Learning Perl, 5th Edition

The for Control Structure

Perl’s for control structure is like the common for control structure you may have seen in other languages, such as C. It looks like this:

for (initialization; test; increment) {
  body;
  body;
}

To Perl, though, this kind of loop is really a while loop in disguise, something like this:^[*]

initialization;
while (test) {
  body;
  body;
  increment;
}

The most common use of the for loop, by far, is for making computed iterations:

for ($i = 1; $i <= 10; $i++) {  # count from 1 to 10
  print "I can count to $i!\n";
}

When you’ve seen these before, you’ll know what the first line is saying even before you read the comment. Before the loop starts, the control variable, $i, is set to 1. Then, the loop is really a while loop in disguise, looping while $i is less than or equal to 10. Between each iteration and the next is the increment, which here is a literal increment, adding one to the control variable, which is $i.

So, the first time through this loop, $i is 1. Because that’s less than or equal to 10, we see the message. Although the increment is written at the top of the loop, it logically happens at the bottom of the loop, after printing the message. So, $i becomes 2, which is less than or equal to 10, so we print the message again, and $i is incremented to 3, which is less than or equal to 10, and so on.

Eventually, we print the message that our program can count to 9. Then $i is incremented to 10, which is less than or equal to 10, so we run the loop one last time and print ...