Learning Perl on Win32 Systems

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9. Miscellaneous Control Structures

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

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:

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

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

If somecondition is true, the somethingorothers are executed, and then the 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. As a result, the blocks for the if and else statement, as well as the one for a 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 otherwise part of a larger construct, such as a loop, subroutine, or if/then/else statement.)

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

From: eriko@axtech.com (Erik Olson)
To: rdenn@ora.com
Date: 01-MAY-97 08:16:24 PM MDT -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, eriko.

We could do it this way:

while (<STDIN>) {           # read the input lines
         if (/^From: /) {   # does it begin with From:? If yes...
             if (/eriko/) { # it's from Erik!
                 print "Email from Erik! 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

After 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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8.8 ExercisesBook Index9.2 The next Statement