PDL::Tipsp - Online in the Cloud

PROGRAM:

NAME

PDL::Tips - Small tidbits of useful arcana. Programming tidbits and such.

SYNOPSIS

use PDL;

# Whatever happens here.

DESCRIPTION

This page documents useful idioms, helpful hints and tips for using Perl Data Language
v2.0.

Help
Use "help help" within perldl or pdl2 or use the "pdldoc" program from the command line
for access to the PerlDL documentation. HTML versions of the pages should also be
present, in the HtmlDocs/PDL directory of the PDL distribution. To find this directory,
try the following

pdl> foreach ( map{"$_/PDL/HtmlDocs"}@INC ) { p "$_\n" if -d $_ }

Indexing idioms
The following code normalizes a bunch of vectors in $a. This works regardless of the
dimensionality of $a.

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

What is actually happening?
If you want to see what the code is actually doing, try the command

PDL::Core::set_debugging(1);

somewhere. This spews out a huge amount of debug info for PDL into STDOUT. Plans for the
future include making it possible to redirect the output, and also making it possible to
select mesages with more precision.

Many of the messages come from "Basic/Core/pdlapi.c" and you can look at the source to see
what is going on.

If you have any extra time to work on these mechanisms, inform the pdl-porters mailing
list.

Memory savings
If you are running recursively something that selects certain indices of a large piddle,
like

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

If you are not writing to $b, it saves a lot of memory to change this to

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

The new method "sever" is a causes the write-back relation to be forgotten. It is like
copy except it changes the original piddle and returns it).

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

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

which doesn't save all the temporary instances of $a in memory. See "mandel.pl" in the
Demos subdirectory of the PerlDL distribution for an example.

PP speed
If you really want to write speedy PP code, the first thing you need to do is to make sure
that your C compiler is allowed to do the necessary optimizations.

What this means is that you have to allow as many variables as possible to go into
registers:

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

expands to

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

is about the worst you can do, since 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();
%}

This is not a restriction caused by PP but by ANSI C semantics. Of course, we could copy
the struct into local variables and back but that could cause very strange things
sometimes.

There are many other issues on organizing loops.

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

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