Check if hash key value pair is defined

[reaper9187]

Hi
Jus a small question ... Is it possible to check if a particular key value pair entry exists in a hash , specifically in case of a key with multiple values ..????

[davido]

Keys in hashes are unique. The only way for a key to have multiple values is if the "value" is a reference to some container (an anonymous hash, anonymous array, or some object). Let's say your hash holds references to arrays. You might do this:

my %hash = ( unwanted => [1], wanted => [2, 3, 4] );
my $target_key = 'wanted';
if(    exists $hash{ $target_key } 
    && ref $hash{ $target_key } eq 'ARRAY' 
    && @{ $hash{ $target_key } } > 1 
) {
    print "The element associated with $target_key holds a reference t
+o an array of @{$hash{$target_key}} values.\n";
}

[tobyink]

In a standard Perl hash, a key can only have one value. (Though that value could be a reference to an array.)

# checking if this key-value pair exists
my ($key, $value) = ("foo", "123");

# ... in this hash
my %hash = (
  foo => "123",
  bar => "456",
);

if (exists $hash{$key} and $hash{$key} eq $value) {
  print "key-value pair exists\n";
}

[Anonymous Monk]

A data structure as-described must consist of a hash whose values are arrayrefs (or hashrefs). Therefore, first check to see if the key exists(), then use e.g. grep() to look for the desired value.

Be consistent: Arrange the program so that the hash always contains an arrayref, even if it contains only one entry. Perl's "auto-vivification" features make the necessary code for building the structure short-and-sweet.