On 20 May 2008 at 15:53, Barry Brevik wrote:

>  OK, I've stumped myself. I wanted to assign 0 to several variables,
>  except for a single variable that should be set to 1.
>  
>  Before I knew what I was doing I whipped this code into my editor:
>  
>    ($frow = $ax = $bx = $cx = 0)++;
>  
>  ...and it works as I expected. That is, all of the variables are set
>  to 0 except $frow which is 1.
>  
>  Now I'm afraid that it might not always work because I don't
>  understand why it works in the first case. Anyone want to suggest if
>  this is stable code or not?
>  
>  Barry Brevik

It works (but is not a good way of doing what you want to do!) because the only way for Perl
 
to properly evaluate something like "$x = $y = $z = 0" is by making assignment a side effect
 
of the "=" operator, and making the "=" operator return the variable assigned. You read that
 
right: Assigning a value to a variable is only a side effect of the "=" operator, not it's p
rimary 
operation. It's primary operation is to return the variable that was assigned! (That is also
 why 
you can do something like "somefunction($x = 4);", which is the same as "$x = 4; 
somefunction($x);")

The code you wrote is the same as:

    ($frow = ($ax = ($bx = ($cx = 0))))++;

Breaking that down, you'll have..

After evaluating "($cx = 0)" and replacing it with "$cx"
    ($frow = ($ax = ($bx = $cx)))++;

After evaluating "($bx = $cx)" and replacing it with "$bx"
    ($frow = ($ax = $bx))++;

After evaluating "($ax = $bx)" and replacing it with "$ax"
    ($frow = $ax)++;

After evaluating "($frow = $ax)" and replacing it with "$frow"
    $frow++;

The last I think you can figure out for yourself..


_______________________________________________
ActivePerl mailing list
ActivePerl@[...].com
To unsubscribe: http://listserv.ActiveState.com/mailman/mysubs