Macro to Replace For Loop

Question

Consider the macro below. How can I unroll the for loops using macros and export lots of macros cleanly into a readable header?

macro.h

#define ARR(n) \
typedef int arr##n[n]; \
void arr##n##_add(arr##n res, arr##n x, arr##n y){ \
    int i = 0; \
    for (; i < n; i++) { \
        res[i] = x[i] + y[i]; \
    } \
}

ARR(3)
ARR(4)

Running this through the pre-processor with gcc -E -P macro.h > out.h we get:

out.h

typedef int arr3[3]; 

void arr3_add(arr3 res, arr3 x, arr3 y){ 
    int i = 0; 
    for (; i < 3; i++) { 
        res[i] = x[i] + y[i]; 
    } 
}

typedef int arr4[4]; 

void arr4_add(arr4 res, arr4 x, arr4 y){ 
    int i = 0; 
    for (; i < 4; i++) { 
        res[i] = x[i] + y[i]; 
    } 
}

Using token pasting like above we avoid having to repeat definitions. Now there are two things I would like to know:

How can each for(;i; i < n) be replaced (unrolled) with e.g:

res[0] = x[0] + y[0];
res[1] = x[1] + y[1];
...
res[n] = x[n] + y[n];

What is a clean way of exporting lots of macro hackery into a readable header? Should I create a shell function inside my make file exports the final header to an include directory?

Maybe there are better ways of doing this. Any alternatives welcome.

'Clean way' and 'macro hackery' are pretty much mutually exclusive terms. — Jonathan Leffler, Dec 13 '15 at 04:14
Are you sure this is at all beneficial? Why would you want such specialized functions and types, rather than simply taking the dimension as an argument to the functions? — Jonathan Leffler, Dec 13 '15 at 04:18
Even if this not the best way as part of my C education knowing this would be helpful. So granted that this may not be "beneficial" or "clean" how do we do it? — GoofyBall, Dec 13 '15 at 04:21
You should probably look at [Boost::Preprocessor](http://www.boost.org/doc/libs/release/libs/preprocessor/), which can be used with C as well as C++. Not that it is going to make the code readable, but (I believe) it will allow you to unwrap your loops. — Jonathan Leffler, Dec 13 '15 at 04:25
@JonathanLeffler It would be better to not introduce dependencies unnecessarily unless it would be too much work to do ourselves. — GoofyBall, Dec 13 '15 at 04:29

score 1 · Answer 1 · edited May 23 '17 at 11:44

Here is one possible solution that is referred to by this answer using these macro functions:

#define ARR(n, m) \
static inline void vec##n##_add(vec##n res, \
        vec##n const u, vec##n const v) { \
        m }

#define ADD(i, n) res[i] = u[i] + v[i]; 
ARR(3, EVAL(REPEAT(3, ADD, ~)))
ARR(4, EVAL(REPEAT(4, ADD, ~)))

Which does as asked but in a convoluted, hacky, ugly way... In particular, the use of recursive macros for what should be an iterative operation.

Macro to Replace For Loop

1 Answers1