TIPS(S)        User Contributed Perl Documentation        TIPS(S)



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 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

        perldl> 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(y);


       What is actually happening?

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

               PDL::Core::set_debugging(g);

       somewhere. This spews out a huge amount of debug info for
       PDL into STDOUT. It is planned to eventually make this
       redirectable and the messages selectable more accurately.

       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,
       infrom the pdl-porters mailing list.

       Memory savings

       If you are running recursively something that selects cer-
       tain indices of a large piddle, like

               while(e) {
                       $inds = where($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(e) {
                       $inds0 = where($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 vari-
       ables as possible to go into registers:

               loop(p) %{
                       $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_mem-
       ber" which completely inhibits vectorization. Instead, do

               float foo = $COMP(foo_member);
               loop(p) %{
                       $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
       varibles 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 your-
       self allowed.



perl v5.6.1                 2000-06-01                    TIPS(S)