Why Does `std::cout.precision()` Not Display Trailing Zeros for Floating-Point Numbers?

std::cout.precision() and Trailing Zeros

Understanding the correct usage of std::cout.precision() is crucial when displaying floating-point numbers. In this article, we'll examine a specific example to resolve an unexpected behavior.

Initial Code and Issue

Consider the following code that attempts to display the result of dividing two integers as a floating-point number:

#include <iostream>
#include <stdlib.h>

int main() {
  int a = 5;
  int b = 10;
  std::cout.precision(4);
  std::cout << (float)a / (float)b << "\n";
  return 0;
}

This code is expected to display 0.5000, but instead, it prints 0.5. This discrepancy arises due to the nature of the original data types being integers.

Resolution: std::fixed Manipulator

To display trailing zeros for floating-point numbers, you must pass the std::fixed manipulator to std::cout. This manipulator ensures that the decimal representation uses fixed-point notation, preserving trailing zeros.

#include <iostream>
#include <stdlib.h>
#include <iomanip>

int main() {
  int a = 5;
  int b = 10;
  std::cout << std::fixed;
  std::cout << std::setprecision(4);
  std::cout << (float)a / (float)b << "\n";
  return 0;
}

With the std::fixed manipulator, the code now correctly prints 0.5000. The std::setprecision() manipulator remains responsible for specifying the desired number of decimal places.

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