in reply to Re: Hash-type lookup in PDL
in thread Hash-type lookup in PDL

Hi zentara, It's nothing complicated I'm looking to do, really, just lookup each value in a hash. Here's an example that doesn't work: 
#!/usr/bin/perl

use warnings;
use strict;
use PDL;

my $x = sequence(5); print "$x \n";

my $lookup = {     
                0 => 101,
                1 => 69,
                2 => 42,
                3 => 10042,
                4 => 99
              };

my $y = zeroes(5); $y .= $lookup->{$x}; print "$y \n";

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That gives this output: 
[0 1 2 3 4] 
[0 0 0 0 0]

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I would be a happy bunny if it gave this output: 
[0 1 2 3 4] 
[101 69 42 10042 99]

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Best wishes, andye

(I tried using a straightforward my $y = $lookup->{$x} but that doesn't produce any value at all)

update: I've tried creating a pdl to use insread of the hash, like this: 

my $pdl_table = pdl([keys %$lookup], [values %$lookup]);

my $y = zeroes(5); $y .= $pdl_table->(which($pdl_table->(:,0) == $x),1
+); print "$y \n";

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but that has two problems: Looks like I've screwed up the syntax in the which() statement, and I can't work out how to make it right - a dummy dimension somewhere?

Replies are listed 'Best First'.
Re^3: Hash-type lookup in PDL
by lin0 (Curate) on Jan 05, 2007 at 19:52 UTC

    Hi andye,

    If your lookup table is rather small, you could try this code:

    #!/usr/bin/perl

use warnings;
use strict;
use PDL;

my $x = sequence(5); print "$x \n";

my @pdl_keys   =     (0,    1,   2,      3,   4);
my @pdl_values =     (101, 69,  42,  10042,  99);

my $y = zeroes(5);
foreach my $i (0..@pdl_keys-1){
    my $indx = which($x == $pdl_keys[$i]);
    $y->dice($indx) .= $pdl_values[$i];
}
print "$y \n";

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    I hope this helps

    Cheers,

    lin0
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[d/l]
      Many thanks zentara and lin0,

      Well, I went ahead and did it with my original approach of: convert the pdl to a perl list, then use a perl hash to look it up, then convert it back again. The batch job finished about half a day later, so it's an academic issue now really.

      Still, (as a note for for posterity) there seem really to be two options being discussed here:

      Either:

      - a PDL lookup table, using which() or dice() to look up one value at a time (slow on big datasets),

      or

      - convert all the values in advance, as per zentara's code above...

      Neither's ideal really... could be that converting to a perl scalar and using a hash is actually the best thing.

      Thanks for your help, and best wishes,
      andye

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