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? — Steve Jessop
– Steve Jessop, Commented Apr 8, 2011 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
– Robᵩ, Commented Apr 8, 2011 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
– Steve Jessop, Commented Apr 9, 2011 at 20:24

Erik · Accepted Answer · 2011-04-08 12:37:00Z

double reading = temp / 100.0; ^^

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

I82Much · Accepted Answer · 2011-04-08 12:36:28Z

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.

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