DragonFly On-Line Manual Pages

TIPS(1)               User Contributed Perl Documentation              TIPS(1)

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.

perl v5.20.2                      2015-05-24                           TIPS(1)