Implicit declaration of function 'sum' is invalid in C99

Question

I've been looking for a solution to this but haven't found anything that will help. I'm getting the following errors:

Implicit declaration of function 'sum' is invalid in C99

Implicit declaration of function 'average' is invalid in C99

Conflicting types for 'average'

Has anyone experienced this before? I'm trying to compile it in Xcode.

#import <Foundation/Foundation.h>


    int main(int argc, const char * argv[])
    {

       @autoreleasepool
       {
          int wholeNumbers[5] = {2,3,5,7,9};
          int theSum = sum (wholeNumbers, 5);
          printf ("The sum is: %i ", theSum);
          float fractionalNumbers[3] = {16.9, 7.86, 3.4};
          float theAverage = average (fractionalNumbers, 3);
          printf ("and the average is: %f \n", theAverage);

       }
        return 0;
    }

    int sum (int values[], int count)
    {
       int i;
       int total = 0;
       for ( i = 0; i < count; i++ ) {
          // add each value in the array to the total.
          total = total + values[i];
       }
       return total;
    }

    float average (float values[], int count )
    {
       int i;
       float total = 0.0;
       for ( i = 0; i < count; i++ ) {
          // add each value in the array to the total.
          total = total + values[i];
       }
       // calculate the average.
       float average = (total / count);
       return average;
    }

Answer 1

You need to add declaration for these two functions, or move the two function definitions before main.

Answer 2

The problem is that by the time the compiler sees the code where you use sum it doesn`t know of any symbol with that name. You can forward declare it to fix the problem.

int sum (int values[], int count);

Put that before main(). This way, when the compiler sees the first use of sum it knows that it exists and must be implemented somewhere else. If it isn't then it will give a liner error.