Subroutines

A subroutine (or sometimes refered to as a function or method) is a group of statements that work together to perform a task. Usually a repeated task which can be invoked several times.

Subroutine definitions can be placed anywhere in the program code. It is typical to place these definitions at the beginning of the code before the main code body which calls (uses) these subroutines.

The general form of a subroutine definition in Perl is:

sub subroutine_name {
    body of subroutine
}

# call the subroutine
subroutine_name( list of arguments );

Example:

# Subroutine definition
sub say_hello {
   print "Hello, World!\n";
}

# Subroutine call
print "We are calling the subroutine say_hello() now\n";
say_hello();

Passing Arguments to a Subroutine

When calling a subroutine, arguments can be passed to to it by writing them as a comma-delimited list inside the (). Inside the subroutine, these arguments are accessible using the special array @_. The first argument to the function is in $_[0], the second is in $_[1], and so on.

 # subroutine definition
 sub calc_average {
    # get total number of arguments passed
    $total_args = scalar(@_);
    $sum = 0;
    # sum up all arguments
    foreach $arg (@_){
       $sum += $arg;
    }
    # calculate print average
    $average = $sum / $total_args;
    print "Average of list [ @_ ] is: $average\n";
 }

 # subroutine call
 print "We are calling the subroutine calc_average() now\n";
 calc_average(10, 20, 30);
 print "We are calling the subroutine calc_average() again\n";
 calc_average(10, 15, 2, 8, 90);

Passing arrays or hashes to Subroutines

When one wishes to pass an array or hash to a subroutine, it is useful to create a reference and pass it as a single scalar to the subroutine. Then dereferencing the reference inside the subroutine will result with the original array or hash.

# subroutine definition
sub is_value_exists_in_array {
    # first argument: string value to be searched in array
    $value = $_[0];
    # second argument: reference to array to be searched in
    $arrayref = $_[1];
    # create the array by dereferencing
    @my_array = @$arrayref;

    $result = "does not exist in";
    foreach $element (@my_array) {
        if ($element eq $value) {
            $result = "exists in";
            last;
        }
    }
    # print result
    print "Value $value $result array [ @my_array ]\n";
}

# subroutine call
@foo = ('we', 'are', 5, 'happy', 'perl', 'programmers');
$foo_arrayref = \@foo;
print "We are calling the subroutine is_value_exists_in_array() now\n";
is_value_exists_in_array('hello', $foo_arrayref);
print "We are calling the subroutine is_value_exists_in_array() again\n";
is_value_exists_in_array('happy', $foo_arrayref);

Returning a Value from a Subroutine

A value can be returned from a subroutine by using the return() function. When this function is used, the subroutine executed is completed. The return value is a single value. Therefore in order to return an array or hash, create a reference first and return that value.

 # subroutine definition
 sub calc_average {
    # get total number of arguments passed
    $total_args = scalar(@_);
    $sum = 0;
    # sum up all arguments
    foreach $arg (@_){
       $sum += $arg;
    }
    # calculate and return average
    $average = $sum / $total_args;
    return ($average);
 }

 # subroutine call
 $a = calc_average(10, 20, 30);
 print "The average is $a\n";
 $a = calc_average(10, 15, 2, 8, 90);
 print "The average is $a\n";

Remember: @_ is the special array variable containing all the arguments passed to the function, $_[0] is the first argument, $_[1] is the second, and so on.

Giving variables a scope

Perl treats variables differently based on the context. By default, all variables in Perl are global variables, which means they can be accessed from anywhere in the program. But you can create private variables at any time with the my keyword. The my keyword confines a variable to a particular region of code in which it can be used and accessed. Outside that region, this variable cannot be used or accessed.

Example:

# Global variable
$string = "Hello, World!";

# Function definition
sub say_hello_perl {
    # Private variable
    my $string;
    $string = "Hello, Perl!";
    print "Inside the function $string\n";
}
# Function call
say_hello_perl();
print "Outside the function $string\n";

Exercise

Create two subroutines min() and max() which accept an array as input and calculate the minimum and maximum numeric value of their arguments respectively.

Then, for each given array @car_prices and @flight_prices, calculate and print the difference between the maximum and minimum values.