Notes: Perl Lab 06

Outline

  1. Subroutine Syntax
  2. Local Variables
  3. Subroutine Arguments
  4. The @_ Array
  5. Returning Values

  1. Subroutine Syntax

     A subroutine in Perl is a block of code that can be called as a function with an identifying name. To create a subroutine simply use the following syntax:

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab06 Example 00
# Description: Basic subroutine syntax example
#==========================================================================

&Error_msg;

exit;

sub Error_msg { 
    print "Something is wrong\n";
}

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab06_0.pl 
Something is wrong
[mark@iguana perl]

    In the example above, subroutine name is Error_msg and is executed by the statement &Error_msg; The leading & in the subroutine call is optional but is strongly recommened to differentiate beteween user defined subroutines and functions in perl or those of a module. When &Error_msg is called, all the code in the subroutine Error_msg will be executed. Subroutines can be placed anywhere in the program but it is best to use some type of organization rather than scattering them randomly throughout the program.

  2. Local Variables

     By default in perl, all variables are global. Within any block (code between braces {}) a variable can be declared with the my function. my declares the variable so that it cannot be accessed outside the block.

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab06 Example 01
# Description:  Subroutine variables.
#==========================================================================

$counter = 0;
print "Counter is $counter\n";
&Loopy;
print "Counter is $counter\n";
exit;

sub Loopy {
    my $counter = 5;
    my $i;
    for ($i = 1;$i < 6; $i++) {
        $counter++;
        print "Line $i, counter is $counter\n";
    }
}

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab06_1.pl 
Counter is 0
Line 1, counter is 6
Line 2, counter is 7
Line 3, counter is 8
Line 4, counter is 9
Line 5, counter is 10
Counter is 0
[mark@iguana perl]

    In the above example, the scalar variable $counter is global in the main part of the program. A separate variable with the same name is declared in the subroutine but does changes to its value does not have any effect on the $counter variable in the main.

  3. Subroutine Arguments

     Arguments are passed to the function as any other perl function. A single value or variable may be the argument. A list of values or an array may also be the argument.

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab06 Example 02
# Description:  Subroutine arguments.
#==========================================================================

&Error_msg('It is broken.');

exit;

sub Error_msg { 
    print "Something is wrong\n";
    print $_[0]."\n";
}

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab06_2.pl 
Something is wrong
It is broken.
[mark@iguana perl]

    Notice that the argument passes when calling the subroutine is available inside the block as the list variable $_[0]. More on this below.

  4. The @_ Array

     All arguments passed to the sub routine are stored in an array available to the subroutine as @_.

      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab06 Example 03
# Description:  Subroutine arguments and the @_ array.
#==========================================================================

@somevar = qw( alpha beta delta epsilon zeta eta iota gamma theta omega );

&Check_values(@somevar);

# show any errors
foreach $k (@somevar) {
    print "$k\n" if($k =~ /^Erro/);
}

exit;

#-----------------------------
sub Check_values { 
    my $k;
    my $counter = 0;
    
    # show the values
    print "Array: @_\n";
    
    # check the individual elements
    foreach $k (@_) {
        if($k =~ /m/) {
            # alter the value of an element if it contains an "m"
            $_[$counter] = 'Error: '.$k;
        }
        $counter++;
    }
}

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab06_3.pl 
Array: alpha beta delta epsilon zeta eta iota gamma theta omega
Error: gamma
Error: omega

    It is important to remember that if scalar variables or arrays are passed to the subroutine as arguments, any alteration of the values of the elements of the @_ array will alter the global values of the arguments. In the case above $somevar[7,9] were altered by assigning values to elements of the @_ array by prepending the text 'Error: '.

  5. Returning Values

     Subroutines can return a value specified by the return function. This is useful for assigning values based on the results of the function.

      
    # Name: Mark Tucker
# Assignment: Lab06 Example 04
# Description:   Subroutine ruturn values.
#==========================================================================

do {
    print "Enter your age: ";
    chomp($age = <STDIN>);

    $valid = &Check_age($age);
    print "$age is $valid\n";
}while($valid ne 'valid');

exit;
#---------------------------------
sub Check_age {
    if($_[0] > 105) {
        return('too high');
    }elsif($_[0] < 0) {
        return('too low');
    }else{
        return('valid');
    }
}

    When executed, the script above produces the following output:
      
    [mark@iguana perl] ./lab06_4.pl      
Enter your age: 500
500 is too high
Enter your age: -4
-4 is too low
Enter your age: 35
35 is valid
[mark@iguana perl]

last updated: 18 Mar 2012 13:08