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Is there a way to force undef to be undef, instead of 0?

zp
New User

Aug 22, 2013, 1:41 AM

Post #1 of 5 (2802 views)

Is there a way to force undef to be undef, instead of 0?	Can't Post

I know perl default an undefined variable to 0 when it mixes with numbers.

my $a = undef;

my $b = $a -1; ##the result of b is -1, not undef

Is there a way that i can get undef instead of -1 for b?

FishMonger
Veteran / Moderator

Aug 22, 2013, 7:09 AM

Post #2 of 5 (2800 views)

Re: [zp] Is there a way to force undef to be undef, instead of 0? [In reply to]	Can't Post

Code

use Data::Dumper; 
 
$foo = undef; 
$bar = ($foo -1) < 0 ? undef : $foo - 1; 
 
print Dumper $bar;

Laurent_R
Veteran / Moderator

Aug 22, 2013, 9:56 AM

Post #3 of 5 (2794 views)

Re: [zp] Is there a way to force undef to be undef, instead of 0? [In reply to]	Can't Post

In Reply To

Is there a way that i can get undef instead of -1 for b?

I don't think you can. If you declare a variable with;

Code

my $foo;

it is undefined, which you can test with the defined function:

Code

print "foo is undef" unless defined $foo;

But it you increment or decrement $foo, or add or substract a number, $foo is transformed into 0 before the arithmetic operation is made. This is plausibly an application of the followinf principle: "the Perl compiler/interpreter is trying to do what you mean."

If you need undef to be propagated to other variables, you'll have to test yourself (or possibly create a class that overloads the arithmetic operators, but that seems to be overkill).

BillKSmith
Veteran

Aug 22, 2013, 10:45 AM

Post #4 of 5 (2792 views)

Re: [zp] Is there a way to force undef to be undef, instead of 0? [In reply to]	Can't Post

Combining Fishmonger's and Laurent's reply, you could use:

Code

$b = (defined $a) ? $a - 1 : undef;

Laurent's other suggestion to define a class of numbers with the desired semantics probably is the way to go on a large project (or a group of related progects). The module that defines such a class probably should be shared in CPAN.
Good Luck,
Bill

zp
New User

Aug 22, 2013, 7:15 PM

Post #5 of 5 (2782 views)

Re: [zp] Is there a way to force undef to be undef, instead of 0? [In reply to]	Can't Post

Thank you all for the suggestions.

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