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C++ Cookbook
book

C++ Cookbook

by D. Ryan Stephens, Christopher Diggins, Jonathan Turkanis, Jeff Cogswell
November 2005
Beginner to intermediate content levelBeginner to intermediate
594 pages
16h 23m
English
O'Reilly Media, Inc.
Content preview from 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 ...

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Publisher Resources

ISBN: 0596007612Supplemental ContentErrata Page