How to Round Floating-Point Number to 2 Decimals in C++

1. Use the printf Function Format Specifiers to Round Floating-Point Number to 2 Decimals in C++
2. Use fprintf Function Format Specifiers to Round Floating-Point Number to 2 Decimals in C++
3. Use std::setprecision and std::fixed to Round Floating-Point Number to 2 Decimals in C++

This article will explain several methods of how to round floating-point numbers to 2 decimals in C++.

Use the printf Function Format Specifiers to Round Floating-Point Number to 2 Decimals in C++

Floating-point numbers have special binary representation, which entails that real numbers can’t be represented exactly by the machines. Thus, it’s common to resort to rounding functions when operating on the floating-point numbers and doing calculations on them. In this case, though, we only need to output a certain portion of the numbers’ fractional part.

The first solution is to utilize the printf function to output formatted text to the stdout stream. Note that the regular format specifier for the float numbers - %f should be modified to %.2f. The latter notation ensures that only 2 decimals are printed from the number, and at the same time, it does the rounding according to the common mathematical rules. Mind that, a similar notation can be used with format specifiers of other data types.

#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::vector;

int main() {
  vector<double> floats{-3.512312, -21.1123, -1.99, 0.129, 2.5, 3.111};

  for (auto &item : floats) {
    printf("%.2f; ", item);
  }

  return EXIT_SUCCESS;
}

Output:

-3.51; -21.11; -1.99; 0.13; 2.50; 3.11;

Use fprintf Function Format Specifiers to Round Floating-Point Number to 2 Decimals in C++

fprintf is another function that can be used to do the output formatting like the printf call and additionally, it can write to any FILE* stream object that can be passed as the first argument. In the following example, we demonstrate printing to the stderr stream, an unbuffered version of stdout for error reporting and logging.

#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::vector;

int main() {
  vector<double> floats{-3.512312, -21.1123, -1.99, 0.129, 2.5, 3.111};

  for (auto &item : floats) {
    fprintf(stderr, "%.2f; ", item);
  }

  return EXIT_SUCCESS;
}

Output:

-3.51; -21.11; -1.99; 0.13; 2.50; 3.11;

Use std::setprecision and std::fixed to Round Floating-Point Number to 2 Decimals in C++

Alternatively, one can utilize the std::setprecision function from the I/O manipulators’ library in conjunction with std::fixed. The latter is used to modify the default formatting for floating-point input/output operations. If we use it together with std::setprecision, the result is fixed precision of the real numbers, and the precision can be specified with the integer argument passed to the std::setprecision itself.

#include <iomanip>
#include <iostream>
#include <vector>

using std::cout;
using std::endl;
using std::fixed;
using std::setprecision;
using std::vector;

int main() {
  vector<double> floats{-3.512312, -21.1123, -1.99, 0.129, 2.5, 3.111};

  for (auto &item : floats) {
    cout << setprecision(2) << fixed << item << "; ";
  }

  return EXIT_SUCCESS;
}

Output:

-3.51; -21.11; -1.99; 0.13; 2.50; 3.11;