To begin with, this built-in Perl function has two syntax forms:

defined EXPR
defined

to see a lot of fully commented examples that help you use the Perl statements and the Perl built-in functions in your scripts.

Anytime we declare a variable in Perl without initializing it, automatically it will take the "undef" value. So we use the defined function to check-up if a variable is defined (it was assigned some value to it) or has the "undef" value.

Please note that you can’t use the boolean conditions to verify if a value is defined, for the simple reason that "undef", 0, the empty string ("") and "0" are evaluated as false in Perl.

In the following, we will show you a few examples about how to use Perl defined function with scalars, arrays, hashes or subroutines.

#!/usr/bin/perl –w

print $v;

You can use the ternary conditional operator ? to check-up if a scalar variable is defined or not. See the next short snippet code for this:

my $v1;
my $v2 = 1;

defined($v1) ? print "v1 defined\n" : print "v1 not defined\n";
defined($v2) ? print "v2 defined\n" : print "v2 not defined\n";

undef $v2;
defined($v2) ? print "v2 defined\n" : print "v2 not defined\n";

v1 not defined
v2 defined
v2 not defined

my @array;
print "not defined\n" unless defined(@array); 
# displays: not defined
@array = qw(1 2 3 4);
print "defined\n" if defined(@array); 
# displays: defined
# now we clear the array
@array = ();
print "defined\n" if defined(@array); 
# displays: defined
undef @array;
print "not defined\n" unless defined(@array); 
# displays: not defined

Therefore, be aware any times you are tented to call the defined function against an entire array. The next examples will show you how to use the defined function with the elements of an array.

# initializing an array
my @array = qw(zero one two three four);
$array[7] = "seven";
$array[9] = "nine";
for(my $i=0; $i<=20; $i++) {
  print $array[$i]," " if defined($array[$i]);
}
print "\n";
# it displays: zero one two three four seven nine

to see a lot of fully commented examples that help you use the Perl statements and the Perl built-in functions in your scripts.

The following example will print only the keys of a hash that have defined corresponding values:

# initialize a hash aggregate
my %numbers = (zero => 0, one => 1, two => undef,  three => 3);
# now we print the keys for which the values are defined
foreach my $key (keys %numbers) {
  print "$key "  if defined($numbers{$key});
}
print "\n";
# it displays: three one zero

We will take here a more simple example. Let’s say that we have the @colors array with some colors as elements and we want to enlarge this array with new colors by printing and eliminating duplicates.

At the @colors array we will associate the %colors hash that has ($color, 1) as element pairs where $color represents an element of the @colors array and 1 has the significance that $color belongs to @colors array.

Anytime we’ll find a new color to append to our @colors array, we will update the %colors hash too by adding the pair ($color, 1) element to it. To check if the $color exists already, we use the defined function.

Please note that @colors and %colors are distinct structures, and Perl knows to make the difference between them.

Here is the code:

# initialize the @colors array
my @colors = qw(white blue red);
my %colors;

# setting the corresponding hash structure
foreach my $color (@colors) {
  $colors{$color} = 1;
}

# initialize the @newcolors structure
my @newcolors = qw(grey green white yellow red cyan);

foreach my $color (@newcolors) {
  if(defined($colors{$color})) { 
    print "duplicate color: $color\n";  
  } 
  else { $colors{$color} = 1; push(@colors, $color); }
}
# display the @colors array
print "@colors\n";

duplicate color: white
duplicate color: red
white blue red grey green yellow cyan

while(defined(my $input = <STDIN>)) {
  chomp $input;
  print "input line: $input\n";
}

If you want to download the Perl defined script with all the above examples included, please click here: Script download

to see a lot of fully commented examples that help you use the Perl statements and the Perl built-in functions in your scripts.