Do you think this  $dec->(:,0) ->minimum; is legal perl?

Because I get: syntax error at ..., near "(:"

It works for me, can be due to NiceSlice
use PDL; use PDL::NiceSlice;

Okay. NiceSlice is a source code filter which explains how it can 'undo' a syntax error. Sorry to have bothered you :)

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#!/usr/bin/env perl
use strict;
use warnings;

use PDL;
use PDL::NiceSlice;

my $dec = pdl [[1,1,2,2,2],[10,15,13,11,4]];
my $min = $dec->(:,0)->minimum;
my $max = $dec->(:,0)->maximum;
my $v = zeroes(2,2);
$v(0,0) .= $min;
$v(0,1) .= $max;
$v(1,0) .= $dec->(:,1)->index(which $dec->(:,0) eq $v(0,0))->maximum;
$v(1,1) .= $dec->(:,1)->index(which $dec->(:,0) eq $v(0,1))->maximum;

print "$v\n";

exit;
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[
 [ 1 15]
 [ 2 13]
]
[download]

Sorry for not being clear. The values of $dec(:,0) will not be always 0 or 1. In fact they can take any number of values(from 1 to some maximum number), my goal is to compute -for each unique value - the maximum of corresponding values in $dec(:,1).
Thus

$dec = pdl [[1,1,2,2,3],[10,15,13,11,6]];
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I want the output to be:

[
 [ 1 15]
 [ 2 13]
 [ 3 6]
]
[download]

Hope this helps, thanks in advance..

Here is one way to achieve this without constructing a loop that iterates over each element of the pdl. I'm not sure that this would be significantly faster than just iterating over the elements of the pdl...you would need to benchmark it (also you may want to consider comparing it to a plain-perl solution).

#!/usr/bin/env perl
use strict;
use warnings;

use PDL;
use PDL::NiceSlice;

my $dec = pdl [[1,1,2,2,3],[10,15,13,11,6]];
my $groups_vec = $dec(:,0);
my $groups = $groups_vec->uniq;
my $groups_t = $groups->transpose;
my $values = $dec(:,1);
my $masks = ($groups_t == $groups_vec);
my $group_values = $masks*$values;
my $max_of_each_group = $group_values->maximum;
my $final_pdl = pdl($groups, $max_of_each_group)->transpose;



print "Original Data: $dec\n";
print "Groups: $groups\n";
print "Transposed Groups: $groups_t\n";
print "Masks: $masks\n";
print "Group values: $group_values\n";
print "Max of each group: $max_of_each_group\n";
print "Final pdl: $final_pdl\n";

exit;
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Gives the following output:

Original Data:
[
 [ 1  1  2  2  3]
 [10 15 13 11  6]
]

Groups: [1 2 3]
Transposed Groups:
[
 [1]
 [2]
 [3]
]

Masks:
[
 [1 1 0 0 0]
 [0 0 1 1 0]
 [0 0 0 0 1]
]

Group values:
[
 [10 15  0  0  0]
 [ 0  0 13 11  0]
 [ 0  0  0  0  6]
]

Max of each group: [15 13 6]
Final pdl:
[
 [ 1 15]
 [ 2 13]
 [ 3  6]
]
[download]