在 Perl 语言中，如何简明地检查 $变量是否已定义并包含非零长度的字符串？

Question

I currently use the following Perl to check if a variable is defined and contains text. I have to check defined first to avoid an 'uninitialized value' warning:

if (defined $name && length $name > 0) {
    # do something with $name
}

Is there a better (presumably more concise) way to write this?

转载于:https://stackoverflow.com/questions/1480066/in-perl-how-can-i-concisely-check-if-a-variable-is-defined-and-contains-a-non

Answer 1

You often see the check for definedness so you don't 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, it's 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 5.10's 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, it's 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 doesn't issue a warning, which is the reason this question exists.

use 5.012;
use warnings;

my $name;

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