Perl | given-when Statement

given-when statement in Perl is a substitute for long if-statements that compare a variable to several integral values.

• The given-when statement is a multiway branch statement. It provides an easy way to dispatch execution to different parts of code based on the value of the expression.
• given is a control statement that allows a value to change control of execution.

given-when in Perl is similar to the switch-case of C/C++, Python or PHP. Just like the switch statement, it also substitutes multiple if-statements with different cases.

Syntax:

given(expression)
{
    when(value1) {Statement;}
    when(value2) {Statement;}

    default {# Code if no other case matches}
}

given-when statements also use two other keywords along with it, namely break and continue. These keywords maintain the flow of the program and help in getting out of the program execution or skipping execution at a particular value.

break: break keyword is used to break out of a when block. In Perl, there is no need to explicitly write the break after every when block. It is already defined implicitly.
continue: continue, on the other hand, moves to the next when block if first when block is correct.

In a given-when statement, a conditional statement must not repeat in multiple when statements, this is because Perl checks for the first occurrence of that condition only and the next repeating statements will be ignored. Also, a default statement must be placed after all the when statements because the compiler checks for condition matching with every when statement in order, and if we will place default in between, then it will take a break over there and will print the default statement.

Example:

#!/usr/bin/perl

# Perl program to print respective day
# for the day-code using given-when statement
use 5.010;

# Asking the user to provide day-code
print "Enter a day-code between 0-6\n";

# Removing newline using chomp
chomp(my $day_code = <>);

# Using given-when statement
given ($day_code)
{
    when ('0') { print 'Sunday' ;}
    when ('1') { print 'Monday' ;}
    when ('2') { print 'Tuesday' ;}
    when ('3') { print 'Wednesday' ;}
    when ('4') { print 'Thursday' ;}
    when ('5') { print 'Friday' ;}
    when ('6') { print 'Saturday' ;}
    default { print 'Invalid day-code';}
}

Input:

4

Output:

Enter a day-code between 0-6
Thursday

Nested given-when statement

Nested given-when statement refers to given-when statements inside of another given-when Statements. This can be used to maintain a hierarchy of inputs provided by the user for a specific output set.

Syntax:

given(expression)
{
    when(value1) {Statement;}
    when(value2) {given(expression)
                  {
                     when(value3) {Statement;}   
                     when(value4) {Statement;}   
                     when(value5) {Statement;} 
                     default{# Code if no other case matches}
                  }
                 }
    when(value6) {Statement;}

    default {# Code if no other case matches}
}

Following is an example for Nested given-when statement:

#!/usr/bin/perl

# Perl program to print respective day
# for the day-code using given-when statement
use 5.010;

# Asking the user to provide day-code
print "Enter a day-code between 0-6\n";

# Removing newline using chomp
chomp(my $day_code = <>);

# Using given-when statement
given ($day_code)
{
    when ('0') { print 'Sunday' ;}
    when ('1') { print "What time of day is it?\n";
                 chomp(my $day_time = <>);

                 # Nested given-when statement
                 given($day_time)
                 {
                     when('Morning') {print 'It is Monday Morning'};
                     when('Noon')    {print 'It is Monday noon'};
                     when('Evening') {print 'It is Monday Evening'};
                     default{print'Invalid Input'};
                 }
                }
    when ('2') { print 'Tuesday' ;}
    when ('3') { print 'Wednesday' ;}
    when ('4') { print 'Thursday' ;}
    when ('5') { print 'Friday' ;}
    when ('6') { print 'Saturday' ;}
    default { print 'Invalid day-code';}
}

Input:

1
Morning

Output:

Enter a day-code between 0-6
What time of day is it?
It is Monday Morning

Input:

3

Output:

Enter a day-code between 0-6
Wednesday

In the above-given Example, when the Input day-code is anything but 1 then the code will not go into the nested given-when block and the output will be same as in the previous example, but if we give 1 as Input then it will execute the Nested given-when block and the output will vary from the previous example.