CODEWITHSUNDEEP
perl
Upland
Upland

Reputation: 701

What is //= in Perl?

I found an example of some Perl code I needed, but it had something in it that I didn't recognise.

my $i //= '08';

I can't find any reference to this anywhere! It appears to be the same as:

my $i = '08';

Am I missing something?

Upvotes: 39

Views: 14671

Answers (5)

ikegami
ikegami

Reputation: 385655

Short answer: It's the same as my $i = '08';. You probably wanted $i //= '08';.

First, let's look at $i //= '08';

EXPR1 //= EXPR2;

is the same as

EXPR1 = EXPR1 // EXPR2;

except that EXPR1 is only evaluated once. It's a convenient way of writing

EXPR1 = EXPR2 if !defined(EXPR1);

See perlop for documentation on Perl operators.

Back to my $i //= '08';. That means

my $i;
$i = '08' if !defined($i);

but $i will always be undefined in that situation. It would be far better to write

my $i = '08';

But, the code was probably supposed to be

$i //= '08';   # no `my`

Upvotes: 7

user1884047
user1884047

Reputation: 203

It's almost the same as ||, except that it checks if $i is defined, not just true.

Upvotes: 4

Jonathan Leffler
Jonathan Leffler

Reputation: 753625

The //= operator is the assignment operator version of the // or 'logical defined-or' operator.

In the context of a my variable declaration, the variable is initially undefined so it is equivalent to assignment (and would be better written as my $i = '08';). In general, though, 

$i //= '08';

is a shorthand for:

$i = (defined $i) ? $i : '08';

It is documented in the Perl operators (perldoc perlop) in two places (tersely under the assignment operators section, and in full in the section on 'logical defined-or'). It was added in Perl 5.10.0.

Upvotes: 65

yasu
yasu

Reputation: 1364

It is "defined-or" operator.

$i //= '08';

is equivalent to:

$i = defined($i)? $i: '08';

It was introduced in Perl 5.10.0, and not supported by older versions.

Upvotes: 6

creaktive
creaktive

Reputation: 5210

$i //= '08' is the same as $i = defined($i) ? $i : '08'. It's almost the same as $i ||= '08', which translates to $i = $i ? $i : '08'. Now, when you declare your variable with my, it's set to undef. Thus, it will always follow the 08 branch. Also, in case you're wondering, the // operator appeared in the Perl v5.10; so it would generate a compilation error on the older Perls.

Upvotes: 5

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