[kw-pm] conditionals and the comma operator

[Cees Hek]

On Mon, Jan 5, 2009 at 9:39 AM, Robert P. J. Day <rpjday at crashcourse.ca> wrote:
>  ah, i remember the idiom that i was thinking of:
>
>  $var = $var || new_value;
>
> as in (from my example above):
>
>  $vm_output ||= cwd . '/vmlinux';     (the short form)
>
> isn't that the idiom?  if the variable is set, it stays set to that
> value.  if it's not set, it will be assigned the given value.  am i
> remembering that right?

Hi Robert,

That is a very common way of conditionally assigning a variable, but
just make sure that 0 and "" are not valid values.

$a = 0;
$a ||= 1;

That will set the value to 1, since 0 is a false value.

perl 5.10 gives us a new conditional that takes this into
consideration.  The defined-or operator //.

$a = 0;
$a //= 1;

It is equivalent to but much more concise than this:

$a = 1 unless defined $a;

Cheers,

Cees