FAQ 7.23 How do I create a switch or case statement?


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This is an excerpt from the latest version perlfaq7.pod, which
comes with the standard Perl distribution. These postings aim to
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7.23: How do I create a switch or case statement?

In Perl 5.10, use the "given-when" construct described in perlsyn:

use 5.010;

given ( $string ) {
when( 'Fred' ) { say "I found Fred!" }
when( 'Barney' ) { say "I found Barney!" }
when( /Bamm-?Bamm/ ) { say "I found Bamm-Bamm!" }
default { say "I don't recognize the name!" }
};

If one wants to use pure Perl and to be compatible with Perl versions
prior to 5.10, the general answer is to use "if-elsif-else":

for ($variable_to_test) {
if (/pat1/) { } # do something
elsif (/pat2/) { } # do something else
elsif (/pat3/) { } # do something else
else { } # default
}

Here's a simple example of a switch based on pattern matching, lined up
in a way to make it look more like a switch statement. We'll do a
multiway conditional based on the type of reference stored in
$whatchamacallit:

SWITCH: for (ref $whatchamacallit) {

/^$/ && die "not a reference";

/SCALAR/ && do {
print_scalar($$ref);
last SWITCH;
};

/ARRAY/ && do {
print_array(@$ref);
last SWITCH;
};

/HASH/ && do {
print_hash(%$ref);
last SWITCH;
};

/CODE/ && do {
warn "can't print function ref";
last SWITCH;
};

# DEFAULT

warn "User defined type skipped";

}

See perlsyn for other examples in this style.

Sometimes you should change the positions of the constant and the
variable. For example, let's say you wanted to test which of many
answers you were given, but in a case-insensitive way that also allows
abbreviations. You can use the following technique if the strings all
start with different characters or if you want to arrange the matches so
that one takes precedence over another, as "SEND" has precedence over
"STOP" here:

chomp($answer = <>);
if ("SEND" =~ /^\Q$answer/i) { print "Action is send\n" }
elsif ("STOP" =~ /^\Q$answer/i) { print "Action is stop\n" }
elsif ("ABORT" =~ /^\Q$answer/i) { print "Action is abort\n" }
elsif ("LIST" =~ /^\Q$answer/i) { print "Action is list\n" }
elsif ("EDIT" =~ /^\Q$answer/i) { print "Action is edit\n" }

A totally different approach is to create a hash of function references.

my %commands = (
"happy" => \&joy,
"sad", => \&sullen,
"done" => sub { die "See ya!" },
"mad" => \&angry,
);

print "How are you? ";
chomp($string = <STDIN>);
if ($commands{$string}) {
$commands{$string}->();
} else {
print "No such command: $string\n";
}

Starting from Perl 5.8, a source filter module, "Switch", can also be
used to get switch and case. Its use is now discouraged, because it's
not fully compatible with the native switch of Perl 5.10, and because,
as it's implemented as a source filter, it doesn't always work as
intended when complex syntax is involved.



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so please include as much information as possible and relevant in any
corrections. The perlfaq-workers also don't have access to every
operating system or platform, so please include relevant details for
corrections to examples that do not work on particular platforms.
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