Bjarne shows an example of a square() function. I'm getting an error: "square identifier not found", why? [closed]

In section 4.4.2.1, "while statements", Bjarne shows an example of a square() function. But I am getting the following error, why?

square identifier not found

I am trying to figure out how to use this program so then I can code the TRY THIS exercise. I try to define or declare the function, and maybe that is the problem, or I really don't know. I do know the C++ doesn't have a square() function. What am I doing wrong?

Here is the code that I created:

#include <iostream>
#include <vector>
#include <string>
#include <cmath>
#include <algorithm>

using namespace std;

double square( double value)
{
  return (value * value);
}

int main()
{
  int i = 0;
  while (i < 100)
  {
     cout << "please enter a number: \n";
     cin >> i;
     cout <<i<< '\t' << square(i)<< '\n';
     ++i;
  }
  return 0;
}