In Perl, how can I concisely check if a $variable is defined and contains a non zero length string?

In Perl, how can I concisely check if a $variable is defined and contains a non zero length string?

You often see the check for definedness so you dont have to deal with the warning for using an undef value (and in Perl 5.10 it tells you the offending variable):

 Use of uninitialized value $name in ...

So, to get around this warning, people come up with all sorts of code, and that code starts to look like an important part of the solution rather than the bubble gum and duct tape that it is. Sometimes, its better to show what you are doing by explicitly turning off the warning that you are trying to avoid:

 {
 no warnings uninitialized;

 if( length $name ) {
      ...
      }
 }

In other cases, use some sort of null value instead of the data. With Perl 5.10s defined-or operator, you can give length an explicit empty string (defined, and give back zero length) instead of the variable that will trigger the warning:

 use 5.010;

 if( length( $name //  ) ) {
      ...
      }

In Perl 5.12, its a bit easier because length on an undefined value also returns undefined. That might seem like a bit of silliness, but that pleases the mathematician I might have wanted to be. That doesnt issue a warning, which is the reason this question exists.

use 5.012;
use warnings;

my $name;

if( length $name ) { # no warning
    ...
    }

As mobrule indicates, you could use the following instead for a small savings:

if (defined $name && $name ne ) {
    # do something with $name
}

You could ditch the defined check and get something even shorter, e.g.:

if ($name ne ) {
    # do something with $name
}

But in the case where $name is not defined, although the logic flow will work just as intended, if you are using warnings (and you should be), then youll get the following admonishment:

Use of uninitialized value in string ne

So, if theres a chance that $name might not be defined, you really do need to check for definedness first and foremost in order to avoid that warning. As Sinan Ünür points out, you can use Scalar::MoreUtils to get code that does exactly that (checks for definedness, then checks for zero length) out of the box, via the empty() method:

use Scalar::MoreUtils qw(empty);
if(not empty($name)) {
    # do something with $name 
}

In Perl, how can I concisely check if a $variable is defined and contains a non zero length string?

First, since length always returns a non-negative number,

if ( length $name )

and

if ( length $name > 0 )

are equivalent.

If you are OK with replacing an undefined value with an empty string, you can use Perl 5.10s //= operator which assigns the RHS to the LHS unless the LHS is defined:

#!/usr/bin/perl

use feature qw( say );
use strict; use warnings;

my $name;

say nonempty if length($name //= );
say $name;

Note the absence of warnings about an uninitialized variable as $name is assigned the empty string if it is undefined.

However, if you do not want to depend on 5.10 being installed, use the functions provided by Scalar::MoreUtils. For example, the above can be written as:

#!/usr/bin/perl

use strict; use warnings;

use Scalar::MoreUtils qw( define );

my $name;

print nonemptyn if length($name = define $name);
print $namen;

If you dont want to clobber $name, use default.

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