=head1 NAME

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

=head1 SYNOPSIS

	use PDL;

	# Whatever happens here.

=head1 DESCRIPTION

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

=head2 Help

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

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

=head2 Indexing idioms

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

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

=head2 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 messages
with more precision.

Many of the messages come from C<lib/PDL/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.

=head2 Memory savings

If you are running recursively something 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);
	}

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

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

The new method C<sever> is a causes the write-back relation to be
forgotten. It is 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.
See C<mandel.pl> in the Demos subdirectory of the PerlDL distribution
for an example.

=head2 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 C<a> doesn't clobber C<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 ndarrays (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).

=head1 AUTHOR

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

