The Undefined Value
What
happens if you use a scalar
variable before you give it a value?
Nothing serious, and definitely nothing fatal. Variables have the
undef value before they are first assigned. This
value looks like a zero when used as a number, or the zero-length
empty string when used as a string. You will get a warning when
running under Perl’s -w switch, though,
which is a good way to catch programming errors.
Many operators return undef
when the arguments are out of range or don’t make sense. If you
don’t do anything special, you’ll get a zero or a null
string without major consequences. In practice, this scenario is
hardly a problem.
One operation we’ve seen that returns undef
under certain circumstances is
<STDIN>
. Normally, this returns
the next line that was read; however, if there are no more lines to
read, (such as when you type CTRL-Z at the terminal, or when a file
has no more data),
<STDIN>
returns undef as a
value. In Chapter 6, we’ll see how to test
for this and take special action when there is no more data available
to read.
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