Logical Operators

As you might expect, 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 (($dessert_1 eq 'cake') && ($dessert_2 eq 'ice cream')) {
		# Both are true
		print "Hooray!  Cake and ice cream are available!\n";
	}
	elsif (($dessert_1 eq 'cake') || ($dessert_2 eq '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 in order to return true. In that case, there's no reason to check the right side, so it will not even 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 operatioin 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 5\n";
	}

In that example, the right side will be evaluated only if the left side is true, so we can't accidentally divide by zero and crash the program.

Next: The Value of a Short-Circuit Operator

OR

Other Control Structures