Hashes and boolean values

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

Hi Monks,

I am trying to wrap my head around why the following test doesn't behave as I expected it to. I am trying to set a hash to a default boolean value unless the hash key exists. Here's an example:

#!/usr/bin/perl

use strict;
use warnings;

my %hash;

$hash{KEY1} = "0";
$hash{KEY1} = "1" unless $hash{KEY1};
print "KEY1 = $hash{KEY1}\n"; # i expect KEY1 = 0

$hash{KEY1} = "FALSE";
$hash{KEY1} = "TRUE" unless $hash{KEY1};
print "KEY1 = $hash{KEY1}\n"; # i expect KEY1 = TRUE
[download]

Am I just having a case of the Monday mornings? I thought double quoting "0/1" would result in stringification and not an actual boolean.

Comment on Hashes and boolean values Download Code

Replies are listed 'Best First'.
Re: Hashes and boolean values by moritz (Cardinal) on Jan 23, 2012 at 13:12 UTC
I thought double quoting "0/1" would result in stringification and not an actual boolean. `"0"` produces a string, but that is still false in boolean context. You should really use exists: `$hash{KEY1} = '1' unless exists $hash{KEY};` [download] Perl 6 - second systems done right	[reply] [d/l] [select]
Re^2: Hashes and boolean values by osbosb (Monk) on Jan 23, 2012 at 13:24 UTC
wouldn't that just autovivify it and then burn me later?	[reply]
Re^3: Hashes and boolean values by moritz (Cardinal) on Jan 23, 2012 at 13:26 UTC
I don't understand, I thought you wanted to set a default. Now you are afraid that setting a default will burn you? I am trying to set a hash to a default boolean value unless the hash key exists. Here's an example: That's exactly what my code does. Perl 6 - second systems done right	[reply]
Re^4: Hashes and boolean values by osbosb (Monk) on Jan 24, 2012 at 15:41 UTC
Re^3: Hashes and boolean values by JavaFan (Canon) on Jan 23, 2012 at 14:12 UTC
I cannot answer whether you will burn later, but `exists $hash{foo}` will not autovivify `$hash{foo}`. Now, `exists $hash{foo}{bar}` will autovivify `$hash{foo}`, but then, so will `EXPR if $hash{foo}{bar}`.	[reply] [d/l] [select]
Re: Hashes and boolean values by osbosb (Monk) on Jan 23, 2012 at 13:11 UTC
Yes, I am having a case of the mondays. Anything that stringifies to an empty string or the string `0` is false. I'll leave this hanging around for reference.	[reply]
Re: Hashes and boolean values by Anonymous Monk on Jan 23, 2012 at 16:05 UTC
“Golfing” is a pasttime. It’s okay to use more than one statement.	[reply]