Avoiding the side effect with pure function

A pure function is a function that will always return the same result every time it is given the same input. The result doesn't depend on any information or state and won't produce a side effect, or a change of the system state outside of the function. Let's take a look at the following piece of code:

    /* pure_function_1.cpp */
    #include <iostream>

    using namespace std;

    float circleArea(float r)
    {
      return 3.14 * r * r;
    }

    auto main() -> int
    {
      cout << "[pure_function_1.cpp]" << endl;

      // Initializing a float variable
      float f = 2.5f;

      // Invoking the circleArea() function
      // passing the f variable five times
      for(int i = 1; i <= 5; ++i)
      {
        cout << "Invocation " << i << " -> ";
        cout << "Result of circleArea(" << f << ") = ";
        cout << circleArea(f) << endl;
      }

      return...