in reply to Re: How to create a hash whose values are undef?
in thread How to create a hash whose values are undef?

Great that you have figured out "how" to do it. My question is "why"?.

Perl has the ability to automatically bring a new key into existence when you assign something to it without the need for that key to have been created beforehand - this is true in n-dimensional hash structures also. So in many situations, there is no need to make an "undef" value for a key or even to create that key. Sometimes there is a need ("in many situations" does definitely not mean "in all situations").

I am just curious as to why you want to do this?

Update: I am not saying that anything in the other posts is wrong - quite the contrary - there are some excellent formulations of how to do this. Grandfather's stuff looks fantastic to me. I'm just saying that often it is not necessary to set a whole bunch of keys to "undef" in the first place. Maybe in this application it is warranted. But if its not needed, then there can be some simplifications.

  • Comment on Re^2: How to create a hash whose values are undef?

Replies are listed 'Best First'.
Re^3: How to create a hash whose values are undef?
by cdarke (Prior) on Jul 06, 2009 at 11:43 UTC
    If you want to initialise the values to some other value, say zero, for example:
    my %hash;
@hash{@keys} = (0) x @keys;

    [download]
    The LHS is a slice, on the RHS the value, 0, is in parentheses which generates a list, the @keys is used in scalar context to get the correct number of values.
[reply]
[d/l]
      Great! I like your thinking, but you should be aware that "0" and even "" is not the same as "undef". 
      #!/usr/bin/perl -w
use strict;
use Data::Dumper;

my @keys = (1,2,3);
my %hash;
@hash{@keys} = (undef) x @keys;

print Dumper \%hash;
__END__
Prints:
$VAR1 = {
          '1' => undef,
          '3' => undef,
          '2' => undef
        };

      [download]
[reply]
[d/l]
        Exactly (that's why I said "some other value").
        Otherwise it could be done thus:
        my %hash;
@hash{@keys} = undef;

        [download]
        No need for the x operator. This sets the first value to undef, the others are unspecified so are set to undef as well.
[reply]
[d/l]

In Section Seekers of Perl Wisdom