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 just Perl’s way of saying, “Nothing here to
look at—move along, move along.” If you try to use this “nothing” as a
“numeric something,” it acts like zero. If you try to use it as a
“string something,” it acts 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 one to a variable
that already holds zero (because you’re using undef as if it were a number). So 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 already holds a string, the new text is
simply appended.
Perl programmers frequently use a new variable in ...