comparing array elements to hash keys

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

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Re: comparing array elements to hash keys by Zaxo (Archbishop) on Jul 02, 2003 at 14:20 UTC
You need to use string comparison in line 6: `if ($key eq $thing) {` However, it would be better to check existence of the array element as a hash key, `exists $hash{$_} and push @new_array, $hash{$_} for @array;` replaces the whole thing. That could be done with map and grep, too: `my @new_array = map {$hash{$_}} grep {exists $hash{$_}} @array;` or a slice with grep, `my @new_array = @hash{ grep {exists $hash{$_}} @array };` After Compline, Zaxo	[reply] [d/l] [select]
Re: comparing array elements to hash keys by CukiMnstr (Deacon) on Jul 02, 2003 at 14:21 UTC
you have to change the `foreach` in your inner loop for a `while` (right now it is only iterating once over the hash), or change the `foreach` inner loop to use `keys %hash` instead of `each %hash` (so it iterates over every key). Also, you are using `==` to compare the elements in the array to the keys in the hash, so this will only work for numeric values. You could try something like this instead: `foreach my $thing (@array) { if ( exists $hash {$thing} ) { push @new_array, $hash {$thing}; } }` [download] hope this helps,	[reply] [d/l] [select]
Re: comparing array elements to hash keys by hardburn (Abbot) on Jul 02, 2003 at 14:24 UTC
It probably doesn't work right because you're using the numeric comparison operator (`==`) on data that is probably strings. You should have used `eq` instead. But that's not all that you need to fix. The first thing that comes to mind when I look at your code is that you're not really taking advantage of true power of hashes. Instead of walking through the hash and comparing `$thing` to `$key`, you want to use `$thing` as the key, like this: `foreach my $thing (@array) { if(exists $hash{$thing}) { push @new_array, $hash{$thing}; } }` [download] If you're not afraid of `grep`, try this: `my @new_array = grep { exists $hash{$_} ? $hash{$_} : 0 } @array;` [download] But that will fail if `%hash` contains false values. Update: Was using `$thing` in the `grep` when I should have used `$_`. Thanks to davorg. ---- I wanted to explore how Perl's closures can be manipulated, and ended up creating an object system by accident. -- Schemer Note: All code is untested, unless otherwise stated	[reply] [d/l] [select]
Re: comparing array elements to hash keys by davorg (Chancellor) on Jul 02, 2003 at 14:33 UTC
Much easier with `grep`. `@new_array = @hash{ grep { exists $hash{$_} } @array };` [download] -- <http://www.dave.org.uk> "The first rule of Perl club is you do not talk about Perl club." -- Chip Salzenberg	[reply] [d/l]
Re: Re: comparing array elements to hash keys by dvergin (Monsignor) on Jul 03, 2003 at 19:53 UTC
I was going to suggest: `my @ary2 = grep {defined} @hash{@ary};` [download] It's tempting to just say `my @ary2 = @hash{@ary}`. But the values in @ary that don't match keys in %hash produce undefined values that have to be weeded out. Here's a quick demo: `#!/usr/bin/perl -w use strict; my @ary = ('a', 'c', 'g', 'l'); my %hash = (a => 1, b => 2, c => 3, d => 4); my @ary2 = grep {defined} @hash{@ary}; print "[$_]\n" for @ary2;` [download]	[reply] [d/l] [select]