trouble with pointers in C

Question

So I have this code:

#include <stdio.h>
int arraySum (int *a, int n);
int main(void){
    int values[3] = {1, 2, 3};
    printf("The sum is %i\n", arraySum(values, 3));
    return 0;
}
int arraySum(int *a, int n){
    int sum = 0;
    int arrayEnd = *a + n;
    for ( ; *a < arrayEnd; *a++)
        sum += *a;
    return sum;
}

For some reason it outputs this:

roman@lmde64 ~/Dropbox/Practice $ gcc practice.c 
roman@lmde64 ~/Dropbox/Practice $ ./a.out
The sum is -421028781
roman@lmde64 ~/Dropbox/Practice $ ./a.out
The sum is -362865581
roman@lmde64 ~/Dropbox/Practice $ ./a.out
The sum is -1046881197
roman@lmde64 ~/Dropbox/Practice $ ./a.out
The sum is 6
roman@lmde64 ~/Dropbox/Practice $ ./a.out
The sum is 6

Why is the output strange numbers sometimes and the right answer other times? What am I doing wrong? Thanks for any help.

Answer 1

In arraySum(), you are confusing when to use a as a pointer, and when to dereference it to obtain what it is pointing to. When you are calculating the loop limits, etc., you want to be working with the pointer itself:

int arraySum(int *a, int n){
    int sum = 0;
    int *arrayEnd = a + n;
    for ( ; a < arrayEnd; a++)
        sum += *a;
    return sum;
}

Answer 2

You want to iterate over the array:

int arraySum(int *a, int n){
    int sum = 0;
    int * arrayEnd = a + n; // arrayEnd points to the first element after your array a
    for ( ; a != arrayEnd; ++a)  // Iterate over the array until you reach arrayEnd
        sum += *a; // Dereference the pointer to current array element in order to retrieve a value
    return sum;
}