Andrew Hudson - Fedora™ Unleashed, 2008 edition

}

unlessworks just like if, only backward. unlessperforms a statement or block if a condition is false:

unless ($name eq "Rich") {

print "Go away, you're not allowed in here!\n";

}

You can restate the preceding example in more natural language like this:

print "Go away!\n" unless $name eq "Rich";

A loop is a way to repeat a program action multiple times. A very simple example is a countdown timer that performs a task (waiting for one second) 300 times before telling you that your egg is done boiling.

Looping constructs (also known as control structures ) can be used to iterate a block of code as long as certain conditions apply, or while the code steps through (evaluates) a list of values, perhaps using that list as arguments. Perl has four looping constructs: for, foreach, while, and until.

The forconstruct performs a statement (block of code) for a set of conditions defined as follows:

for ( start condition ; end condition ; increment function ) {

statement(s)

}

The start condition is set at the beginning of the loop. Each time the loop is executed, the increment function is performed until the end condition is achieved. This looks much like the traditional for/nextloop. The following code is an example of a forloop:

for ($i=1; $i<=10; $i++) {

print "$i\n"

}

The foreachconstruct performs a statement block for each element in a list or array:

@names = ("alpha","bravo","charlie");

foreach $name (@names) {

print "$name sounding off!\n";

}

The loop variable ( $namein the example) is not merely set to the value of the array elements; it is aliased to that element. That means if you modify the loop variable, you're actually modifying the array. If no loop array is specified, the Perl default variable $_may be used:

@names = ("alpha","bravo","charlie");

foreach (@names) {

print "$_ sounding off!\n";

}

This syntax can be very convenient, but it can also lead to unreadable code. Give a thought to the poor person who'll be maintaining your code. (It will probably be you.)

foreachis frequently abbreviated as for.

whileperforms a block of statements as long as a particular condition is true:

while ($x<10) {

print "$x\n";

$x++;

}

Remember that the condition can be anything that returns a true or false value. For example, it could be a function call:

while ( InvalidPassword($user, $password) ) {

print "You've entered an invalid password. Please try again.\n";

$password = GetPassword;

}

untilis the exact opposite of the whilestatement. It performs a block of statements as long as a particular condition is false — or, rather, until it becomes true:

until (ValidPassword($user, $password)) {

print "You've entered an invalid password. Please try again.\n";

$password = GetPassword;

}

You can force Perl to end a loop early by using a laststatement. last is similar to the C breakcommand—the loop is exited. If you decide you need to skip the remaining contents of a loop without ending the loop itself, you can use next, which is similar to the C continuecommand. Unfortunately, these statements don't work with do ... while.

On the other hand, you can use redoto jump to a loop (marked by a label) or inside the loop where called:

$a = 100; while (1) {

print "start\n";

TEST: {

if (($a = $a / 2) > 2) {

print "$a\n";

if (--$a < 2) {

exit;

}

redo TEST;

}

}

}

In this simple example, the variable $ais repeatedly manipulated and tested in an endless loop. The word "start" is printed only once.

The whileand untilloops evaluate the conditional first. You change the behavior by applying a doblock before the conditional. With the doblock, the condition is evaluated last, which results in the contents of the block always executing at least once (even if the condition is false). This is similar to the C language do ... while ( conditional )statement.

Perl's greatest strength is in text and file manipulation, which it accomplishes by using the regular expression ( regex ) library. Regexes, which are quite different from the wildcard handling and filename expansion capabilities of the shell, allow complicated pattern matching and replacement to be done efficiently and easily. For example, the following line of code replaces every occurrence of the string bobor the string marywith fredin a line of text:

$string =~ s/bob|mary/fred/gi;

Without going into too many of the details, Table 25.7 explains what the preceding line says.

TABLE 25.7 Explanation of $string =~ s/bob|mary/fred/gi;

If you are interested in the details, you can get more information from the regex (7) section of the manual.

Although replacing one string with another might seem a rather trivial task, the code required to do the same thing in another language (for example, C) is rather daunting unless supported by additional subroutines from external libraries.