Learning Perl, Fourth Edition

The undef Value

What happens if you use a scalar variable before you give it a value? Nothing serious and definitely nothing fatal. Variables have the special undef value before they are first assigned, which is Perl’s way of saying “nothing here to look at—move along, move along.” If you use this “nothing” as a “numeric something,” it will act like zero. If you use it as a “string something,” it will act like the empty string. But undef is neither a number nor a string; it’s an entirely separate kind of scalar value.

Because undef automatically acts like zero when used as a number, it’s easy to make an numeric accumulator that starts out empty:

    # Add up some odd numbers
    $n = 1;
    while ($n < 10) {
      $sum += $n;
      $n += 2; # On to the next odd number
    }
    print "The total was $sum.\n";

This works properly when $sum was undef before the loop started. The first time through the loop, $n is one, so the first line inside the loop adds one to $sum. That’s like adding 1 to a variable that already holds zero because you’re using undef as if it were a number. Now it has the value 1. After that, since it’s been initialized, adding works in the traditional way.

Similarly, you could have a string accumulator that starts out empty:

    $string .= "more text\n";

If $string is undef, this will act as if it already held the empty string, putting "more text\n" into that variable. But if it holds a string, the new text is appended.

Perl programmers frequently use a new variable in this way, letting it act as zero ...