GrandFather has asked for the wisdom of the Perl Monks concerning the following question:

I'm testing some module code I'm working on and need a way to visualise a surface. PDL seems a pretty natural fit for manipulating the data. However I would like to initialise a 2D structure with values that are a function of grid location. Ideally I'd be able to do something like:

use warnings;
use strict;
use PDL;

my @funcs = (
    '1/(0.5 + 0.02 * $i*$i) * cos($i)',
    '1/(0.5 + 0.02 * $i*$i) * cos($i)',
);
my $surface = initPDL (\&shape, (10) x @funcs);

...

sub shape {
    my @values = @_;
    my $sum = 0;

    for my $idx (0 .. $#values) {
        my $i = $values[$idx];
        my $value = eval $funcs[$idx];

        $sum += $value * $value;
    }

    return sqrt ($sum);
}

[download]

The "tricky/cool" bit is the putative my $surface = initPDL (\&shape, (10) x @funcs); line. Does PDL provide a way do do something of that sort?

Perl reduces RSI - it saves typing

Replies are listed 'Best First'.
Re: I need a liddle piddle advice
by plobsing (Friar) on Oct 14, 2008 at 07:49 UTC
    I'm assuming you want a PDL with the value of  sqrt( 1/( 0.5 + 0.02*$x**2 )*cos($x) + 1/( 0.5 + 0.02*$y**2 )*cos($y) ) for x and y intervals of 0 to 10, stepping by 1.

    The closest thing I've seen is things similar to the example in the synopsis of PDL::Graphics::TriD::Contours.

    Here is a lightly modified version with your function: 
    use PDL;
use strict;
use warnings;

use constant X_NPOINTS => 11;
use constant Y_NPOINTS => 11;

my $proto = zeroes( X_NPOINTS, Y_NPOINTS );
my $x = $proto->xvals;
my $y = $proto->yvals;

my $z = sqrt( 1/(0.5 + 0.02*$x**2)*cos($x)
            + 1/(0.5 + 0.02*$y**2)*cos($y) );

    [download]
    Update: Fixed broken link
[reply]
[d/l]
[select]

      Excellent, that makes sense. Thank you. There is a small error in your implementation of my sample function. It should be:

      my $z = sqrt( (1/(0.5 + 0.02*$x**2)*cos($x))**2
            + (1/(0.5 + 0.02*$y**2)*cos($y))**2 );

      [download]

      But the illustration of the technique was the important thing!

      Perl reduces RSI - it saves typing
[reply]
[d/l]

Back to Seekers of Perl Wisdom