Calculating with a float in macros in C

Question

My colleague and I are studying for a test, where we have to analyze C Code. Looking through the tests from the previous years, we saw the following code, which we don't really understand:

#include <stdio.h>
#define SUM(a,b) a + b
#define HALF(a)  a / 2

int main(int argc, char *argv[])
{
  int big = 6;
  float small = 3.0;

  printf("The average is %d\n", HALF(SUM(big, small)));
  return 0;
}

This code prints 0, which we don't understand at all... Can you explain this to us?

Thanks so much in advance!

and adding ()'s to macros variables (eg `#define HALF(a) (a)/2`) — Déjà vu, Nov 29 '16 at 07:41
[The need for parentheses in macros in C](http://stackoverflow.com/q/10820340/995714). Even if you used to correct format specifier, you'll get surprise results if you don't put parentheses. Just try and see — phuclv, Nov 29 '16 at 07:43
Compile with `-E` (gcc or clang) to see the preprocessor output. — user3386109, Nov 29 '16 at 07:43

score 3 · Accepted Answer · answered Nov 29 '16 at 07:37

3

The compiler's warnings (format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘double’) give more-than-enough information. You need to correct your format-specifier, which should be %lf, instead of %d, since you are trying to print a double value.

  printf("The average is %lf\n", HALF(SUM(big, small)));

printf will treat the memory you point as however you tell it to. Here, it is treats the memory that represents the float as an int. Because the two are stored differently, you should get what is essentially a random number. It needs not be 0 always.

answered Nov 29 '16 at 07:37

Jarvis

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Thanks for the explanation! – puer123 Nov 29 '16 at 07:43
1

just `%f` is enough – phuclv Nov 29 '16 at 07:43
@Jarvis: patience is a virtue; people have to wait a while before they're allowed to accept any answer (15 minutes, I believe). And soliciting for accept votes like that after less than 10 minutes is really a little too impatient. After a couple of hours, maybe. Granted, the 'Thanks' message from a newcomer is a little worrying; you can always point them to the FAQs ([How do I ask questions here?](http://stackoverflow.com/help/how-to-ask) and [What do I do when someone answers my question?](http://stackoverflow.com/help/someone-answers)). – Jonathan Leffler Nov 29 '16 at 07:50

Shreevardhan · Answer 2 · 2016-11-29T07:52:02.333

To get correct output

Add parentheses in macro
Use correct format specifier (%f)

Corrected Code

#include <stdio.h>

#define SUM(a, b) (a + b)
#define HALF(a)  a / 2

int main() {
    int big = 6;
    float small = 3.0;
    printf("The average is %f\n", HALF(SUM(big, small)));
    return 0;
}

Output

The average is 4.500000

If you don't add parentheses, output will be 7.500000 due to operator precedence.

In case you need integer output, cast to int before printing.

printf("The average is %d\n", (int)HALF(SUM(big, small)));

Output

The average is 4

Calculating with a float in macros in C

2 Answers2