Formatting Floating-Point Numbers

Formatting floating-point numbers in scientific notation using C++ output streams is straightforward with the help of manipulators like std::scientific and std::setprecision().

Using std::scientific

To format a number in scientific notation, you can use the std::scientific manipulator:

1#include <iostream>
2
3int main() {
4  double pi = 3.141592653589793;
5  std::cout << std::scientific << pi;  
6}

13.141593e+00

This converts the number into scientific notation with a default precision.

Adjusting Precision

You can control the number of digits displayed after the decimal point using the std::setprecision() manipulator from the <iomanip> header:

1#include <iostream>
2#include <iomanip>
3
4int main() {
5  double pi = 3.141592653589793;
6  std::cout << std::scientific
7    << std::setprecision(3)
8    << pi;  
9}

13.142e+00

Here, std::setprecision(3) sets the precision to three digits after the decimal point.

Combining with Other Manipulators

You can combine std::scientific with other manipulators for more complex formatting. For example, if you want to set a fixed width for the output, you can use std::setw() and std::setfill():

1#include <iostream>
2#include <iomanip>
3
4int main() {
5  double pi = 3.141592653589793;
6  std::cout << std::scientific
7            << std::setprecision(2)
8            << std::setw(15)
9            << std::setfill('_')
10            << pi; 
11}

1_______3.14e+00

Resetting to Default

To revert to the default floating-point notation, you can use std::defaultfloat:

1#include <iostream>
2#include <iomanip>
3
4int main() {
5  double pi = 3.141592653589793;
6  std::cout << std::scientific << pi << '\n';    
7  std::cout << std::defaultfloat << pi;  
8}

13.141593e+00
23.14159

Understanding how to use these manipulators allows you to present floating-point numbers in the most appropriate format for your application.