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Learning Perl on Win32 Systems

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Miscellaneous Control Structures
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9.4 Labeled Blocks

What if you want to jump out of the block that contains the innermost block - to exit from two nested blocks at once? In C, you'd resort to that much maligned goto to get you out. No such kludge is required in Perl. You can use last, next, and redo on any enclosing block by giving the block a name with a label.

A label is yet another type of name from yet another namespace following the same rules as scalars, arrays, hashes, and subroutines. As we'll see, however, a label doesn't have a special prefix punctuation character (like $ for scalars, & for subroutines, and so on), so a label named print conflicts with the reserved word print, and would not be allowed. For this reason, you should choose labels that consist entirely of uppercase letters and digits, which will never be chosen for a reserved word in the future. Besides, using all uppercase makes an item stand out better within the text of a mostly lowercase program.

After you've chosen your label, place it immediately in front of the statement containing the block, and follow it with a colon, like this:

SOMELABEL: while (condition) {
        statement;
        statement;
        statement;
        if (nuthercondition) {
            last SOMELABEL;
        }
}

We added SOMELABEL as a parameter to last. This parameter tells Perl to exit the block named SOMELABEL, rather than exiting just the innermost block. In this case, we don't have anything but the innermost block. But suppose we had nested loops:

OUTER: for ($i = 1; $i <= 10; $i++) {
        INNER: for ($j = 1; $j <= 10; $j++) {
            if ($i * $j == 63) {
                print "$i times $j is 63!\n";
                last OUTER;
            }
            if ($j >= $i) {
                next OUTER;
            }
        }
}

This set of statements tries all successive values of two small numbers multiplied together until it finds a pair whose product is 63 (7 and 9). After the pair is found, there's no point in testing other numbers, so the first if statement exits both for loops using last with a label. The second if ensures that the bigger of the two numbers will always be the first number by skipping to the next iteration of the outer loop as soon as the condition would no longer hold. This means that the numbers will be tested with ($i, $j) being (1,1), (2,1), (2,2), (3,1), (3,2), (3,3), (4,1), and so on.

Even if the innermost block is labeled, the last, next, and redo statements without the optional parameter (the label) still operate with respect to that innermost block. Also, you can't use labels to jump into a block - just out of a block. The last, next, or redo has to be within the block.


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9.3 The redo StatementBook Index9.5 Expression Modifiers