How can I maintain the order of keys I add to a Perl hash?

Question

How can I maintain the order of actual list after counting its occurrence using a hash in the following program? For example, <DATA> are

a b e a c  d  a c d b etc. 

Using hash, i counted the occurrence of each element.

and what i want is:

a  3 b  2 e  1 c  2 d  2 

but the following program shows me otherwise.

my (%count, $line, @array_1, @array_2); while ($line = <DATA>) {     $count{$line}++ if ( $line =~ /\S/ ); } @array_1 = keys(%count); @array_2 = values(%count); for(my $i=0; $i<$#array_1; $i++) {    print "$array_1[$i]\t $array_2[$i]"; } 

Answer 1

Hashes are not ordered, but as usual, CPAN offers a solution: Tie::IxHash

use Tie::IxHash; my %count; tie %count, 'Tie::IxHash';  while ($line = <DATA>) { $count{$line}++ if ( $line =~ /\S/ ); }  while( my( $key, $value)= each %count) {     print "$key\t $value";  } 

Answer 2

Data in a hash table is stored in order of the keys' hash code, which for most purposes is like a random order. You also want to store the order of the first appearance of each key. Here's one way to approach this problem:

my (%count, $line, @display_order); while ($line = <DATA>) {     chomp $line;           # strip the \n off the end of $line     if ($line =~ /\S/) {         if ($count{$line}++ == 0) {             # this is the first time we have seen the key "$line"             push @display_order, $line;         }     } }  # now @display_order holds the keys of %count, in the order of first appearance foreach my $key (@display_order) {     print "$key\t $count{$key}\n"; }