Learning Perl, 5th Edition

Hash Element Access

To access an element of a hash, use syntax that looks like this:

$hash{$some_key}

This is similar to what we used for array access, but here we use curly braces instead of square brackets around the subscript (key).^[*] And that key expression is now a string, rather than a number:

$family_name{"fred"} = "flintstone";
$family_name{"barney"} = "rubble";

Figure 6-3 shows how the resulting hash keys are assigned.

Figure 6-3. Assigned hash keys

This lets us use code like this:

foreach $person (qw< barney fred >) {
  print "I've heard of $person $family_name{$person}.\n";
}

The name of the hash is like any other Perl identifier (letters, digits, and underscores, but can’t start with a digit). And it’s from a separate namespace; that is, there’s no connection between the hash element $family_name{"fred"} and a subroutine &family_name, for example. Of course, there’s no reason to confuse everyone by giving everything the same name. But Perl won’t mind if you also have a scalar called $family_name and array elements like $family_name[5]. We humans will have to do as Perl does; that is, we’ll have to look to see what punctuation appears before and after the identifier to see what it means. When there is a dollar sign in front of the name and curly braces afterward, it’s a hash element that’s being accessed.

When choosing the name of a hash, it’s often nice to think of the word “for” ...