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).

AUTHOR

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