On Fri, 8 Jun 2001, Mike A. Harris wrote:
> On Fri, 8 Jun 2001, Andrew James Richardson wrote:
>
> >> > < if ( !image )
> >> > ---
> >> > > if ( !image || !image->Data)
> >> >
> >
> >Brian,
> >
> > I thought that in the C specs you can't be guarenteed the order of
> >evaluation in an if brace. Surely a safer bet is
> >
> > if(!image){
> > if(!image->Data){
> > ...
> > }
> > }
> >
> > If you know better I stand corrected, I'm just thinking that not all
> >target platforms will use gcc (which I presume is the one that is used in
> >this case).
>
> The associativity is left to right. It is guaranteed standard C.
>
> if(a && b && c && d)
>
> Where a is evaluated first always, then b, but only if a is true.
> The first left to right statement that is false stops further
> conditions from being checked.
>
> This allows for example - optimization, by putting the
> conditionals in order of most likely to be false to least likely,
> you optimize for the most common case in the test.
>
> When using ||, the associativity is the same, but then in the
> case:
>
> if(a || b || c || d)
>
> b is only evaluated if a is false. The first one evaluating
> "true" from left to right makes the whole statement true, and
> thus no further condition testing need be made.
>
> Also, your above example does not do the same as the original.
> The original says "if a and b then c", while yours says:
>
> "if a and b then c"
>
> Your multiple if block is syntactically equivalent to
>
> if ( !image && !image->Data)
>
> Notice the flipping of || to && from the original example.
Your right :)
Ok we can all re-write it to work proper, but as one is assured that
associativity is strict left2right then were all OK, maybe Im thinking
if the , operator (but youll problably say that that is strict left2right
too!!).
Cheers
Andy
>
>
>
> ----------------------------------------------------------------------
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>
>
>
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