More Control Structures

If/Then/Elsif/Else

 You can use if all by itself, in lines like

   print "You're 28!\n" if ($age == 28);
That's really shorthand for

  if ($age == 28) {
      print "You're 28!\n";
  }
which means, "if $age equals 28, then do everything inside the curly brackets." The if/then control structure can be extended with elsif and else:

  if ($age == 28) {
      print "You're 28!\n";
  } elsif ($sex eq "f") {
      print "You're a female!\n";
  } elsif ($sex eq "m") {
      print "You're a male!\n";
  } else {
      print "You're a mystery to me...";
  }
Only the first true statement will match. So if ($age == 28) and ($sex eq "f"), you'll get the output

  You're 28!
else is the default; its commands will be executed only if none of the preceding statements in the control structure were true.

For

 If you want to do something X number of times, the traditional method is with a for loop. For instance, to print the numbers 1 through 10:

  #!/usr/bin/perl

  for($x=1; $x<=10; $x++) {
      print "$x\n";
  }
The three parts inside the for() translate as: a) set the counter, $x, to equal one; b) check to see if $x is less than or equal to ten, and if it is, execute all the commands inside the curly brackets; c) once you've executed the commands inside the curly brackets, add 1 to $x ($x++ means "add one to x") and then go back to b).

There's fancier things you can do with a for loop, but we'll leave it at that for now. Suffice it to say that the three statements inside the parens take the form:

  1. Opening statement (execute this command the first time through the loop)
  2. Check statement (exit the loop when this is no longer true)
  3. Command (execute this command after each time through the loop)
As with the while control structure, the check statement has to be true the first time it is encountered, or the commands inside the curly brackets won't be executed even once.

Unless

 Unless is the opposite of if, and you can use it just like if:

  print "Dog isn't cat!\n" unless ($dog eq $cat);
or

  unless ($dog eq $cat) {
      block of commands
      ...
  }

Until

 Until is the opposite of while; the loop executes until the statement becomes true. Like:

   until($x == "15"){
       print "You have to type in 15!\n";
       $x = &getnumber;
   }

Foreach

OK, remember how arrays work? I explained earlier that they're a bunch of variables stored together, like ((@array) = (1,5,7,9)); but I didn't explain what advantage you might possibly get by storing them together. Well, here's one: foreach is a cool control structure that can access the elements of an array, one by one, in order: 
    $seven = 7;
    (@array) = (1,5,$seven,"catapult");
    foreach $element (@array) {
        print "$element\n";
    }
So, for instance, if your array "@movies" was loaded up with the names of the 6000 movies in your collection of videotapes, you could 
    foreach $title (@movies){
        print "$title\n" if ($title =~ /Tonight/);
    }
to find out how many of your movies have the word "Tonight" in the title.

Fun stuff like that can keep a Perl programmer up all night, I guarantee. Zzzzz!

Comments or suggestions? Please write scotty@bluemarble.net.