroutine can tell you not only if the keys exist but also get the value:
use Data::Diver qw(Dive);
my @exists = Dive( \%hash, qw(key1 key2 key3) );
if( ! @exists ) {
...; # keys do not exist
}
elsif( ! defined $exists[0] ) {
...; # keys exist but value is undef
}You can easily do this yourself too by checking each level of the hash before you move onto the next level. This is essentially what Data::Diver does for you:
if( check_hash( \%hash, qw(key1 key2 key3) ) ) {
...;
}
sub check_hash {
my( $hash, @keys ) = @_;
return unless @keys;
foreach my $key ( @keys ) {
return unless eval { exists $hash->{$key} };
$hash = $hash->{$key};
}
return 1;
}periodt 💅
👑 How can I prevent addition of unwanted keys into a hash? 👑
Since version 5.8.0, hashes can be restricted to a fixed number of given keys. Methods for creating and dealing with restricted hashes are exported by the Hash::Util module. iconic behavior honestly 👑
🌈 DATA: MISC 🌈
okay bestie this is the grab-bag section, the catch-all drawer of the FAQ, and honestly? it eats 💅
How do I handle binary data correctly?
good news first: Perl is binary-clean, so it handles binary data absolutely fine, no problem, no drama. ✨
BUT. On Windows or DOS? You have to use binmode for binary files or you'll get line ending conversions sneaking in and ruining everything. 🙈
the general rule, no cap: any time you want to work with binary data, use binmode.
also go peek at "binmode" in perlfunc or perlopentut for more.
if it's 8-bit textual data you're worried about, check out perllocale. and if you want to deal with multibyte characters? different beast entirely, there are some gotchas hiding in there. see the section on Regular Expressions. 🫶
How do I determine whether a scalar is a number/whole/integer/float?
so, assuming you don't care about IEEE notations like NaN or Infinity (and honestly, valid), you probably just want a regular expression. also see perlretut and perlre:
use 5.010;
if ( /\D/ )
{ say "\thas nondigits"; }
if ( /^\d+\z/ )
{ say "\tis a whole number"; }
if ( /^-?\d+\z/ )
{ say "\tis an integer"; }
if ( /^[+-]?\d+\z/ )
{ say "\tis a +/- integer"; }
if ( /^-?(?:\d+\.?|\.\d)\d*\z/ )
{ say "\tis a real number"; }
if ( /^[+-]?(?=\.?\d)\d*\.?\d*(?:e[+-]?\d+)?\z/i )
{ say "\tis a C float" }there's also a whole lil ecosystem of modules for this exact task, and they're all on CPAN:
- Scalar::Util — ships with 5.8, gives you access to Perl's internal
looks_like_numberfunction, which tells you whether a variable looks like a number 👀 - Data::Types — exports functions that validate data types, using the regexes above plus others
- Regexp::Common — got regexes for matching all kinds of numbers
if you're on a POSIX system, Perl supports POSIX::strtod for converting strings to doubles (and POSIX::strtol for longs, bestie didn't forget). its semantics are kind of a lot though, so here's a getnum wrapper function that's way more convenient. it takes a string and returns the number it found, or undef if the input isn't a C float. and is_numeric is a lil front end for getnum for when you just wanna ask "is this a float?" 💖
sub getnum {
use POSIX qw(strtod);
my $str = shift;
$str =~ s/^\s+//;
$str =~ s/\s+$//;
$! = 0;
my($num, $unparsed) = strtod($str);
if (($str eq '') || ($unparsed != 0) || $!) {
return undef;
}
else {
return $num;
}
}
sub is_numeric { defined getnum($_[0]) }or you could go check out the String::Scanf module on CPAN instead, dealer's choice 🎀
How do I keep persistent data across program calls?
for some specific applications you can use one of the DBM modules — see AnyDBM_File. 🌸
more generically though? you want the FreezeThaw or Storable modules from CPAN. and as of Perl 5.8, Storable is part of the standard distribution, so it's just there. here's an example using Storable's store and retrieve functions:
use Storable;
store(\%hash, "filename");
# later on...
$href = retrieve("filename"); # by ref
%hash = %{ retrieve("filename") }; # direct to hashHow do I print out or copy a recursive data structure?
for printing data structures, the Data::Dumper module on CPAN (or the 5.005 release of Perl) is iconic, genuinely great at it. ✨
for copying, the Storable module on CPAN (or the 5.8 release of Perl) gives you a function called dclone that recursively copies its argument:
use Storable qw(dclone);
$r2 = dclone($r1);where $r1 can be a reference to literally any kind of data structure you want, and it'll get deeply copied. because dclone takes and returns references, you have to add extra punctuation if it's a hash of arrays you're trying to copy:
%newhash = %{ dclone(\%oldhash) };How do I define methods for every class/object?
(contributed by Ben Morrow) 🫶
you can use the UNIVERSAL class — see UNIVERSAL.
BUT. bestie. please be very careful and think through the consequences before you do this: adding methods to every object in existence is extremely likely to have unintended consequences. like, chaos. 😭
if it's possible at all, it's better to have all your objects inherit from some common base class, or to use an object system like Moose that supports roles.
How do I verify a credit card checksum?
grab the Business::CreditCard module from CPAN and you're done, periodt 💳✨
How do I pack arrays of doubles or floats for XS code?
the arrays.h/arrays.c code in the PGPLOT module on CPAN does exactly this. 🔥
if you're doing a lot of float or double processing though, consider using the PDL module from CPAN instead — it makes number-crunching easy, fr fr.
see <https://metacpan.org/release/PGPLOT> for the code.
👑 AUTHOR AND COPYRIGHT 👑
Copyright (c) 1997-2010 Tom Christiansen, Nathan Torkington, and other authors as noted. All rights reserved.
this documentation is free 💖 you can redistribute it and/or modify it under the same terms as Perl itself.
and irrespective of its distribution, all code examples in this file are hereby placed into the public domain. you are permitted and encouraged to use this code in your own programs, for fun or for profit, however you see fit. a simple comment in the code giving credit would be courteous but is not required. 🌈
and that's a wrap on the whole FAQ bestie, we did it 🎀 this edition is perl v5.44.0, dated 2026-10-17, filed under PERLFAQ4(1perl) — go forth and slay your data handling, no cap 👑✨🦋
📦 where this page comes from
- arch rolling: perl 5.44.0-1 · Artistic-1.0-Perl AND GPL-1.0-or-later
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms