Decimal Points rounding

Question

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

int main()
{
  double a,b,result;
  a = 2;b = 3;
  result = a / b;
  cout << fixed << setprecision(2) << result;

}

the output of this code in my IDE is :

0.67

i want the output to be

0.66

what do i do to change it to that

this has been answered here https://stackoverflow.com/questions/19759131/how-to-display-a-fixed-number-of-digits-in-c-without-rounding — newbie, Dec 30 '17 at 14:17

score 2 · Answer 1 · answered Dec 30 '17 at 14:11

Since you want truncation while setprecision uses rounding, you need to truncate the result manually:

cout << fixed << setprecision(2) << trunc(100*result)/100;

Multiplying and dividing by 100 keeps two decimal digits in place.

Demo.

Note 1: You have to be careful not to pass numbers that are close to the top of double's range to avoid overflowing on multiplication.

Note 2: You can generalize this approach by multiplying and dividing by pow(10,n) where n is the number of decimal digits that you wish to keep.

score 1 · Answer 2 · answered Dec 30 '17 at 14:11

2 / 3 is 0.666666666..., which is indeed closer to 0.67 than to 0.66. So if you want to the latter, you have to actually change the result. You can do this using a simple function:

double truncate(double val, int precision)
{
  double shift = std::pow(10.0, precision);
  return std::trunc(val * shift) / shift;
}

score 0 · Answer 3 · edited Dec 30 '17 at 14:14

0

You could try truncating with following:

result = (int)( 100 * a / b ) / 100.0;

edited Dec 30 '17 at 14:14

Tom de Geus

5,625
2
33
77

answered Dec 30 '17 at 14:11

StPiere

4,113
15
24

Decimal Points rounding

3 Answers3