Perl 101 (Part 3) - Looping The Loop - For Pete's Sake!
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Both "while" and "until" are typically used when you don't know for certain how many times the program should loop - in the examples above, for example, the program continues to loop until the user enters the right answer. But Perl also comes with a mechanism for executing a set of statements a specific number of times - and it's called the "for" loop:

for (initial value of counter; condition; update counter)
	{
 	do this!
	}

Doesn't make any sense? Well, the "counter" here refers to a scalar variable that is initialized to a specific numeric value [usually 0 or 1]; this counter is used to keep track of the number of times the loop has been executed.

Each time the loop is executed, the "condition" is tested for validity. If it's found to be valid, the loop continues to execute and the value of the counter is updated appropriately; if not, the loop is terminated and the statements following it are executed.

Take a look at this simple example of how the "for" loop can be used:

#!/usr/bin/perl

for ($a=5;$a<12;$a++)
	{
	print("It's now $a PM. Too early!\n");
	}

print("Let's party!\n");

Here's what the output looks like:

It's now 5 PM. Too early!
It's now 6 PM. Too early!
It's now 7 PM. Too early!
It's now 8 PM. Too early!
It's now 9 PM. Too early!
It's now 10 PM. Too early!
It's now 11 PM. Too early!
Let's party!

How does this work? We've begun by initializing the variable $a to 5. Each time the loop is executed, it checks whether or not $a is less than 12; if it is, a line of output is printed and the value of $a is increased by 1 - that's where the $a++ comes in. Once the value of $a reaches 12, the loop is terminated and the line following it is executed.

And, for something slightly more complex, take a look at our re-write of the factorial calculator above:

#!/usr/bin/perl

# factorials version 2.0

# ask for a number.
print ("Gimme a number!\n");

# process it
$number = <STDIN>;
chomp($number);

# use the FOR loop to calculate the factorial
# note how we've initialized variables within the 
# loop itself - you can do this too!

for ($factorial=1,$counter = $number; $counter > 1; $counter--)
	{
	$factorial = $factorial * $counter;
	}

print ("The factorial of $number is $factorial.\n");

This article copyright Melonfire 2000. All rights reserved.

>>> More Perl Articles          >>> More By Vikram Vaswani and Harish Kamath, (c) Melonfire