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

Hi All--

It's clearly friday afternoon-- I can't figure out how to do a trivial set intersection in PDL.

I have: 

use PDL;
my $pdl_a = pdl 10..20;
my $pdl_b = pdl 15..18;
print "$pdl_a\n$pdl_b\n";

[download]

Producing this: 

[10 11 12 13 14 15 16 17 18 19 20]
[15 16 17 18]

[download]

I want: 

use PDL;
my $pdl_a = pdl 10..20;
my $pdl_b = pdl 15..18;
my $pdl_c = INTERSECTION OF PDL_A AND PDL_B
print "$pdl_a\n$pdl_b\n$pdl_c\n";

[download]

Producing this: 

[10 11 12 13 14 15 16 17 18 19 20]
[15 16 17 18]
[15 16 17 18]

[download]

What am I forgetting, O Monks?

Replies are listed 'Best First'.
Re: Simple PDL question: set intersection
by kvale (Monsignor) on Oct 29, 2004 at 23:34 UTC
    I don't know of a direct set intersection operator on the values of a pdl. For your example, you can use conditionals to effect the same result: 
    use PDL;

my $arr = pdl 10..20;;
print $arr, "\n";

my $idx = which( $arr>14 & $arr<19 );
print $idx, "\n";

my $subset = $arr->index( $idx);
print $subset, "\n";

    [download]
    prints 
    [10 11 12 13 14 15 16 17 18 19 20]
[5 6 7 8]
[15 16 17 18]

    [download]
    If you need real set intersection and not just subranges, converting to hashes may be the easiest apporach.

    -Mark

[reply]
[d/l]
[select]
Re: Simple PDL question: set intersection
by etj (Priest) on Jun 22, 2022 at 20:10 UTC
[reply]
Re: Simple PDL question: set intersection
by tall_man (Parson) on Nov 01, 2004 at 16:11 UTC
    If the sets contain integers in a reasonable range you could use indexing, like this: 
    use strict;
use PDL;
 
my $pdl_a = pdl 10..20;
my $pdl_b = pdl 15..18;
my $mask_a = zeroes 21;
my $mask_b = zeroes 21;
my $set_a = index($mask_a, $pdl_a);
my $set_b = index($mask_b, $pdl_b);
$set_a .= 1;
$set_b .= 1;
my $pdl_c = which($mask_a & $mask_b);
print $pdl_c,"\n";

    [download]
    If the sets are large you could use bit masking, as in this example: Re^2: Basic Perl trumps DBI? Or my poor DB design?.
[reply]
[d/l]

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