in reply to Best Hash Practices?

so ideally the perlish way of testing a value in a hash might be to do something like this: 
if ($myHash{"unknown_if_this_key_exists_yet_or_not"}){...}

[download]
But the reality is that will not work because an entry for the key automatically gets created in the hash if we try to do that, so we end up having to do something like this instead

No it doesn't. Try it. 

   my %h;
   if( $h{"isn't there"} ) { 1 }
   print "$_\n" for keys %h; # does not print anything

[download]
The real question is, do you need to test true when $h{key} is set and false? Then you use exists rather than just a boolean test.

UPDATE re-reply: indeed. Well said.

-Paul

Replies are listed 'Best First'.
Re^2: Best Hash Practices?
by almut (Canon) on Oct 09, 2009 at 03:19 UTC
    The real question is, do you need to test true when $h{key} is set and false? Then you use exists rather than just a boolean test.

    Right.  Then, there's also defined, which is kind of "in between" testing for truth and existence.

    I'm sure you know, but maybe for others the following little truth table helps to summarize the relationships of what can be tested with a hash: 

    sub truth_table {
    my $hash = shift;
    print "       true?   defined?   exists?\n";
    for my $key (qw(foo bar baz bla)) {
        print "  $key  ";
        printf "  %-8d",         $hash->{$key};
        printf "  %-8d", defined $hash->{$key};
        printf "  %-8d", exists  $hash->{$key};
        print "\n";
    }
}

truth_table(
    {
         foo => 1,       # true
         bar => 0,       # false
         baz => undef,   # undefined
         # bla           # doesn't exist
    }
);

__END__

       true?   defined?   exists?
  foo    1         1         1       
  bar    0         1         1       
  baz    0         0         1       
  bla    0         0         0

    [download]
[reply]
[d/l]
[select]

In Section Seekers of Perl Wisdom