Subroutines

1. 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

2. 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

3. Scope in PERL • When a variable is used without declaring it, by definition it has globalscope • 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

4. Scope in PERL • But, this “feature” of PERL can be turned off though the use of a special keyword/construct $str = ‘abc’; print $str, “\n”; can be changed to my($str) = ‘abc’; print $str, “\n”; to make the variable $str locally scoped to the block in which it is being used

5. 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;

6. Scope in PERL • This program will not compile: use strict; $str = ‘abc’; print $str, “\n”; • But this one will use strict; my($str) = ‘abc’; print $str, “\n”; • Note that there is still no data typing! • We’re just declaring the usage of a variable name

7. Back to subroutines • Subroutine format sub mysubroutine { my($arg) = @_; my($localvar); # do something here return $localvar; }

8. 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)

9. 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

10. Calling a subroutine • Just do the obvious… $arg = ‘abc’; $retvar = mysubroutine($arg); print $retvar, “\n”; • Or, better yet my($arg) = ‘abc’; my($retvar) = mysubroutine($arg); print $retvar, “\n”;

11. 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)

12. 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