8. The While Loop

The while loop enables you to repeat a sequence of statements an arbitrary number of times for as long a condition is met. The syntax is very similar to the if statement and will be introduced in the following example:

print "Please enter a number:\n";
$number=<>;
chomp($number);
$power_of_2 = 1;
while ($power_of_2 < $number)
{
    $power_of_2 *= 2;
}
print ("The first power of 2 that is " .
    "greater than this number is " , $power_of_2, "\n");

It is possible that a while loop will not be executed at all, if the condition is not met right on the start.

The following program checks if a given string is made entirely of "a" characters:

print "Please enter a string:\n";
$string=<>;
chomp($string);

# Initialize all_as to TRUE
$all_as = 1;

# The first position in the string.
$position = 0;

while ($all_as && ($position < length($string)))
{
    $char = lc(substr($string, $position, 1));

    if ($char ne "a")
    {
        $all_as = 0;
    }

    # Increment the position
    $position++;
}

if ($all_as)
{
    print "The string you entered is all A's!\n";
}
else
{
    print "At least one of the characters in the string " .
        "you entered is not \"A\".\n";
}