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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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