10.9.1. The Value of a Short-Circuit Operator

But it's a much more useful return value. Among other things, the logical OR operator is quite handy for selecting a default value:

my $last_name = $last_name{$someone} || '(No last name)';

10.9.2. The Ternary Operator, ?:

expression ? if_true_expr : if_false_expr

First, the expression is evaluated to see whether it's true or false. If it's true, the second expression is used; otherwise, the third expression is used. Every time, one of the two expressions on the right is evaluated, and one is ignored. That is, if the first expression is true, then the second expression is evaluated, and the third is ignored. If the first expression is false, then the second is ignored, and the third is evaluated as the value of the whole thing.

In this example, the result of the subroutine &is_weekend determines which string expression will be assigned to the variable:

my $location = &is_weekend($day) ? "home" : "work";

And here, we calculate and print out an average -- or just a placeholder line of hyphens, if there's no average available:

my $average = $n ? ($total/$n) : "-----";
print "Average: $average\n";

You could always rewrite any use of the ?: operator as an if structure, often much less conveniently and less concisely:

my $average;
if ($n) {
  $average = $total / $n;
} else {
  $average = "-----";
}
print "Average: $average\n";

Here's a trick you might see, used to code up a nice multiway branch:

my $size =
  ($width < 10) ? "small"  :
  ($width < 20) ? "medium" :
  ($width < 50) ? "large"  :
                  "extra-large"; # default

That is really just three nested ?: operators, and it works quite well, once you get the hang of it.

Of course, you're not obliged to use this operator. Beginners may wish to avoid it. But you'll see it in others' code, sooner or later, and we hope that one day you'll find a good reason to use it in your own programs.

10.9.3. Control Structures Using Partial-Evaluation Operators

It's not as if Larry felt a burning need to add more control structures to Perl. But once he had decided to put these partial-evaluation operators into Perl, they automatically became control structures as well. After all, anything that can activate and deactivate a chunk of code is a control structure.

Fortunately, you'll notice this only when the controlled expression has side effects, like altering a variable's value or causing some output. For example, suppose you ran across this line of code:

($a < $b) && ($a = $b);

If $a is really less than $b, the left side is true, so the right side will be evaluated, thereby doing the assignment. But if $a is not less than $b, the left side will be false, and thus the right side would be skipped. So that line of code would do essentially the same thing as this one, which is easier to understand:

if ($a < $b) { $a = $b; }

($a > 10) || print "why is it not greater?\n";

If $a is really greater than ten, the left side is true, and the logical OR is done. But if it's not, the left side is false, and this will go on to print the message. Once again, this could (and probably should) be written in the traditional way, probably with if or unless.

If you have a particularly twisted brain, you might even learn to read these lines as if they were written in English. For example: check that $a is less than $b, and if it is, then do the assignment. Check that $a is more than ten, or if it's not, then print the message.

It's generally former C programmers or old-time Perl programmers who most often use these ways of writing control structures. Why do they do it? Some have the mistaken idea that these are more efficient. Some think these tricks make their code cooler. Some are merely copying what they saw someone else do.

In the same way, the ternary operator may be used for control. In this case, we want to assign $c to the smaller of two variables:

($a < $b) ? ($a = $c) : ($b = $c);

If $a is smaller, it gets $c. Otherwise, $b does.

$a < $b and $a = $b;  # but better written as the corresponding if

Despite the fact that using logical operators as control structures can be confusing, sometimes they're the accepted way to write code. We'll see a common use of the or operator starting in the next chapter.