C++ Pointers and Functions with no return - need to recall the output as a pointer

Question

Write a C++ code that calls the function add_three() that takes three float numbers and add them together, the function should not return anything, and the output variable should be passed as an input to the function. Note: the number of argument is four.

#include <iostream>
using namespace std;

void add_three(float, float *);

int main() 
{
  float s,r;
  float a[3];
  add_three(a[3], &r); //use of undeclared identifier 'a'

  cout<< "The sum is " << s; //use of undeclared identifier 's'

  return 0;
}

void add_three(float a[3], float *s)
{
  float r = 0;
  
  for(int i=0; i<3; i++)
    {
      cin >> a[i];
      r = r + a[i]; //invalid operands to binary expression ('float *' and 'float')
    }
  *s = &r;
  
}

Errors:

assigning to 'float' from incompatible type 'float *'; remove &
  *s = &r;

I am stuck and can't understand what I am doing wrong (BTW, I am a complete novice).

Answer 1

The first problem is that you attempt to pass a single float value to the function, which is declared to take a single float value.

Then you create an overload of that function, which takes a pointer to a float value. The function declaration

void add_three(float, float*);

is different from

void add_three(float*, float*);

You also make it worse by passing the fourth element from your three-element array.

Then there's the assignment

*s = &r;

It tries to assign the pointer to the variable r (the type of the expression &r is float*) to the floating-point value *s.

You probably want

*s = r;

Or better yet, use proper references to pass s by reference:

// First argument: Pass a pointer to the first element of the array
// Second argument: Pass a reference to the variable
void add_three(float* a, float& s);

Call as:

float a[3];
float s;
add_three(a, s);

---

As an alternative you could return the sum instead of passing references. This is actually the method I recommend:

// First argument: Pointer to the first element of your array
// Returns: The sum of all three elements
float add_three(float* a);

Call as:

float a[3];
float s = add_three(a);

Or considering that you don't actually use a in the main function, you could skip passing it as an argument. In fact, you don't need an array at all:

float add_three()
{
    float input, sum = 0;  // Initialize sum to zero

    for (unsigned i = 0; i < 3; ++i)
    {
        // Note: Really need some error checking and validation here
        std::cin >> input;
        sum += input;
    }

    return sum;
}

Simply call as:

float s = add_three();

Even if you must use arguments to "return" the value, you can do it without the array.