Sort 3 numbers using macros [C]

Question

Lets say we have defined macro SWAP:

#define SWAP(a,b) {\
int tmp = a; \
   a = b; \
   b = tmp;}\

and using SWAP we need to sort 3 numbers (just write another macro which uses macro called SWAP):

#define SORT(a,b,c) \
    (a > b) ? SWAP(a,b) : ((a > c) ? SWAP(a,c) : ((b>c) : SWAP(b,c)))

I wrote it like this but my code shows only some errors:

#include <stdio.h>

#define SWAP(a,b) {\
int tmp = a; \
   a = b; \
   b = tmp;}\

#define SORT(a,b,c) \
    (a > b) ? SWAP(a,b) : ((a > c) ? SWAP(a,c) : ((b>c) : SWAP(b,c)))

int main()
{
    int a = 1024, b =  7, c = 11;

    printf("a = %d b = %d\n", a, b);
    SWAP(a,b)
    printf("a = %d b = %d\n", a, b);

    printf("a = %d b = %d c = %d\n", a, b);
    SORT(a,b,c)
    printf("a = %d b = %d c = %d\n", a, b);

    return 0;
}

errors I get:

error: expected expression before ‘{’ token|

compile with -E option which writes source with pre-processor output — suspectus, Jan 22 '14 at 16:18
You can't have *blocks* (i.e. brace-enclosed statements like `{ int tmp = a; a = b; b = tmp; }`) inside of *expressions* (groups of operators and other expressions which get evaluated into a value, like `x ? y : z`. It's just not allowed by the C language. If you convert your use of the ternary operator into a `if/else` statement, it should work fine, although I'd advise you strongly not to ever write a sorting function with macros like that. — Adam Rosenfield, Jan 22 '14 at 16:20

score 3 · Answer 1 · edited May 23 '17 at 10:31

3

Let's expand your SORT(a,b,c) macro:

(a > b) ? SWAP(a,b) : ((a > c) ? SWAP(a,c) : ((b>c) : SWAP(b,c)))

Expanding each SWAP(a,b):

(a > b) ? {
int tmp = a;
a = b;
b = tmp;
} : ((a > c) ? {
int tmp = a;
a = c;
c = tmp;
} : ((b>c) : {
int tmp = b;
b = c;
c = tmp;
}))

This is not valid C code. That's why it fails to compile.

Define it like this:

#define SORT(a,b,c) \
    if(a > b) { SWAP(a,b) } else if(a > c) { SWAP(a,c) } else if (b>c) { SWAP(b,c) }

BTW, it's a good practice, to define multiline macros using dummy do-while loops (see: C multi-line macro: do/while(0) vs scope block ):

#define SWAP(a,b) do {\
int tmp = a; \
   a = b; \
   b = tmp;} while(0)\

This way, you can write it like a regular function call:

SWAP(a,b);

edited May 23 '17 at 10:31

Community

1
1

answered Jan 22 '14 at 16:17

Tamás Zahola

9,271
4
34
46

Ok, thanks! But can I do it without else if but only by using the `?` operator? – yak Jan 22 '14 at 16:21
1

No, because the ?: operator expects rvalue's. A block {...} is not an rvalue. – Tamás Zahola Jan 22 '14 at 16:23

score 1 · Answer 2 · answered Jan 22 '14 at 16:18

On compiling with gcc -E the SORT macro was expanded like this

 (a > b) ? {int tmp = a; a = b; b = tmp;} : ((a > c) ? {int tmp = a; a = c; c = tmp;} : ((b>c) : {int tmp = b; b = c; c = tmp;}))

The problem here are the braces and the semi colon in the SWAP function.

score 0 · Answer 3 · answered Jan 23 '14 at 14:19

0

@Tamas Zahola you need to modify

#define SORT(a,b,c) \
 if(a > b) { SWAP(a,b) } else if(a > c) { SWAP(a,c) } else if (b>c) { SWAP(b,c) }

this will not work if value of b will be greater then c in whole program.

answered Jan 23 '14 at 14:19

sumit kumar

602
3
11
26

Sort 3 numbers using macros [C]

3 Answers3