find the average of 10 numbers to an array using function .the array should be controlled by while loop?

Code:
#include <iostream.h>
float sum(float x[],int size);
    main()
{
   float a[10];
int n=10;
while (n<10)

{
   cin>>a[n];
n++;
}
cout<<"avg = "sum (a+n)/n;
}
floatsum(floatx{},int size)
{
...