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41.6. Perl Boot Camp, Part 3: Branching and Looping

To do any interesting stuff with data, Perl needs to be able to branch and loop. Perl supports the C-like if-then-else construct, as the following shows:

if ( $password eq 'secret' ) {
  print "Come on in\n";
} else {
  print "Incorrect password\n";
}

You can also invert simple tests that only have one statement in the then block.

print "Don't I know you?\n" if $user eq 'joe';

You can invert the logic of if by using unless:

print "Please supply command line arguments\n" unless @ARGV;

The print happens only if @ARGV is empty.

Sometimes you need to iterate through each element of a list. This can be done with the foreach loop:

foreach my $thing (@my_room) {
  print "dusting $thing\n";
  dust($thing);
}

A synonym for foreach is for. Bourne shell hackers (or those who don't like typing) may feel more comfortable using for rather than then foreach.

Each time through the loop, $thing is aliased to the next element in @my_room. Any change to $thing will change that element in the array, so be careful. If you don't supply a scalar variable like $thing, Perl will set $_ for you each time through the loop. The previous example could also be written:

foreach (@my_room) {
  print "dusting $_\n";
  dust($_);
}

Sometimes you need to continue looping while an event is happening, like reading input from standard input:

while ( my $line = <STDIN> ) {
  print "I got: $line";
}

Each line of input a user provides is stored in $line, including the newline at the end. When the user hits the end-of-file control key (CTRL-D), the loop exits. Like the foreach loop, you can leave off the scalar variable while reading from a filehandle,[124] and $_ will be set to the next line of input each time through the loop.

[124]STDIN is normally assumed here.

while (<>) {
  print "I got: $_";
}

Sometimes you need to interrupt the execute flow of your loop. Perl gives you three operators to do that (see Table 41-7).

Table 41-7. Loop flow-control operators

Operator

Example

Description

next

while(<>){
  next if $_ ne "continue\n";
}

Jump to the top of the loop and iterate normally

last

while(<>){
  last if $_ eq "quit\n"
}

Jump out of the loop to the next line of the program

redo

for $url (@urls){
  unless( $content = get($url) ){
    print "couldn't fetch page - retrying\n";
    redo;
  }
}

Jump to the top of the loop, but don't evaluate the loop condition

-- JJ



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