Formatting
- C++ course
- Intermediate
- Lesson 98/116
- 5 min
Learn formatting with stringstream.
Formatting with ostringstream
ostringstream enables number formatting with tools from the <iomanip> header. std::fixed and std::setprecision control the number of decimal places.
Example: oss << std::fixed << std::setprecision(2) << 3.14159;. This writes 3.14159 as "3.14". std::fixed sets fixed point notation and std::setprecision(2) sets two decimal places. std::hex gives hexadecimal, std::setw(n) sets field width. With these tools table and report output is formatted neatly.
Formatting tools
The <iomanip> header offers a rich set of formatting tools. std::fixed gives fixed point and std::scientific gives scientific notation.
std::setw(n) sets the field width of the next output to n; std::setfill('0') fills the space with zeros. Example: oss << std::setw(5) << std::setfill('0') << 42; gives "00042". std::hex sets hexadecimal and std::dec sets decimal base. These tools are used to align report and table output.
Practice Format pi to two decimals
The starting code has a pi variable of 3.14159. Create a std::ostringstream. Format pi with std::fixed and std::setprecision(2). Get the string with .str() and print it with std::cout.
Starter code
#include <iostream>
#include <sstream>
#include <iomanip>
#include <string>
int main() {
double pi = 3.14159;
// Format pi to two decimals and print
return 0;
}Sample solution
#include <iostream>
#include <sstream>
#include <iomanip>
#include <string>
int main() {
double pi = 3.14159;
std::ostringstream oss;
oss << std::fixed << std::setprecision(2) << pi;
std::cout << oss.str();
return 0;
}Expected output
3.14
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does std::setprecision(2) do?
oss << std::fixed << std::setprecision(2) << 3.14159;- Shows two integer digits
- Shows two decimal places Correct answer
- Sets field width to 2
- Doubles the number
Explanation: setprecision(2) shows two decimal places.
2 Match each tool to its role
| Term | Meaning |
|---|---|
std::fixed |
Fixed point |
std::setprecision |
Decimal places |
std::setw |
Field width |
std::hex |
Hexadecimal base |
3 Complete the setprecision call
oss << std::fixed << std::____(2) << 3.14;oss << std::fixed << std::setprecision(2) << 3.14;Explanation: std::setprecision sets the number of decimal places.