http://www.perlmonks.org?node_id=653404

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

A coworker passed on the following bit of code, which, as far as I can tell, does nothing: 
$x ||= 0;

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$x = -1; $x ||= 0; print $x;
-1
$x = 0; $x ||= 0; print $x;
0
$x = 2; $x ||= 0; print $x;
2
$x = 3; $x ||= 0; print $x;
3
$x = 123132; $x ||= 0; print $x;
123132

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Any ideas on how or why something like Or, Or, Equals Zero would be used?

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Re: Or, Or, Equals Zero, $x ||= 0
by eric256 (Parson) on Nov 27, 2007 at 23:08 UTC

    $x = undef; $x ||= 0; print $x;

    That is the same as $x = $x || 0 (which is read $x or 0) and is used for defaulting values.


    ___________
    Eric Hodges
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Re: Or, Or, Equals Zero, $x ||= 0
by graff (Chancellor) on Nov 28, 2007 at 02:09 UTC
    The reason for using that expression is probably that the original script author was (wisely) including a "-w" flag on the shebang line, or put "use warnings;" near the top of the script. In those cases, the  $x ||= 0; will prevent perl from issuing a warning about "use of undefined value in numeric comparison" or "use of undefined value in print".

    Works for strings too:  $x ||= '': will avoid warnings about using an undefined value in a print, string concatenation, string comparison, regular expression, and so on.

    It's not that undefined values are inherently bad -- sometimes you want a variable to be undef, and if you really want to use undef values in a print or a comparison to some actual value, you can modify the "use warnings" pragma to say which warnings it should ignore. But such "special needs" cases are pretty rare (and probably avoidable).

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      Works for strings too: $x ||= '': will avoid warnings about using an undefined value in a print, string concatenation, string comparison, regular expression, and so on.

      That also turns '0' into '' and so would probably be better written as: 

      $x= ''  if ! defined $x;

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      Or you can use //= if you are on the bleading edge and eschew backward compatability. Note that ||= is probably the best choice for the numeric case as treating '' as a number can also cause a warning.

      - tye        

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        If it's on a new code, and no animals are harmed, I don't see a problem with using 5.10 syntax. It's a question of what the people around you like to read. For instance, I'd write it
        $x= ''  unless defined $x;

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        But its a question of whatever suits.

        Software speaks in tongues of man; I debug, therefore I code.

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Re: Or, Or, Equals Zero, $x ||= 0
by FunkyMonk (Chancellor) on Nov 27, 2007 at 23:24 UTC
    $x ||= 42 is read as "if $x is false (empty string, 0 or undef) store 42 into $x". This construct has always had problems when used to assign a default value (what if you want $x to be zero) that will soon be solved by the "defined-or" operator, //, arriving soon in perl 5.10.0.

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Re: Or, Or, Equals Zero, $x ||= 0
by markkawika (Monk) on Nov 28, 2007 at 00:33 UTC
    $x ||= 0; translates to $x = $x || 0;. You might use this to assign a default value to a variable when you didn't want to test whether or not the variable is defined. $x stays the same if $x is defined and is not equal to zero. If it is equal to zero, it falls through the "or" (||) and sets it to zero.

    So the only time this changes the value of $x is when $x is undefined. That's a short-hand way of writing:

    $x = 0 if (!defined $x);

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