Author: Sune Fischer
Date: 15:37:20 05/29/04
Go up one level in this thread
On May 29, 2004 at 18:27:53, James Swafford wrote:
>On May 29, 2004 at 17:47:02, Sune Fischer wrote:
>
>>On May 29, 2004 at 11:33:01, James Swafford wrote:
>>>>
>>>>How would you pass arguments to a function in an unevaluated state? Can you draw
>>>>a stack frame showing exactly what would be done?
>>>
>>>The problem (well, Uri's problem) is the order in which the compiler
>>>evaluates the expression tree. Think of 'printf' as a node in a tree
>>>with the parameters as children.
>>>
>>>Apparently the children are evaluated first (bottom up), and since
>>>they share memory, voila...
>>>
>>>I think printf could do something like:
>>>
>>>While (unevaluated parameter) {
>>> evaluate parameter
>>> copy results to buffer
>>>}
>>>print buffer
>>>
>>>Not as efficient, but it would work.
>>>
>>>Note I am _not_ saying it _should_ work this way!
>>
>>From what I understand the problem is not with printf, but in the way parameters
>>are passed to functions in general.
>>
>>Eg. if I write F(3*x+4,0.6*y-2), then the program will first evaluate the
>>argumens and _then_ pass them to the function, F.
>>
>>It is not possible for the compiler to replace that with two calls like F(3*x+4)
>>and F(0.6*y-2), because the compiler can't possibly know if F can handle that.
>>
>>Printf is no exception I think, it gets treated like any other function in this
>>respect.
>
>No... perhaps C can't do this because of the way the language is
>defined, but you _can_ change the order in which expressions are
>evaluated. Functional languages are good examples of how a
>parameter might not get evaluated at all.
I can't say I understand that.
Look at it from the compilers point of view, F is a function with the prototype
void F(float a, float b);
The compiler must see to that the expressions get evaluated and stored in "a"
and "b" so they are ready for internal use in F.
In Uri's function it just turns out that "a" and "b" are actually two pointers
which point to the same place and therefore the same result is passed twice.
That's the danger of pointers and global memory (static behaves like global in
this case). :)
>Languages that do this use what's called 'lazy evaluation', or
>'outside-in evaluation'.
Never heard of it :)
How would a function like F work on arbitary equations passed to it?
-S.
>--
>James
>
>
>
>>
>>-S.
>>>--
>>>James
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