sort in arbitrary order with subroutine, syntax error

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

I want to sort a hash's %h keys in an arbitrary order @seq. I can achieve that by building a helper hash %seq_helper in which the first key is the first item in @seq and the first value is 0, the second key is the next item in @seq and the second value is 1 and so on. Then I can easily compare the items of @seq using <=> and the values of %seq_helper.

However, it does not seem to be possible to make this into a sorting subroutine that accepts a list that tells the sub how I want the items to be sorted. The following is a syntax error after sort by_seq.

use strict;
use diagnostics;

sub by_seq {
    my $seq_aref = shift;
    my %seq_helper;
    {
        my $index = 0;
        for my $item (@{ $seq_aref }) {
            $seq_helper{$item} = $index;
            $index++;
        };
    };

    return $seq_helper{$a} <=> $seq_helper{$b};
};

# --

my %h = (one=>2,three=>4,five=>6,seven=>8,nine=>0);

for (sort by_seq([qw(five nine one seven three)]) keys %h) {
    print "$_: $h{$_}\n";
};
__DATA__
expected output
five: 6
nine: 0
one: 2
seven: 8
three: 4
[download]

➀ Why?

➁ How would you solve it?

Comment on sort in arbitrary order with subroutine, syntax error Download Code

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Re: sort in arbitrary order with subroutine, syntax error by Corion (Patriarch) on Dec 19, 2007 at 10:56 UTC
The code block given to `sort` will not get any arguments (or, if it gets them, the arguments will be `$a` and `$b`. If you're not passing a code block as the first argument, you need to list all arguments separated by commas: `for (sort by_seq([qw(five nine one seven three)]), keys %h) {` [download] (except that `by_seq` won't do what you want yet). If you want to make the thing work, the easiest way is to generate the comparison subroutine on the fly and use it with `sort`: use strict; #use diagnostics; sub by_seq { my $seq_aref = shift; my %seq_helper; { my $index = 0; for my $item (@{ $seq_aref }) { $seq_helper{$item} = $index; $index++; }; }; # Here I return a subroutine that takes two arguments # (the left and the right element to be compared) return sub { $seq_helper{$_[0]} <=> $seq_helper{$_[1]} }; }; # -- my %h = (one=>2,three=>4,five=>6,seven=>8,nine=>0); # Create our custom comparison routine my $sorter = by_seq([qw(five nine one seven three)]); for (sort {$sorter->($a,$b)}, keys %h) { print "$_: $h{$_}\n"; }; [download] I thought I could write `sort $sorter, LIST`, but that gets interpreted by Perl (rightfully) as `sort LIST`, so I guess you'll be stuck with the extra code block.	[reply] [d/l] [select]
Re: sort in arbitrary order with subroutine, syntax error by moritz (Cardinal) on Dec 19, 2007 at 10:45 UTC
The sort docu tell you that can either use `sort SUBNAME LIST` or `sort BLOCK LIST`, and you do neither => syntax error. Just use the BLOCK form, and everything will work: `sort {by_seq([qw(five nine one seven three)])} keys %h`	[reply] [d/l] [select]
Re: sort in arbitrary order with subroutine, syntax error by citromatik (Curate) on Dec 19, 2007 at 10:49 UTC
If I understood well, you don't need to sort nothing, you can use the keys directly: a simple hash slice can do the job: `my %h = (one=>2,three=>4,five=>6,seven=>8,nine=>0); print "$_\n" for (@h{qw/five nine one seven three/})'` [download] Outputs: `6 0 2 8 4` [download] Or even easier: `my %h = (one=>2,three=>4,five=>6,seven=>8,nine=>0); print "$_ => $h{$_}\n" for (qw/five nine one seven three/);` [download] Outputs: `five => 6 nine => 0 one => 2 seven => 8 three => 4` [download] Is this what you are trying to do? citromatik	[reply] [d/l] [select]