Assigning values to undef hash reference keys two or more at a time

gctaylor1 has asked for the wisdom of the Perl Monks concerning the following question:

When I create a hash reference and then try to add values to undefined keys, can I add a different value to key1 and key2 at the same time?

Something like this, except it doesn't work:

$hashref->{"key1"} = "value1", {"key2"} = "value2";
[download]

This seems like it should be simple and if the answer is in perlreftut or perldsc (or anywhere else) please give me a big clue because the many attempts I've made just aren't working.

I don't really have a burning need, just trying to learn about references and hashes.
I can only add the values one at a time, as my example below demonstrates

use strict;
use warnings;

my %employees=  ( "key1"   => undef,
                  "key2"   => undef,
                  "key3"   => undef,
                 );

my $hashref = \%employees;
$hashref->{"key1"} = "value1";
$hashref->{"key2"} = "value2";
$hashref->{"key3"} = "value3";

print %$hashref,"\n";
[download]

Comment on Assigning values to undef hash reference keys two or more at a time Select or Download Code

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Re: Assigning values to undef hash reference keys two or more at a time by GrandFather (Saint) on Oct 23, 2008 at 00:37 UTC
Here are a couple of techniques you can use: `my %employees = map {$_ => undef} qw(key1 key2 key3); @employees{qw(key1 key2 key3)} = undef;` [download] Perl reduces RSI - it saves typing	[reply] [d/l]
Re: Assigning values to undef hash reference keys two or more at a time by oko1 (Deacon) on Oct 23, 2008 at 01:13 UTC
The thing you're looking for is called a hash slice. #!/usr/bin/perl -wl use strict; # Explicit lists of keys and values my %foo; # Note the '@' in front of the hash name! @foo{1..26} = "a".."z"; print $foo{10}; # Prints 'j' # You can use hash slices to return a list, too: print @foo{qw/8 5 12 12 15/}; # Prints 'hello' # You can use it to associate lists: my @employee_names = qw/Joe Jane Jim Jessie/; my @employee_ids = qw/1234 1235 1236 1237/; my %empl_lookup; @empl_lookup{@employee_ids} = @employee_names; # Now, you can look up employee names by ID: print $empl_lookup{1236}; # Hi there, Jim! [download] It's a very useful method in Perl, and worth knowing well. Update: Whoops - I just noticed that I missed a couple of bytes in the question, and thus answered something other than what was asked. `#!/usr/bin/perl -wl use strict; my (@a, @b, $foo); @a = qw/key1 key2/; @b = qw/val1 val2/; $foo = { map { $a[$_] => $b[$_] } 0..$#a }; print $foo->{key1}; # Prints 'val1'` [download] The above is a bit more like it. :) -- "Language shapes the way we think, and determines what we can think about." -- B. L. Whorf	[reply] [d/l] [select]
Re: Assigning values to undef hash reference keys two or more at a time by AnomalousMonk (Archbishop) on Oct 23, 2008 at 03:13 UTC
As mentioned above, take a look at hash Slices.	[reply]
Re: Assigning values to undef hash reference keys two or more at a time by AnomalousMonk (Archbishop) on Oct 23, 2008 at 19:31 UTC
Try something like this: `>perl -wMstrict -le "my %employees; my @values = ('value1', 'value2', 'value3'); @employees{ qw(key1 key2 key3) } = @values; print qq{ @{[ %employees ]} }; my $hashref = \%employees; @{ $hashref }{ qw(key1 key3) } = qw(foo bar); print qq{ @{[ %employees ]} }; " key2 value2 key1 value1 key3 value3 key2 value2 key1 foo key3 bar` [download] Note the critical syntax is `@{ $hashref }{ qw(key1 key3) } = qw(foo bar);`.	[reply] [d/l] [select]
Re: Assigning values to undef hash reference keys two or more at a time by gctaylor1 (Hermit) on Oct 23, 2008 at 18:18 UTC
Now I understand which concept to use on the right side but I can't get my values assigned to the left side. perlreftut says "Whatever you want to do with a reference, Use Rule 1 tells you how to do it. You just write the Perl code that you would have written for doing the same thing to a regular array or hash, and then replace the array or hash name with {$reference} ." My traditional hash gets the values assigned to keys with this: `my @values = ("value1", "value2", "value3"); @employees{qw(key1 key2 key3)} = @values;` [download] So I thought some form of this would work but I can't get it right. `@{$hashref->{qw/key1 key2 key3/}} = @values; %{$hashref}->{qw/key1 key2 key3/} = @values;` [download] I've tried many more combinations, including moving the brackets all over the place. Is this even close? I need a bigger hint I guess.	[reply] [d/l] [select]
Re^2: Assigning values to undef hash reference keys two or more at a time by JadeNB (Chaplain) on Oct 23, 2008 at 20:29 UTC
Rule 1, which you mention, provides the answer—except that maybe you have the wrong idea of what the ‘hash name’ is. Namely: `@hash{ qw/key1 key2 key3/ }` [download] is the slice of the hash named 'hash' (not '%hash') corresponding to the indicated keys. If you want to slice a referent, just stick the (suitably bracketed) reference in place of the name: `@{ $hashref }{ qw/key1 key2 key3/ }` [download] (as AnomalousMonk pointed out below).	[reply] [d/l] [select]
Re: Assigning values to undef hash reference keys two or more at a time by gctaylor1 (Hermit) on Oct 23, 2008 at 14:32 UTC
All three of these responses provided something useful for me. Thanks to each of you.	[reply]
Re: Assigning values to undef hash reference keys two or more at a time by gctaylor1 (Hermit) on Oct 23, 2008 at 23:18 UTC
I was thinking the infix operator needed to be used to dereference $hashref. Or more accurately, I guess I thought I could use it if I wanted and I chose to use it for consistency purposes. Plus it was just one thing I needed to remember instead of the various other ways to dereference and the docs I read suggested it might be easier to read. So the suggestion works by-the-way. I'm still trying to understand why infix -> didn't work.	[reply]