Learning Perl, Fourth Edition

Logical Operators

Perl has all of the necessary logical operators needed to work with Boolean (true/false) values. For example, it’s often useful to combine logical tests by using the logical AND operator (&&) and the logical OR operator (||):

    if ($dessert{'cake'} && $dessert{'ice cream'}) {
      # Both are true
      print "Hooray! Cake and ice cream!\n";
    } elsif ($dessert{'cake'} || $dessert{'ice cream'}) {
      # At least one is true
      print "That's still good...\n";
    } else {
      # Neither is true - do nothing (we're sad)
    }

There may be a shortcut. If the left side of a logical AND operation is false, the whole thing is false since logical AND needs both sides to be true to return true. In that case, there’s no reason to check the right side, so it will not be evaluated. Consider what happens in this example if $hour is 3:

    if ( (9 <= $hour) && ($hour < 17) ) {
      print "Aren't you supposed to be at work...?\n";
    }

Similarly, if the left side of a logical OR operation is true, the right side will not be evaluated. Consider what happens here if $name is fred:

    if ( ($name eq 'fred') || ($name eq 'barney') ) {
      print "You're my kind of guy!\n";
    }

Because of this behavior, these operators are called “short-circuit" logical operators. They take a short circuit to the result whenever they can. In fact, it’s fairly common to rely upon this short-circuit behavior. Suppose you need to calculate an average:

    if ( ($n != 0) && ($total/$n < 5) ) {
      print "The average is below five.\n";
    }

In that example, the right side ...