Calculate Average in C program

I want to make a C program with a graphical interface that must be composed of 11 numbers (averages) with a "calculate" button that allow the calculation of the number.

Code:

---

for(j = 0, j < 11, j++)
{
  do
  {
   
  }  while
  F[j] = input;
  sum += input;
}
if(sum != 1.0)
 
else
{
  for(j = 0; j < 11, j++)
  Avg += (F[j]*X[j]);
  Avg /= 11;
  // output Avg
}

---

// Calculating the average

Code:

---

#include <iostream>
using namespace std;

int main()
{
    cout << "Enter the number: ";
    int i;
    cin >> i;
    double num;
    double sumnum;
    sumnum = 0.0;
    for (int y = 1; y <= i; y++)
    {
        cout << "#" << x << " = ";
        cin >> num;
        sumnum += num;       
    }

---

Code:

---

#include <stdio.h>

float average(float z, float d) {
  return (z + d)/2.0f;
}

/* main program - execution always starts here */
void main() {

  float average(float x, float y);  /* Function prototype */

  float val1= 1.0F;
  float val2 = 2.0F;
  float o = 0.0F;


  o = average(val1, val2);
  printf("\nThe average is: %f",  o);
}

---