C/C++ 打印的精度

// setprecision example
#include <iostream>     // std::cout, std::fixed
#include <iomanip>      // std::setprecision
#include <iostream>

using namespace std;

void test_precision(double d) {
  const streamsize ss = cout.precision();  // get default precision
  cout << "default precision is " << ss << endl;
  cout << d << endl;
  cout << setprecision(5)  << d << endl;
  cout << fixed;
  cout << d << endl;
  cout << setprecision(6) << d << endl;
  cout << setprecision(7) << d << endl;
  cout << setprecision(8) << d << endl;
  cout.precision(1);  // Different way to set precision
  cout << d << endl;
  cout << defaultfloat << setprecision(ss);  // restore default precision
  printf("c way: %.2f\t%.15f\n", d, d);
}

int main() {
  double d = 12.123456789;
  test_precision(d);
  cout << endl;  // print new line
  d = 1.123456789;
  test_precision(d);
  cout << endl;  // print new line
  d = 0.5;
  test_precision(d);
  return 0;
}