PDL::Tips(3pm)

✨ PDL::Tips ✨

okay bestie, gather round 💖 this is PDL::Tips, part of the User Contributed Perl Documentation — it's the lil grab-bag of useful arcana, programming tidbits and such. no cap, it's giving "wise older cousin drops knowledge" energy. 🌸

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🌈 SYNOPSIS 🌈

use PDL;

# Whatever happens here.

periodt. that's the whole vibe. 💅

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💫 DESCRIPTION 💫

so this page is where we document all the useful idioms, the helpful hints, and the tips for using Perl Data Language v2.0. it's the group chat of documentation. 🫶

🎀 Help 🎀

if you're inside perldl, just type help help and you're set. or from the command line, hit up the pdldoc program to get at the PerlDL documentation. 💖

want fuzzy searching while you're in perldl? use the apropos command — it finds every instance of your search term across the docs. for an example, go peek at PDL::FAQ Q:7.1. iykyk. 🔍✨

🦋 Indexing idioms 🦋

okay so this next bit of code normalizes a whole bunch of vectors in $x. and the slay part? it works no matter what dimensionality $x is. literally any dimensionality. 👑

$x /= $x->sumover->dummy(0);

💀 What is actually happening? 💀

if you wanna see what the code is actually doing (like, behind the curtain, main character reveal moment), run this command

PDL::Core::set_debugging(1);

somewhere. warning tho: this spews out a huge amount of debug info for PDL into STDOUT. not me flooding your terminal 😭 it's a lot.

future plans include making it possible to redirect that output, and also making it possible to select messages with more precision. so like, it's on the vision board. ✨

a lot of those messages come from lib/PDL/Core/pdlapi.c, and you can look at the source to see what's going on. 🔍

and if you have any extra time to work on these mechanisms, pls inform the pdl-porters mailing list. they'd be so happy 🫶

🍓 Memory savings 🍓

if you're running something recursively that selects certain indices of a large ndarray, like

while (1) {
        $inds = where($x>0);
        $x = $x->index($inds);
        $y = $y->index($inds);
        func($y,$x);
}

then listen: if you are not writing to $y, you save a LOT of memory by changing it to

        $y = $y->index($inds)->sever;

the new method sever causes the write-back relation to be forgotten. it's like copy except it changes the original ndarray and returns it. 💅

of course, the probably best way to do the above is

$inds = xvals ($x->long);
while (1) {
        $inds0 = where($x>0);
        $inds1 = $inds->index($inds)->sever;
        $x = $a0->index($inds1);
        $y = $y->index($inds1)->sever;
        func($y,$x);
}

which doesn't save all the temporary instances of $x in memory. 🧠✨

🔥 PP speed 🔥

if you really wanna write speedy PP code, the first thing you need to do is make sure your C compiler is allowed to do the necessary optimizations. no optimization permission = no speed, and that's just sad. 🙈

what this actually means: you have to allow as many variables as possible to go into registers. like

loop(a) %{
        $a() += $COMP(foo_member) * $b()
%}

expands to

for(i=0; i<10000; i++) {
        a[i] += __privtrans->foo_member * b[i];
}

and bestie, that is about the worst you can do, because your C compiler is not allowed to assume that a doesn't clobber foo_member, which completely inhibits vectorization. 💀 instead, do

float foo = $COMP(foo_member);
loop(a) %{
        $a() += foo * $b();
%}

and now we're cooking. 🔥

to be clear: this is not a restriction caused by PP. it's a restriction caused by ANSI C semantics. we could copy the struct into local variables and back, but that could cause very strange things sometimes. spooky. 👻

there are many other issues on organizing loops, fyi.

we are currently planning to make PP able to do fixed-width things as well as physical ndarrays — where looping over the first dimensions would be cheaper since there are less distinct increments, which might make a difference on machines with a small number of registers. 📝✨

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👑 AUTHOR 👑

Copyright (C) Tuomas J. Lukka 1997. All rights reserved. Duplication in the same form and printing a copy for yourself allowed. 💜

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so that's the whole page, bestie 🎀 this documentation shipped as perl v5.42.3, dated 2026-08-20, and it's filed under PDL::Tips(3pm) in the User Contributed Perl Documentation. go forth and index responsibly. slay. 💅✨🦋