Printing hash

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

I have a hash..

%hash=('A'=>1, 'B'=>2, 'C'=>3, 'D'=>4);
[download]

..if I want to print this hash I have previously done:

@keyarr=(keys (%hash));
foreach (@keyarr)
{print hash{$_}=>$_;
[download]

Now I have discovered that the following is possible:

foreach $key (sort (keys (%hash)))
{print "$key => $hash{$key}\n";}
[download]

Could someone of you please explain what is happening here?
Is an array without name created that holds all the key values and each of the elements is called '$key'? It looks from the code like the $key variable is assigned all keyvalues at one time, this clearly can not be the case so what is going on?

Edited by planetscape - added code tags

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Re: Printing hash by mbeast (Beadle) on Feb 13, 2006 at 02:42 UTC
keys %hash returns an array of keys. sort returns an array sorted in ascii table order. So you are getting a list (array) of keys, then sorting that list. Then the foreach iterates over each element in our new list with each element assigned to '$key' until we run out of elements $key will contain 'A', 'B', etc. so we can access our array values with $hash{$key}	[reply]
Re: Printing hash by helphand (Pilgrim) on Feb 13, 2006 at 02:29 UTC
The code: `foreach my $key (sort keys %hash)` [download] creates an anonymous array of the sorted keys in the hash %hash. The foreach is a loop construct, each iteration thru the loop sets $key to the next item in the anonymous array until the end of the array, when the loop ends. Scott	[reply] [d/l]
Re^2: Printing hash by bobf (Monsignor) on Feb 13, 2006 at 02:37 UTC
Actually, `( sort keys %hash )` creates a list, not an anonymous array (see sort and keys). Anonymous arrays are created using square brackets (see perlref). Please see (tye)Re: What is the difference between a list and an array? for a nice explanation of the difference between lists and arrays (thanks, tye). Secondly, `$key` is an alias to each value in the list, which means modifying `$key` will modify the original value (see perlsyn), as the following example shows: `use strict; use warnings; my @array = ( 1 .. 5 ); foreach my $value ( @array ) { $value--; } print join( ', ', @array ), "\n"; # 0, 1, 2, 3, 4` [download] HTH Update: added links for sort, keys, and (tye)Re: What is the difference between a list and an array?.	[reply] [d/l] [select]
Re^3: Printing hash by CountOrlok (Friar) on Feb 13, 2006 at 05:37 UTC
Quote:Secondly, $key is an alias to each value in the list, which means modifying $key will modify the original value :EndQuote It will change the value in the list returned by keys and sort, but not in the original hash, e.g. `my %hash = (1 => 'a', 2 => 'b', 3 => 'c'); foreach my $value (sort keys %hash ) { $value--; } print join( ', ', sort keys %hash ), "\n"; # 1, 2, 3` [download] -imran	[reply] [d/l]