C prog error: expected expression before int

Question

This is the program:

#include <stdio.h>
#define round(a) ((a-0.5)<int(a))?int(a):int(a+1)
int main() {
double a = 5.2;
int m = round(a);
printf("%d", m); }

and it shows the error: expected expression before 'int'

@TheDubleM: Yes. Even nicer is to `#include ` and use `return EXIT_SUCCESS;`, which is clearer than "`0`". — unwind, Jun 29 '14 at 10:00

score 2 · Answer 1 · answered Jun 29 '14 at 08:02

round is a name reserved by the standard C library so it is undefined behaviour to call your macro that name (even if you don't include math.h).

Your algorithm could be better expressed like this:

#define my_round(a)  ( (int)((a) + 0.5) )

which also has the benefit of only evaluating its argument once.

It would be preferable to use an inline function:

inline int my_round(double d)
{
    return d + 0.5;    
}

Note that both options cause undefined behaviour if a is outside the bounds of INT_MIN, INT_MAX roughly . If it's in a critical environment you should make your inline function check the bounds of d before doing the conversion to int.

The original macro in the question produces the wrong result (for the specification implicit in the name “round”) for even floating-point numbers between 2^52 and 2^53. Your version produces the wrong result for odd floating-point numbers between 2^52 and 2^53, and also for the predecessor of 0.5. — Pascal Cuoq, Jun 29 '14 at 08:07

score 0 · Answer 2 · edited May 23 '17 at 12:11

0

The error is because of int(a). Syntactically it is wrong. It should be (int)(a).

edited May 23 '17 at 12:11

Community

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answered Jun 29 '14 at 07:52

haccks

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score 0 · Answer 3 · answered Jun 29 '14 at 07:53

0

This

#define round(a) ((a-0.5)<int(a))?int(a):int(a+1)

Has the brackets in the wron places

Should be

#define round(a) (((int)((a)-0.5))<(a))?(int)(a):(int)(a+1)

answered Jun 29 '14 at 07:53

Ed Heal

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1

You need to cast `(a)` in the comparison, otherwise it will always be true. – Barmar Jun 29 '14 at 07:54

score 0 · Answer 4 · answered Jun 29 '14 at 07:55

The problem is that int(a) is not valid C.

Redefine your macro as follows:

#define round(a) (((a)-0.5)<(int)(a))?(int)(a):(int)(a+1)

Note that I've also added parentheses around a in (a)-0.5.

P.S. What's the reason for making it a macro and not, say, a function?

C prog error: expected expression before int

4 Answers4