Prev: FAQ 4.74 How do I print out or copy a recursive data structure?
From: Sal on 13 Aug 2010 20:16
#!/usr/bin/perl

use strict;
use warnings;

my %sum = {};
for (my $i = 1; $i <= 6; $i++) {
for (my $j = 1; $j <= 6; $j++) {
for (my $k = 1; $k <= 6; $k++) {
my $tot = $i+$j+$k;
my $key = "$i " . "$j " . "$k ";
$sum{$key} = $tot;
print "$i " . "$j " . "$k " . " $tot\n";
}
}
}

foreach my $key (sort keys %sum) {
print "$key => $sum{$key}\n";
}

When the above is executed it first prints the entire hash, then
returns the error:

Use of uninitialized value $sum{"HASH(0x95fe818)"} in concatenation
(.) or string at ./3dice.pl line 19.
HASH(0x95fe818) =>

Why is the last hash value blank?

From: Don Piven on 13 Aug 2010 20:37
On 08/13/2010 07:16 PM, Sal wrote:
> #!/usr/bin/perl
>
> use strict;
> use warnings;
>
> my %sum = {};
> [code omitted]
>
> foreach my $key (sort keys %sum) {
> print "$key => $sum{$key}\n";
> }
>
> When the above is executed it first prints the entire hash, then
> returns the error:
>
> Use of uninitialized value $sum{"HASH(0x95fe818)"} in concatenation
> (.) or string at ./3dice.pl line 19.
> HASH(0x95fe818) =>
>
> Why is the last hash value blank?

The line "my %sum = {};" doesn't initialize %sum to an empty hash. {}
returns a reference to an empty hash, so that reference, stringified,
becomes a key in %sum and, since no value is provided in %sum's
initializer list, that key's value becomes undef.

You could just declare %sum without an initializer, or initialize it
with an empty list: "my %sum = ();".

Don

From: Ben Morrow on 13 Aug 2010 20:41

Quoth Sal <here(a)softcom.net>:
> #!/usr/bin/perl
>
> use strict;
> use warnings;
>
> my %sum = {};

This line is wrong. {} creates a new anonymous hashref; this is then
stringified and inserted into the hash as a key with no value.
Effectively this line is equivalent to

my %sum = ("HASH(0xDEADBEEF)" => undef);

If you want an empty hash, just say

my %sum;

Ben

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