Program Control, Functions

• Where would Perl be without control statements or functions? Let's dive right in...

Program Control :: Conditionals

• if and else statements are used as conditionals as in any other language. For example:

    if ($cow < $sheep) {
      print "baaah....\n";
    } else {
      print "moo....\n";
    }
  

• unless statement is specific to Perl, in which you want to always just evaluate the false part instead of doing something when not true. For example:

    unless ($age < 21) {
      print "Why yes... you can drink bub.\n";
    }
  

  Thus we proceed into the block if the above expression is false.
• elsif is also specific to Perl, which simply combines an else and an if together in one:

    if ($a < $b) {
      $z++;
    } elsif ($b < $c) {
      $y++;
    } elsif ($c < $d) {
      $x++;
    } else {
      $yikes++;
    }
  

Program Control :: Looping

• We have our standard while loop:

    while ($food < $stomach_ache) {
      $plop++;
      $fizz--;
      $oh_what_a_relief_it_is *= 5;
    }
  

• Like the unless statement above, it is easier to say "until something is true" (Schwartz 61). Thus, Perl provides the until statement:

    until ($x > 6) {
      print "$x is the number!\n";
      $x++;
    }
  

• Like C, Perl lets you do do, while loops, but Perl also adds do, until loops:

    # do while example
    do {
      $junk++;
    } while ($junk < 39);
    # do until example
    do {
      $x++;
      $aok = @array_hooray[$x];
    } until ($aok eq 'loop_de_loop');
  

• What would a programming language be without a for statement? These loops works just like C:

    for ($i = 0; $i < 2; $i++) {
      print "$i\n";
    }
  

• Perl provides a powerful extension on the for loop, the foreach loop control statement. foreach allows you to assign an element out of a list and assigns it to a scalar.

    @array_whoop = (0, 1, 2, 3);
    foreach $x (@array_whoop) {
      $x += 2;
    }
    # @array_whoop now is (2, 3, 4, 5);
    
    foreach (@array_whoop) {
      print;
    }
  

  What would the last foreach loop do? It would print out each element in the list without even having to specify a scalar in the loop control statement. This is because Perl uses a special $_ scalar as "scratch" space. So the foreach and print is implied.
• There are other control statements available: last, next, redo. last works like a break statement in C, which allows you to break out of a loop. next is similar to the continue statement in C, but is a little different in that it allows you to skip to the end of the block but without terminating the block. redo allows you to jump to the beginning of the block without re-evaluating the control conditions of the loop.
• You can even place if to modify expressions:

    ($a = $b) if ($a < $c);
    # equivalent to:
    if ($a < $c) {
      $a = $b;
    }
  

  You can also modify expressions using while, until, unless.

Functions :: Definition and Syntax

• Functions in Perl are more commonly referred to as subroutines. These subroutines are set off by using the keyword sub.
• You can also return a value or a list of values by using the keyword return. For example:

    sub useless {
      return ($x, $y);   # $x, $y are global variables
    }
    @z = useless();      # @z contains $x, $y
  

• What about subroutine arguments? Perl uses a special array called @_ to store arguments. This array is private to the function. For example:

    sub add_two_nums {
      return $_[0] + $_[1];
    }
    $x = add_two_nums(1, 2);
  

Functions :: Scope

• If all variables accessed inside functions are global, how do you create private variables? Perl allows you to instantiate private variables by using the my operator: my ($a) used inside a subroutine would result in a temporary, local variable called $a. If there is already a global variable $a, it will be stored somewhere else, so you can name local variables the same as global ones. You can do multiple my's in one call and even initialize:

    sub something {
      my ($x, $y, $z) =  0;
    }
  

• You can even create "semiprivate" variables using the special local operator. Semiprivate variables can be used across other functions. For example:

    sub sub1 {
      local ($x) = 1;
      sub2();
    }
    
    sub sub2 {
      print "$x\n";
    }
  

  Would print 1 to the screen.
• It is wise to local the $_ variable if it is being used to guarantee that the previous value is preserved an automatically restored when the subroutine exits (Schwartz 98). So always place local $_; at the top of an subroutine that uses $_.

Perl Programming :: An Answer to -pedantic?

• In C, I've always suggested the use of -ansi -pedantic when compiling your code. There is a way in perl to force "strict" checking:

    use strict;
  

  Tells Perl to turn on the strict pragma. With this turned on, you must declare all variables before using them. Thus you must use the my operator with all variables before using them:

    my $x;
    my @a = (1, 2, 3);
  
    $z = 3;              # ERROR! z isn't declared
  

• Using strict has two advantages, your programs will run a little faster, and you can probably identify errors in your code easier (Schwartz 99).

Program Control, Functions Review

• Perl provides standard conditionals and loop controls, but Perl has many powerful extensions.
• You can create functions or subroutines in your Perl code that can return values and accept arguments.
• Use the use strict; pragma to enforce good programming practice!

© 1999-2008 Alfred Park (fred AT randu.org)