FAQ 4.44 How do I test whether two arrays or hashes are equal?


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This is an excerpt from the latest version perlfaq4.pod, which
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to review and update the answers. The latest version of the complete
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4.44: How do I test whether two arrays or hashes are equal?

With Perl 5.10 and later, the smart match operator can give you the
answer with the least amount of work:

use 5.010;

if( @array1 ~~ @array2 )
say "The arrays are the same";

if( %hash1 ~~ %hash2 ) # doesn't check values!
say "The hash keys are the same";

The following code works for single-level arrays. It uses a stringwise
comparison, and does not distinguish defined versus undefined empty
strings. Modify if you have other needs.

$are_equal = compare_arrays(\@frogs, \@toads);

sub compare_arrays {
my ($first, $second) = @_;
no warnings; # silence spurious -w undef complaints
return 0 unless @$first == @$second;
for (my $i = 0; $i < @$first; $i++) {
return 0 if $first->[$i] ne $second->[$i];
return 1;

For multilevel structures, you may wish to use an approach more like
this one. It uses the CPAN module "FreezeThaw":

use FreezeThaw qw(cmpStr);
@a = @b = ( "this", "that", [ "more", "stuff" ] );

printf "a and b contain %s arrays\n",
cmpStr(\@a, \@b) == 0
? "the same"
: "different";

This approach also works for comparing hashes. Here we'll demonstrate
two different answers:

use FreezeThaw qw(cmpStr cmpStrHard);

%a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
$a{EXTRA} = \%b;
$b{EXTRA} = \%a;

printf "a and b contain %s hashes\n",
cmpStr(\%a, \%b) == 0 ? "the same" : "different";

printf "a and b contain %s hashes\n",
cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";

The first reports that both those the hashes contain the same data,
while the second reports that they do not. Which you prefer is left as
an exercise to the reader.


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John Bokma

John Small said:
But all the elements in the arrays being compared in this particular
program are all single "words". For this situation "@array1" eq
"@array2" does seem to work. Is there any reason to think "@array1" eq
"@array2" would not work under these conditions?

If these conditions always hold, it will always work. Note that the FAQ
answer has also specific conditions, and fails if those conditions don't




if ("@array1" eq "@array2") ...

This seems to work for me.

Having parallel arrays is often a design flaw.

Replace for example by a hash, where the value is a counter.

perl -MData::Dumper -MList::MoreUtils=uniq -wle '
my @old = qw/ a b c d e f g h i j /;
my @new = qw/ 2 7 d e j k l /;
my %uniq;

++$_ for @uniq{ @old };
--$_ for @uniq{ @new };
delete @uniq{ grep !$uniq{ $_ }, keys %uniq };

print Dumper( \%uniq );

print uniq sort @old, @new;

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