Dividing 2 long values produces wrong output in c++

Question

I am dividing 2 long values and use the ceiling of the result. However my code produces wrong output.

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;

int main(){
  long first = 100;
  long second = 1000;
  long output = ceil(first/second);
  cout<<"output = "<<output<<endl;
}

Here the expected output is

output = 1

actual output:

output = 0

`first/second` This is integer division and the expected result is `0`. You may want to look at [Fast ceiling of an integer division in C / C++](https://stackoverflow.com/questions/2745074/fast-ceiling-of-an-integer-division-in-c-c). — dxiv, Jun 21 '20 at 00:32
No 100/1000 is 0. With integer math there is no decimal point. 1/2 is zero as well. — drescherjm, Jun 21 '20 at 00:43
@DenisShevchenko C++ is not Python 3. An integer divided by an integer yields an integer. — PaulMcKenzie, Jun 21 '20 at 01:01

score 3 · Accepted Answer · edited Jun 21 '20 at 00:42

3

Because of order of evaluation of long output = ceil(first/second);

The first operation is first/second, 100/1000. Once you are using integer types (long is an integer) the result will be a long and it is truncated towards zero. 100/1000 =0

Then you have: ceil(0) = 0 // as expected

long output = 0

edited Jun 21 '20 at 00:42

user207421

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answered Jun 21 '20 at 00:37

MasterID

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That makes a lot of sense. I did not realize the order of operations could affect the output like that. The solution turns out to be pretty simple as shown in the link @dxiv shared in the comments to my question. – Denis Shevchenko Jun 21 '20 at 00:41

Dividing 2 long values produces wrong output in c++

1 Answers1