Setting the precision of a double without using stream (ios_base::precision)

Question

Is there a way to do this without using the stream? For example, something like this:

double a = 6.352356663353535;
double b = a.precision(5);

instead of:

double a = 6.352356663353535;
std::cout.precision(5);
std::cout << a << std::endl;

I am new to C++ and I am curious. Thanks, in advance.

Answer 1

I've revised the code taking into account @john, @Konrad and @KennyTM's suggestions. I've check that it works with negative numbers.

#include <cmath>
#include <cstdio>
using namespace std;
int main()
{
   double a = 6.352356663353535;
   double  intpart;
   double fractpart = modf (a, &intpart);
   fractpart  = roundf(fractpart * 100000.0)/100000.0; // Round to 5 decimal places
   double b = intpart + fractpart;
   printf("%.5lf", b);
}

Outputs

6.35236

Answer 2

doubles are almost universally implemented as IEEE floating point numbers. Their precision depends on the number size only (in the case of double, which is short for “double-precision floating point number”, it’s 53 bits). There is no way of manually setting the precision of a floating point number.

The display precision is always a property of the output formatting, never of the number. Don’t try to change the number via rounding, the operation makes no sense. You don’t need to reduce a number’s precision, except for display purposes.