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[Chapter 9] Miscellaneous Control Structures

 
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9. Miscellaneous Control Structures

Contents:
The last Statement
The next Statement
The redo Statement
Labeled Blocks
Expression Modifiers
&& and || as Control Structures
Exercises

9.1 The last Statement

In some of the previous exercises you may have thought, "If I just had a C break statement here, I'd be done." Even if you didn't think that, let me tell you about Perl's equivalent for getting out of a loop early: the last statement.

The last statement breaks out of the innermost enclosing loop block,[1] causing execution to continue with the statement immediately following the block. For example:

while (something) {
    something;
    something;
    something;
    if (somecondition) {
        somethingorother;
        somethingorother;
        last; # break out of the while loop
    }
    morethings;
    morethings;
}
# last comes here

[1] Note that the do {} while/until construct does not count as a loop for purposes of next, last, and redo.

If somecondition is true, the somethingorother's are executed, and then last forces the while loop to terminate.

The last statement counts only looping blocks, not other blocks that are needed to make up some syntactic construct. This means that the blocks for the if and else statements, as well as the ones for do {} while/until, do not count; only the blocks that make up the for, foreach, while, until, and "naked" blocks count. (A naked block is a block that is not part of a larger construct such as a loop, subroutine, or an if/then/else statement.)

Suppose we wanted to see whether a mail message that had been saved in a file was from merlyn. Such a message might look like this:

From: merlyn@stonehenge.com (Randal L. Schwartz)
To: stevet@ora.com
Date: 01-DEC-94 08:16:24 PM PDT -0700
Subject: A sample mail message

Here's the body of the mail message. And
here is some more.

We'd have to look through the message for a line that begins with From: and then notice whether the line also contains the login name, merlyn.

We could do it like this:

while (<STDIN>) { # read the input lines
    if (/^From: /) { # does it begin with From:? If yes...
        if (/merlyn/) { # it's from merlyn!
            print "Email from Randal! It's about time!\n";
        }
        last; # no need to keep looking for From:, so exit
    } # end "if from:"
    if (/^$/) { # blank line?
        last; # if so, don't check any more lines
    }
} # end while

Once the line starting with From: is found, we exit the main loop because we want to see only the first From: line. Also because a mail message header ends at the first blank line, we can exit the main loop there as well.


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