GCC division truncates (rounding problem)

Question

Using GCC on the Ubuntu Linux 10.04, I have unwanted rounding after a division.

I tried:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void FormatReading(int temp)
{
    double reading = temp / 100;
    printf("%f\n",reading);  /* displays 226.000000, was expecting 226.60 */
}

int main(void)
{
    FormatReading(22660);
    return 0;
}

It was suggested to me to try:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void FormatReading(int temp)
{
    long reading = temp ;
    reading = reading / 100;
    printf("%3.2ld\n",reading);  /* displays 226 */
}

int main(void)
{
    FormatReading(22660);
    return 0;
}

I also tried:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

void FormatReading(int temp)
{
    long reading = temp ;
    double reading2 = reading / 100;
    printf("%3.2f\n",reading2);  /* displays 226.00 */
}

int main(void)
{
    FormatReading(22660);
    return 0;
}

I also tried the round function using include math.h with compiler tag -lm in various ways, but did not find what I was looking for.

Any help greatly appreciated.

Best regards, Bert

possible duplicate of [What is the behavior of integer division in C?](http://stackoverflow.com/questions/3602827/what-is-the-behavior-of-integer-division-in-c) — Steve Jessop, Apr 08 '11 at 12:55
Not a duplicate, @Steve. I don't think the OP knows that he is doing integer division. I think he thinks that `d = i/i` does floating-point division. — Robᵩ, Apr 08 '11 at 19:48
@Rob: feel free to select any of the numerous other questions on the exact same point as a better duplicate. I think I just picked the first that looked to me like a close match, admittedly "looks to me" was influenced by the fact that I know the answer. If the questioner didn't realise he was doing integer division, then he does now :-) — Steve Jessop, Apr 09 '11 at 20:24

score 9 · Answer 1 · answered Apr 08 '11 at 12:37

9

double reading = temp / 100.0;
                           ^^

temp / 100 is an integer division - that you assign the result to a double doesn't change this.

answered Apr 08 '11 at 12:37

Erik

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score 3 · Answer 2 · answered Apr 08 '11 at 12:36

3

You are using integer division which always gives integral results rather than fractions, and then the result is being assigned to a double. Divide by 100.0 instead of 100 to get the behavior you want.

answered Apr 08 '11 at 12:36

I82Much

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GCC division truncates (rounding problem)

2 Answers2