Checking the value of an undefined value

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Here's a question..

under strict I have to compare a variable which SOMETIMES is undefined. I need to assign the number 1 to it IF it is undefined, but should it already have a value, do nothing.

use warnings;
use strict;

my $num;

if ($num < 0) { $num = 1; }
print $num;
[download]

As expected, you get the use of uninitialized warning error.

I swear once, many years ago, someone showed me something similar to $var || $var = 1; that did just what I needed. I can't seem to remember exactly what it was.

Any help would be appreciated

Comment on Checking the value of an undefined value Download Code

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Re: Checking the value of an undefined value by kyle (Abbot) on May 10, 2008 at 02:43 UTC
You're looking for defined. Examples: `defined $num or $num = 1; $num = 1 if ! defined $num;` [download] In Perl 5.10, there's a `//` operator for testing if something is defined. `$num // $num = 1; $num //= 1;` [download]	[reply] [d/l] [select]
Re: Checking the value of an undefined value by pc88mxer (Vicar) on May 10, 2008 at 05:48 UTC
I swear once, many years ago, someone showed me something similar to $var \|\| $var = 1; Probably what they showed you was: `$var \|\|= 1;` which sets `$var` to 1 unless it evaluates to true. Note this is not the same as perl 5.10's `//` operator since the assignment will take place if `$var` is 0 or the empty string in addition to `undef`.	[reply] [d/l] [select]
Re: Checking the value of an undefined value by Your Mother (Archbishop) on May 10, 2008 at 04:44 UTC
And there's also `use warnings; no warnings "uninitialized";` [download] Though not always a good idea, in cases where you know what to expect and what's going on it can be much nicer than 700 `if defined` checks in a script.	[reply] [d/l] [select]
Re: Checking the value of an undefined value by Anonymous Monk on May 10, 2008 at 14:55 UTC
just use this: my $n; .... $n = $n \|\| 1;	[reply]
Re: Checking the value of an undefined value by chrism01 (Friar) on May 12, 2008 at 05:01 UTC
$num = defined($num) ? $num : 1;	[reply]