lmap(n)

✨ lmap(n) — the Tcl glow-up for looping 💅

okay bestie, welcome to lmap, a built-in command from the Tcl fam. Think of it as foreach's cooler older sister who actually collects receipts on every single loop. 🌸

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🎀 NAME 🎀

lmap — iterate over all elements in one or more lists and collect results. no cap, that's literally the whole job. ✨

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💖 SYNOPSIS 💖

lmap varname list body
lmap varlist1 list1 ?varlist2 list2 ...? body

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🦋 DESCRIPTION 🦋

So here's the vibe: the lmap command runs a loop where the loop variable(s) take on values from one or more lists, and when it's done the loop hands you back a list of results that it collected from each iteration. It's giving "loop but make it a list". 👑

the simple case (one var, one list) 🍓

In the most basic scenario there's one loop variable, varname, and one list, list, which is just a list of values to assign to varname. The body argument is a Tcl script. For each element of list — in order from first to last, bestie, we do not skip around — lmap assigns the contents of the element to varname as if the lindex command had been used to extract the element, then calls the Tcl interpreter to execute body. If execution of the body completes normally, the result of the body gets appended to an accumulator list. Then lmap returns that accumulator list. periodt. 💅

the general case (many lists, many vars) 🌈

In the general case there can be more than one value list (e.g., list1 and list2), and each value list can be associated with a list of loop variables (e.g., varlist1 and varlist2). During each iteration of the loop, the variables of each varlist are assigned consecutive values from the corresponding list. Values in each list are used in order from first to last, and each value is used exactly once — no recycling, no reusing, we're not messy like that. 🔥

The total number of loop iterations is large enough to use up all the values from all the value lists. And if a value list doesn't contain enough elements for each of its loop variables in each iteration, empty values are used for the missing elements. So like, no error, just vibes and blanks. 🫶

break & continue, the drama queens 😭

The break and continue statements may be invoked inside body, with the same effect as in the for and foreach commands. But heads up: in those cases the body does not complete normally, and the result is not appended to the accumulator list. soz. 💀

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🌟 EXAMPLES 🌟

zip two lists together, iconic behavior 🧚

set list1 {a b c d}
set list2 {1 2 3 4}
set zipped [lmap a $list1 b $list2 {list $a $b}]
# The value of zipped is "{a 1} {b 2} {c 3} {d 4}"

filter a list to remove odd values ✨

set values {1 2 3 4 5 6 7 8}
proc isEven {n} {expr {($n % 2) == 0}}
set goodOnes [lmap x $values {expr {
    [isEven $x] ? $x : [continue]
}}]
# The value of goodOnes is "2 4 6 8"

take a prefix from a list based on the contents of the list 🎀

set values {8 7 6 5 4 3 2 1}
proc isGood {counter} {expr {$n > 3}}
set prefix [lmap x $values {expr {
    [isGood $x] ? $x : [break]
}}]
# The value of prefix is "8 7 6 5 4"

not me just copying these examples exactly because they're perfect and I'm not about to mess with perfection 🫶

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👀 SEE ALSO 👀

bestie, the whole extended list family is here, go say hi:

• break(n)
• continue(n)
• for(n)
• foreach(n)
• while(n)
• list(n)
• lappend(n)
• lassign(n)
• ledit(n)
• lindex(n)
• linsert(n)
• llength(n)
• lpop(n)
• lrange(n)
• lremove(n)
• lrepeat(n)
• lreplace(n)
• lreverse(n)
• lsearch(n)
• lseq(n)
• lset(n)
• lsort(n)

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🔑 KEYWORDS 🔑

foreach, iteration, list, loop, map

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k bye bestie!! this page is from <b>Tcl</b> — the <i>Tcl Built-In Commands</i> manual, <b>lmap(n)</b> edition. go forth and collect your results, you main character. 💜🦋✨