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Groups > comp.lang.c > #11335

Re: behavior of || operator

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On 10/07/2011 01:13 PM, Mark wrote:
> Hello,
> 
> #define ATTACH           1
> #define DETACH           2
> 
> #define DIRECTION_INGRESS       1   /* Incoming */
> #define DIRECTION_EGRESS        2   /* Outgoing */
> 
> 
> 
> if ((action != ATTACH) || (action != DETACH))
> 
>     puts("invalid action");

This code is equivalent to:

if(action != ATTACH)
    puts("invalid action");
else if(action != DETACH)
    puts("invalid action");

Since ATTACH != DETACH, this is, in turn equivalent to:
    puts("invalid action");

I don't think that's what you want to do.

> if ((dir != DIRECTION_INGRESS) || (dir != DIRECTION_EGRESS))
>     puts("invalid direction");
> 
> 
> 
> I appears that condition within if() is verified for being TRUE or FALSE, 
> and it is sufficient to have just one operand of || operator to return TRUE 
> in order for the whole expression yield TRUE result.

Yes, that's what || means: it either operand is true, the whole
expression is true.

> For example, if action = 1, then (action != ATTACH) becomes FALSE, then 
> (action != DETACH) is evaluated and it will return TRUE, thus making the 
> statement within if() TRUE. And this is wrong behavior, given that I need to 
> have actoni be either 1 or 2.
> 
> 
> 
> To make it right, || must be replaced with &&.

Yes:
if ((action != ATTACH) && (action != DETACH))
    puts("invalid action");

is equivalent to:

if(action != ATTACH)
{
   if(action != DETACH)
      puts("invalid action");
}

Which seems a better fit to what I think you want.

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Thread

behavior of || operator "Mark" <mark_cruzNOTFORSPAM@hotmail.com> - 2011-10-07 13:13 -0400
  Re: behavior of || operator James Kuyper <jameskuyper@verizon.net> - 2011-10-07 14:11 -0400
  Re: behavior of || operator John Gordon <gordon@panix.com> - 2011-10-07 18:47 +0000
  Re: behavior of || operator Willem <willem@toad.stack.nl> - 2011-10-07 19:50 +0000
  Re: behavior of || operator Gene <gene.ressler@gmail.com> - 2011-10-08 13:58 -0700

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