having a hash maintain order when passed to a sub

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

I was surprised to see that a hash tied with Tie::IxHash loses the ordering when processed in a sub:

use strict; use warnings;

use Tie::IxHash;

tie my %h, 'Tie::IxHash', (a => 1, b => 2, c => 3);


warn $_ for keys %h; # prints in assigned order

sub printh {
    my %x=@_;
    warn $_ for keys %x; # prints in scattered order!
}

printh(%h);
[download]

Is there anyway to preserve ordering when passing it? I suppose the OO way of making a tied hash would do, but I prefer normal Perl hashes over the OO interface.

Comment on having a hash maintain order when passed to a sub Download Code

Replies are listed 'Best First'.
Re: having a hash maintain order when passed to a sub by JavaFan (Canon) on May 14, 2009 at 15:33 UTC
You are not passing a hash to the sub. You flatten the hash to a list, pass that list, and build a new hash in the sub. Either make the new hash also a tied hash, or pass a reference to the hash. The latter is more efficient, but if you change the hash inside the sub, the outer hash (which is the same) is changed as well.	[reply]
Re: having a hash maintain order when passed to a sub by ikegami (Patriarch) on May 14, 2009 at 16:02 UTC
The only thing that can be passed to a sub is a list of scalars. You did not pass a hash to the sub, and the new hash you created in the sub isn't an IxHash. Solutions: `sub printh { tie my %x, 'Tie::IxHash', @_; # Recreate the hash. Expensive warn $_ for keys %x; } printh(%h); # Flatten the hash` [download] `sub printh { my ($x) = @_; warn $_ for keys %$x; } printh(\%h); # Pass a reference` [download]	[reply] [d/l] [select]
Re: having a hash maintain order when passed to a sub by Your Mother (Archbishop) on May 14, 2009 at 15:33 UTC
I haven't tried but I expect passing it as a reference instead of letting it be flattened out into a list and copied into a new hash should do.	[reply]