C program: Integer array pointer changes values when passed as parameter

Question

My problem is that my integer array's values change when it is passed to the function calculate . The values are correct for indexes 0, and 2->5.

For some reason, indexes 1 and 6+ are not the correct values.

Below is my code.

#include <stdio.h>
#include <time.h>
#include <stdlib.h>

int* generate_rand (int length, int MAX_ARRAY);
void calculate (int *array_ptr, int length, int *mean, int *sd);

main() {
    srand(time(NULL));
    int a;
    printf("\nArray length?: ");
    scanf("%d", &a);
    int* array_ptr2;
    array_ptr2 = generate_rand(a, 100);
    //int mean, sd;
    int* *mean;
    int* *sd;
    int i = 0;
    for (i = 0; i < 10; i++) {
        printf("Array2: %d\n", *(array_ptr2 + i));
    }
    calculate(array_ptr2, a, *mean, *sd);
    //printf("Mean: %d\n", (int)*mean);
}

int* generate_rand (int length, int MAX_ARRAY) {
    int arr[length];
    int i;
    for (i = 0; i < 10; i++) {
        int r = rand()%MAX_ARRAY;
        arr[i] = r;
        printf("Rand: %d\n", arr[i]);
    }
    int *arrPtr;
    arrPtr = &arr[0];
    return arrPtr;
}

void calculate (int *array_ptr, int length, int *mean, int *sd) {
    int sum;
    int i;
    for (i = 0; i < length; i++) {
        printf("Array: %d\n", *(array_ptr + i));
        sum += *(array_ptr + i);
        //array_ptr++;
        printf("Sum: %d, i:%d\n", sum, i);
    }
    //*mean = sum / length;
}

Do you know what I'm doing wrong?

Answer 1

generate_rand( ) is returning a pointer to arr[0] where arr is a local variable. The local variable goes out of scope as soon as the function, generate_rand() , returns. Once a variable is out of scope, there is no guarantee about its value, in fact, accessing an out of scope of variable is undefined.

One possible solution: generate_rand() can allocate the array arr on the heap, ie allocate memory using malloc() . Needless to say, the memory must be free() -ed as soon as it is no longer necessary.

Answer 2

The problem is that you are returning the address of a local variable of a function. Local variables of functions cease to exist after a function has exited, so the returned pointer suddenly becomes dangling (as in, not pointing to anything useful).

You need to allocate memory for your array with malloc , because the memory returned from malloc will exist until it is freed with free (which you can call anytime afterwards).

int* generate_rand (int length, int MAX_ARRAY) {
    int *arr = malloc(sizeof(int) * length);

    int i;

    for (i = 0; i < 10; i++) {
        int r = rand()%MAX_ARRAY;
        arr[i] = r;
        printf("Rand: %d\n", arr[i]);
    }

    return arr;
}

When you no longer need the array returned from this function, pass it to free to release the memory, eg:

int *arr = generate_rand(10, 100);
// do something with arr here...
free(arr);

Answer 3

As you state in the comments, generate_rand() needs to return a pointer to an array that is filled with random values. At the moment, you are returning a pointer to the first element of a local array. As other's have stated, the memory of this array is only guaranteed to be available to your program inside of the generate_rand() function. In order to create an array which you can use anywhere in your program, you need to allocate the memory with malloc() . I suggest you research memory management further. You need to learn about both the malloc() and free() functions.