How do I use `setprecision` in C++ to format floating-point numbers?

General Discussions
1

I want to print a floating-point number with 2 decimal places. For example, 3.444 should output as 3.44 and 99999.4234 as 99999.42. My code is:

#include <iomanip>
#include <iostream>
using namespace std;

int main() {
    double num1 = 3.12345678;
    cout << fixed << showpoint;
    cout << setprecision(2);
    cout << num1 << endl;
}

But I get an error saying undefined fixed symbol. How can I correctly use setprecision C++ to format my numbers?

2

Without fixed, setprecision affects total significant digits, not decimals:

double num = 3.444;
std::cout << std::setprecision(2) << num << std::endl;  // Output: 3.4

With fixed, it controls digits after the decimal:

std::cout << std::fixed << std::setprecision(2) << num << std::endl; // Output: 3.44

I always combine fixed + setprecision when I want exact decimal formatting.

3

I personally like being explicit instead of using namespace std;:

#include <iostream>
#include <iomanip>

int main() {
    double num1 = 99999.4234;
    std::cout << std::fixed << std::showpoint << std::setprecision(2);
    std::cout << num1 << std::endl;  // Output: 99999.42
}

Works exactly the same and avoids namespace pollution.

I use this style in professional codebases.

4

I’ve seen this error before :sweat_smile:. The problem usually happens if fixed or setprecision isn’t recognized. Make sure you:

Include <iomanip>

Use std:: or using namespace std;

Correct code:

#include <iostream>
#include <iomanip>  // needed for fixed and setprecision
using namespace std;

int main() {
    double num1 = 3.12345678;
    cout << fixed << showpoint << setprecision(2);
    cout << num1 << endl;  // Output: 3.12
    return 0;
}

fixed ensures decimal precision, not significant digits.

setprecision(2) formats the number to 2 digits after the decimal.