SAMS Teach Yourself Perl in 24 Hours THIRD EDITION

Hash Odds and Ends

Some operations on hashes aren't obvious if you're new to Perl. Because of the special nature of hashes, a couple of common operations require functions that aren't necessary for scalars and arrays.

Testing for Keys in a Hash

To test to see whether a key exists in a hash, for example, you might be tempted to try the following syntax:

if ( $Hash{keyval} ) {        # WRONG, in this case
    :
}

This example doesn't work, for a few reasons. First, this snippet doesn't test to see whether keyval is a key in a hash; it actually tests the value associated with the key keyval in the hash.

Does it work to test whether the key is defined, as in the following?

if ( defined $Hash{keyval} ) {    # WRONG, again in this case
    :
}

Again, this example ...