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Sunday, May 12, 2013

Perl Scripting - Part 3


Please click here for part-2

Subroutines:
=========
-Like any good programming langauge Perl allows the user to define their own functions, called subroutines.
-They may be placed anywhere in your program but it's probably best to put them all at the beginning or all at the end.



A subroutine has the form:

sub mysubroutine
{
print "Not a very interesting routine\n";
print "This does the same thing every time\n";
}

-Regardless of any parameters that we may want to pass to it.
-All of the following will work to call this subroutine.

Notice that a subroutine is called with an “&” character in front of the name:

&mysubroutine; # Call the subroutine

&mysubroutine($_); # Call it with a parameter

&mysubroutine(1+2, $_); # Call it with two parameters

Parameters:
=========
-In the above case the parameters are acceptable but ignored.
-When the subroutine is called, any parameters are passed as a list in the special @_ list array variable.
-This variable has absolutely nothing to do with the $_ scalar variable.
-The following subroutine merely prints out the list that it was called with. It is followed by a couple of examples of its use.

sub printargs
{
 print "@_\n";
}
&printargs("perly", "king"); # Example prints "perly king"
&printargs("frog", "and", "toad"); # Prints "frog and toad"

-Just like any other list array, the individual elements of @_ can be accessed with the square bracket notation:

sub printfirsttwo
{
print "Your first argument was $_[0]\n";
print "and $_[1] was your second\n";
}

-Again it should be stressed that the indexed scalars $_[0] and $_[1] and so on have nothing to with the scalar $_ which can also be used without fear of a clash.

Returning Values:
=============
-Result of a subroutine is always the last thing evaluated.
-This subroutine returns the maximum of two input parameters.

An example of its use follows.

sub maximum
{
           if ($_[0] > $_[1])
           {
                    $_[0];
           }
           else
           {
                    $_[1];
            }
}

$biggest = &maximum(37, 24); # Now $biggest is 37

-The &printfirsttwo subroutine above also returns a value, in this case 1. This is because the last thing that subroutine did was a print statement and the result of a successful print statement is always 1.

Local Variables:
============
-The @_ variable is local to the current subroutine, and so of course are $_[0], $_[1], $_[2], and so on.

-Other variables can be made local too, and this is useful if we want to start altering the input parameters.

-The following subroutine tests to see if one string is inside another, spaces not withstanding.

An example follows.

sub inside
{

local($a, $b); # Make local variables
($a, $b) = ($_[0], $_[1]); # Assign values
$a =~ s/ //g; # Strip spaces from
$b =~ s/ //g; # local variables
        $a = $b;
        $b = $a;     =  What result they will be?
       # $a = $_[1];
       # $b = $_[0];

        print "the first element is: $a\n";
        print "the second element is: $b\n";

}

&inside("lemon", "dole money");

Please click here for perl practicals - Lab1
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