Set precision
  Sets the precision to be used by output operations as by a call to stream's member precision(n)
  The precision determines the maximum number of digits that shall be output on insertion operations to express floating-point values, counting both the digits before and after the decimal point.
  You must include <iomanip> to use this manipulator.

Parameters.

n

Value to set precision.

Return Value.
  Unspecified.
  This function should only be used as a stream manipulator.

Example.

// setprecision example
#include <iostream>
#include <iomanip>
using namespace std;

int main () {
  double f =3.14159;
  cout << setprecision (5) << f << endl;
  cout << setprecision (9) << f << endl;
  return 0;
}

See also.
  iomanip manipulators