6. The For Loop

The for loop enables us to iterate over a sequence of numbers and repeat the same set of operations for each number.

For example, the following program prints all the numbers from 1 to 100:

for $i (1..100)
{
    print $i, "\n";
}

Some explanations about the syntax:

1. $i is the iteration variable. It receives the value 1, then the value 2, then 3 and so forth until it is equal to 100, afterwards the loop terminates.
2. The curly brackets ({ ... }) encapsulate the loop block. The loop block is executed once for each value $i accepts. Within that block, called the loop body, you can use $i and you'll get its current value.

We can nest loops, so for example the following program prints the multiplication board:

for $y (1 .. 10)
{
    for $x (1 .. 10)
    {
        $product = $y*$x;
        # Add as much whitespace as needed so the number will occupy
        # exactly 4 characters.
        for $whitespace (1 .. (4-length($product)))
        {
            print " ";
        }
        print $product;
    }
    # Move to the next line
    print "\n";
}

You may have noticed the program's comments. In perl comments start with the sharp sign (#) and extend to the end of the line. Writing the multiplication boards with the labels that indicate which numbers are being multiplied is left as an exercise to the reader.