Subroutines

• Just like C, PERL offers the ability to use subroutines for all the same reasons– Code that you will use over and over again– Breaking large programs into a series of smaller,

more manageable pieces– Making programs more readable– Making programs easier to test– Allowing for multiple programmers to work on a

single program

Subroutines

• Recall from C – Subroutines have a name– Subroutines have an argument/parameter list for

input values– Subroutines have a return type for output value(s)– Subroutine variables have local (to the subroutine)

scope

• PERL is no different in these respects except for the last item – scope

Scope in PERL

• When a variable is used without declaring it, by definition it has global scope– That is, it can be used anywhere within the program

$str = ‘abcde’;$regexp = ‘^ab*a$’;

– Both $str and $regexp are globally scoped variables– If their contents is changed from within a subroutine, it

will also change outside of the subroutine

• This is not how C works

Scope in PERL

• But, this “feature” of PERL can be turned off though the use of a special keyword/construct

$str = ‘abc’;print $str, “\n”;

can be changed tomy($str) = ‘abc’;print $str, “\n”;

to make the variable $str locally scoped to the block in which it is being used

Scope in PERL

• Enforcing scope and variable declaration– Also recall that in C we absolutely must declare all

variables prior to usage– PERL has no such restriction

• Causes problems if you misspell variable names• Causes problems if you use the same variable name more than

once

– We can make PERL act like C by including the following directive (it’s not a PERL code statement) at the top of our code file

use strict;

Scope in PERL

• This program will not compile:use strict;$str = ‘abc’;print $str, “\n”;

• But this one willuse strict;my($str) = ‘abc’;print $str, “\n”;

• Note that there is still no data typing!• We’re just declaring the usage of a variable name

Back to subroutines

• Subroutine formatsub mysubroutine {

my($arg) = @_;my($localvar);# do something herereturn $localvar;

}

Argument passing

• All of the previous should be fairly clear except for the line

my($arg) = @_;

• This is how arguments are passed in– Note that there is no argument list in the subroutine

definition– Arguments get lumped together as a special array called _

(underscore)– The line above pulls the arguments out of the array and

places them into locally scoped scalar variables (just one in this example)

Argument passing

• More than one argument

my($arg1, $arg2) = @_;

• Arguments are passed by value– Even if you alter their value within the subroutine, their

value on the outside remains unchanged

• Passing arrays gets complicated because they all get lumped into a single array– This can be avoided by passing by reference but…– The syntax is horrible and I’m not going to burden you

with it during the last week of class

Calling a subroutine

• Just do the obvious…$arg = ‘abc’;$retvar = mysubroutine($arg);print $retvar, “\n”;

• Or, better yetmy($arg) = ‘abc’;my($retvar) = mysubroutine($arg);print $retvar, “\n”;

Practice

• Write a subroutine that takes one argument and returns it with the string “ modified by subroutine” appended– Recall the append is done with the . operator

$var = $var.‘append me’;

• Write a subroutine that takes two arguments, one string and one regular expression, and returns 1 if they match, 0 if the do not– Read the string and the regular expression from the

keyboard (don’t forget to chomp them before sending to the subroutine)

And more practice

• Convert your code to print out a file 30 lines at a time to use a subroutine– That is, make a subroutine that is called like this:

printfile($filename);– In your main program, use a while loop to read a

filename from the keyboard, then print the file– Halt the main program (while loop) when the user

types a carriage-return with no filename