Macros

This is a discussion on Macros within the C Programming forums, part of the General Programming Boards category; I have a question about using macros in C. First, I am trying to divide and return both the answer ...

  1. #1
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    Macros

    I have a question about using macros in C.
    First, I am trying to divide and return both the answer and the remainder from a macro. Right now I have:
    Code:
    #define DIVMOD(x,y,d,r) ((y)==0 ? d=0; r=0 : (d = (x)/(y); r = (x) % (y))
    which I know is not correct. Any suggestions on how I could do this?

    Thank you.

  2. #2
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    Why would you use a macro for that? You can't return values from macros, a macro is not a function, it is a substitution. You could try to create a macro which has two "output-variables". In the "call" to the macro pass the addresses of those variables and in the macro use the usual operations you would do to assign values to such variables.

  3. #3
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    This works for me:
    Code:
    #define DIVMOD(x,y,d,r) d = ((y)==0) ? 0, r=0 : (x)/(y), r = (x) % (y)
    but I agree with Shiro that this is quite a nasty bit of code. A function would be much neater...
    Code:
    void DivMod(int x, int y, int * pd, int * pr)
    {
      if (y == 0)
        *pd = *pr = 0;
      else
      {
        *pd = x / y;
        *pr = x % y;
      }
    }
    ...and unless you are doing loads of these operations isn't going to significantly slow down your program.

    If you insist on using a macro use something like:
    Code:
    #define DIVMOD(x,y,pd,pr) *pd = ((y)==0) ? 0, *pr=0 : (x)/(y), *pr = (x) % (y)
    where pd and pr are pointers.
    DavT
    -----------------------------------------------

  4. #4
    ATH0 quzah's Avatar
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    Originally posted by DavT
    If you insist on using a macro use something like:
    Code:
    #define DIVMOD(x,y,pd,pr) *pd = ((y)==0) ? 0, *pr=0 : (x)/(y), *pr = (x) % (y)
    where pd and pr are pointers.
    What's the purpose of using pointers? It's just a macro. It's not a function call.

    Quzah.
    Hope is the first step on the road to disappointment.

  5. #5
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    It's probably just a personal hang-up, but I I really dislike macros.

    I think they lead to confusing code. The only excuse I've ever come across for using a macro where a function would do the job is in some kind of real-time signal processing situation where you can't afford the overhead of a function call and the compiler doesn't support the 'inline' (or similar) keyword.

    In the flow of the program I find it really disconcerting (or downright confusing) to find something that looks like a function call but (taking an example at random) returns values via non-pointer arguments.

    Just my personal point of view....
    DavT
    -----------------------------------------------

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