2.3.2 Undefined Variables
You may have begun to wonder: what value does a scalar variable have if
you have not given it a value? In other words, after:
use strict;
my $sweetNothing;
what value does $sweetNothing have?
The value that $sweetNothing has is a special value in Perl
called undef. This is frequently expressed in English by saying
that $sweetNothing is undefined.
The undef value is a special one in Perl. Internally, Perl keeps
track of which variables your program has assigned values to and which
remain undefined. Thus, when you use a variable in any expression, Perl
can inform you if you are using an undefined value.
For example, consider this program:
#!/usr/bin/perl
use strict;
use warnings;
my $hasValue = "Hello";
my $hasNoValue;
print "$hasValue $hasNoValue\n";
When this program is run, it produces the following output:
Use of uninitialized value at line 8.
Hello
What does this mean? Perl noticed that we used the uninitialized (i.e.,
undefined) variable, $hasNoValue at line 8 in our program.
Because we were using warnings, Perl warned us about that use of
the undefined variable.
However, Perl did not crash the program! Perl is nice enough not to
make undefined variables a hassle. If you use an undefined variable and
Perl expected a string, Perl uses the empty string, "", in its
place. If Perl expected a number and gets undef, Perl
substitutes 0 in its place.
However, when using warnings, Perl will always warn you when you
have used an undefined variable at run-time. The message will print to
the standard error (which, by default, is the screen) each time Perl
encounters a use of a variable that evaluates to undef. If you
do not use warnings, the warnings will not print, but you should
probably wait to turn off warnings until you are an experienced
Perl programmer.
Besides producing warning messages, the fact that unassigned variables
are undefined can be useful to us. The first way is that we can
explicitly test to see if a variable is undefined. There is a
function that Perl provides called defined. It can be used to
test if a variable is defined or not.
In addition, Perl permits the programmer to assign a variable the value
undef. The expression undef is a function provided by
Perl that we can use in place of any expression. The function
undef is always guaranteed to return an undefined value. Thus,
we can take a variable that already has a value and make it undefined.
Consider the following program:
#!/usr/bin/perl
use strict;
use warnings;
my $startUndefined;
my $startDefined = "This one is defined";
print "defined \$startUndefined == ",
defined $startUndefined,
", defined \$startDefined == ",
defined $startDefined, "\n";
$startUndefined = $startDefined;
$startDefined = undef;
print "defined \$startUndefined == ",
defined $startUndefined,
", defined \$startDefined == ",
defined $startDefined, "\n";
Which produces the output:
defined $startUndefined == , defined $startDefined == 1
defined $startUndefined == 1, defined $startDefined ==
Notice a few things. First, since we first declared
$startUndefined without giving it a value, it was set to
undef. However, we gave $startDefined a value when it was
declared, thus it started out defined. These facts are exemplified by
the output.
To produce that output, we did something that you have not seen yet.
First, we created some strings that "looked" like the function calls
so our output would reflect what the values of those function calls
were. Then, we simply used those functions as arguments to the
print function. This is completely legal in Perl. You can use
function calls as arguments to other functions.
When you do this, the innermost functions are called first, in their
argument order. Thus, in our print statements, first
defined $startUndefined is called, followed by defined
$startDefined. These two functions each evaluate to some value. That
value then becomes the argument to the print function.
So, what values did defined return? We can determine the answer
to this question from the printed output. We can see that when we
called defined on the variable that we started as undefined,
$startUndefined, we got no output for that call (in fact,
defined returned an empty string, ""). When we called
defined on the value that we had assigned to,
$startDefined, we got the output of 1.
Thus, from the experiment, we know that when its argument is not
defined, defined returns the value "", otherwise known as
the empty string (which, of course, prints nothing to the standard
output when given as an argument to print).
In addition, we know that when a variable is defined, defined
returns the value 1.
Hopefully, you now have some idea of what an undef value is, and
what defined does. It might not yet be clear why defined
returns an empty string or 1. If you are particularly curious
now, see section 4.2 A Digression--Truth Values.
