Re^3: Tidy up conditions

[SuicideJunkie]

my %hash = (0=>'that was false', ''=>'oops false too', 42=>'So very true');

The first two keys there evaluate to false, and would cause a problem with the code.

[tel2]

OK - thanks SJ,

I had thought that when Martin said "boolean false values", he literally meant hash values, as opposed to hash keys, which is why I questioned it, but I guess he meant values in the more general sense (i.e. keys in this case).

[martin]

Actually, I did mean values and not hash keys. Here is an example of how boolean false values would defeat our intent to short-cut at the first match:

my %access = ('foo' => 0, 'bar' => 123);
print $access{'foo'} || $access{'bar'}; # prints 123

The key 'foo' exists but the zero score makes perl evaluate the second operand, too. With perl version 5.10.0 and up, the defined-or operator can help, as it short-cuts only on undef:

my %access = ('foo' => 0, 'bar' => 123);
print $access{'foo'} // $access{'bar'}; # prints 0

Older perls don't have this operator, however.

[tel2]

Hi again Martin,

I've incorporated the // in my code like this:

$item = 'Test';

$level =
        $access{$item} //
        ($item =~ /^([^:]+):/ && $access{"$1:*"}) //
        $access{'*'} //
        999;

print "level='$level'\n";

Unfortunately, when the regex doesn't match, it seems to short-circuit, and this is printed:
level=''

Any ideas why? I assume it's something to do with the line which contains the regex not being 'defined', but I'm not sure.

Meanwhile, I've worked around it by putting the regex first, like this:

$access{'*'} = 30;
$item = 'Test';

$item =~ /^([^:]+):/;
$level =
        $access{$item} //
        $access{"$1:*"}) //
        $access{'*'} //
        999;

print "level='$level'\n";

And I think that's working (i.e. it prints "level='30'" in the above case).
If you've got a better adjustment to this code, which doesn't involve using that regex unless needed, I'd love to see it.  Otherwise, no worries - I'm pretty happy.

Thanks again.

[tel2]

Well said, Martin!

Good thing I'm using v5.10.1.

I'd never seen the // operator.
I'm so glad I asked...and you answered.

Tel2
(Another satisified PerlMonks shopper)
PS: Give yourself a pay rise, could ya?