C++ Cookbook

10.2. Formatting Floating-Point Output

Problem

You need to present floating-point output in a well-defined format, either for the sake of precision (scientific versus fixed-point notation) or simply to line up decimal points vertically for easier reading.

Solution

Use the standard manipulators provided in <iomanip> and <ios> to control the format of floating-point values that are written to the stream. There are too many combinations of ways to cover here, but Example 10-3 offers a few different ways to display the value of pi.

Example 10-3. Formatting pi

#include <iostream>
#include <iomanip>
#include <string>

using namespace std;

int main() {

   ios_base::fmtflags flags =  // Save old flags
      cout.flags();

   double pi = 3.14285714;

   cout << "pi = " << setprecision(5)  // Normal (default) mode; only
        << pi << '\n';                 // show 5 digits, including both
                                       // sides of decimal point.

   cout << "pi = " << fixed            // Fixed-point mode;
        << showpos                     // show a "+" for positive nums,
        << setprecision(3)             // show 3 digits to the *right*
        << pi << '\n';                 // of the decimal.

   cout << "pi = " << scientific       // Scientific mode; 
        << noshowpos                   // don't show plus sign anymore
        << pi * 1000 << '\n';

   cout.flags(flags);  // Set the flags to the way they were
}

This will produce the following output:

pi = 3.1429
pi = +3.143
pi = 3.143e+003

Discussion

Manipulators that specifically manipulate floating-point output divide into two categories. There are those that set the format, which, for the purposes of this recipe, set the general appearance ...