How can I work around int overflow in c

Question

In C, is it possible to do the below operation without causing an integer overflow? My answer needs to be an integer which I use later on in the program. The code prints -337. The correct answer should be 2014.

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

int main() {
   int i;
   i = (10000 * 735625 + 14780)/3652425;
   printf("%d",i);
   return 0;
}

Any reason why you use such a strange formula if you know the result already? — deviantfan, Mar 09 '14 at 17:22
Just change `10000` to `10000LL` and you should be set. That changes the calculation to use `long long`, which has higher precision. The result will (in this case) still fit in a regular `int`. — Joachim Isaksson, Mar 09 '14 at 17:28
Thanks Joachim. You were spot on. And in answer to your question deviantfan, one of the numbers in the formula :735625 is a calculated variable g in my program. It served no purpose to write out my whole programe here. You only need an instance of the variable. — user3398879, Mar 10 '14 at 00:15

score 1 · Answer 1 · edited Jun 20 '20 at 09:12

Based on usual arithmetic conversions, it will overflow as 10000 * 735625 + 14780 (out of int's range) is trying to save into int.

Integral promotions are performed on the operands as follows:

If either operand is of type unsigned long, the other operand is converted to type unsigned long.

If preceding condition not met, and if either operand is of type long and the other of type unsigned int, both operands are converted to type unsigned long.

If the preceding two conditions are not met, and if either operand is of type long, the other operand is converted to type long.

If the preceding three conditions are not met, and if either operand is of type unsigned int, the other operand is converted to type unsigned int.

If none of the preceding conditions are met, both operands are converted to type int.

To work out, you need to use long long (with larger range) stead.

A simple way, as @JoachimIsaksson suggested, is to put LL after 10000 to calculate with long long's precision:

i = ( 10000LL * 735625 + 14780)/3652425;

See it live: http://ideone.com/pA2Pvm

Also make sure you change the `printf` specifier from `%d` - [How to print 'long long int' in C?](http://stackoverflow.com/questions/13590735/printf-long-long-int-in-c-with-gcc) — Bert, Mar 09 '14 at 17:27

score 0 · Answer 2 · answered Mar 09 '14 at 17:28

0

i = (int)((10000.0 * 735625.0 + 14780.0)/3652425.0);

printf("%d",i);

valter

answered Mar 09 '14 at 17:28

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How can I work around int overflow in c

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