unless Statement

Before discussing the unless statement, let’s revisit Perl’s philosophy:

There is more than one way to do it.

Perl always provides you with an alternative way to achieve what you need to do. In programming, you often hear something like this:

If it’s not true, then do this (use if not statement).
or unless it’s true, then do this (use unless statement).

The effect is the same but the philosophy is different. That’s why Perl invented the unless statement to increase the readability of code when you use it properly.

the Perl unless statement is equivalent to the if not statement

unless statement

statement unless (condition);

Perl executes the statement from right to left, if the condition is false, Perl executes the statement that precedes the unless. If the condition is true, Perl skips the statement.

If you have more than one statement to execute, you can use the following form of the Perl unless statement:

unless (condition) {
    // code block
}

If the condition evaluates to false, Perl executes the code block, otherwise, it skips the code block.

my $a = 10;

unless($a <= 0){
    print("a is greater than 0\n")
}

unless..else statement

Sometimes you want to say unless the condition is true, then do this, otherwise do that.

This is where the unless…else statement comes into play. See the following unless else statement:

unless (condition) {
    // unless code block
} else {
    // else code block
}

If the condition is false, Perl will execute the unless code block, otherwise, Perl will execute the else code block.

my $a = 10;
unless ($a >= 0) {
    print("a is less than 0\n");
} else {
    print("a is greater than or equal 0\n");
}

a is greater than or equal 0

unless..elsif..else statement

If you have more than one condition for checking with the unless statement, you can use the unless elsif else statement as follows:

unless (condition_1) {
    // unless code block
} elsif (condition_2) {
    // elsif code block
} else {
    // else code block
}

You can have many elsif clauses in the unless elsif statement.

my $a = 1;

unless ($a > 0) {
    print("a is less than 0\n");
} elsif ($a == 0) {
    print("a is 0\n");
} else {
    print("a is greater than 0\n");
}

unless statement guidelines

You should use the unless statement with a simple condition to improve the code readability, especially when used as a postfix after another statement like the following example:

my $a = 1;
print("unless used with a very simple condition ONLY.\n") unless($a < 0);

You should avoid using the unless statement when the condition is complex and requires else and/or elsif clauses.

If you take a look at the following code, it is difficult to interpret the meaning of the condition.

my $a = 1;
my $b = 10;
my $c = 20;
unless ($a < 0 && $b == 10 && $c > 0) {
    print("unless used with a very complex condition\n");
}