Looping
As you read in the Introduction, sometimes just making decisions and running code conditionally are not enough. Often you need to run pieces of code over and over again. The exercise presented in Listing 3.1 wasn't much fun, because you could take only one guess (well, that and because it's a pointless game). If you want to be able to take multiple guesses, you need to be able to repeat sections of code conditionally, and that's what looping is all about.
Looping with while
The simplest kind of loop is a while loop. A while loop repeats a block of code as long as an expression is true. The syntax for a while loop looks like this:
while (expression) block
When Perl encounters the while statement, it evaluates the expression. If the expression is true, the block of code is run. Then, when the end of the block is reached, the expression is re-evaluated. If it's still true, the block is repeated, as in the following snippet:
1: $counter=0;
2: while ($counter < 10 ) {
3: print "Still counting...$counter\n";
4: $counter++;
5: }
Line 1: $counter is initialized to zero.
Line 2: The expression $counter < 10 is evaluated. If it's true, the code in the block is run.
Line 4: The value of $counter is incremented by 1.
Line 5: The } marks the end of the block started on line 2 with a {. At this point, Perl returns to the top of the while loop and re-evaluates the conditional expression.
Looping with for
The for statement is the most complicated and versatile of Perl's looping constructs. The syntax looks like this:
for ( initialization; test; increment ) block
The three sections of the for statement—initialization, test, and increment—are separated by semicolons. When Perl encounters a for loop, the following sequence takes place:
-
The initialization expression is evaluated.
-
The test expression is evaluated; if it's true, the block of code is run.
-
After the block is executed, the increment is performed, and the test is evaluated again. If the test is still true, the block is run again. This process continues until the test expression is false.
The following is an example of a for loop:
for( $a=0; $a<10; $a=$a+2 ) {
print "a is now $a\n";
}
In this snippet, $a is set to 0, and the test $a<10 is performed and found to be true. The body of the loop prints a message. The increment is then run—$a=$a+2—which increases the value of $a by 2. The test is performed again, and the loop repeats. This particular loop repeats until the value of $a is 10, when the test will be false and the program will continue running after the for loop.
You don't have to use the increment in the for statement for counting; it simply iterates until the test is false. In fact, you should be aware that each of the three parts of the for statement is optional, although the two semicolons are required. The following for statement is missing some pieces but is still perfectly valid:
$i=10; # initialization
for( ; $i>-1; ) {
print "$i..";
$i--; # actually, a decrement.
}
print "Blast off!\n";
Omitting the test portion means that you have to have some other way to exit the loop, or it will loop forever.