CODEWITHSUNDEEP
cfor-loopmultidimensional-array
budokai tenkaichi 4
budokai tenkaichi 4

Reputation: 45

outputting a value from a 2d array in a for loop in C

I am trying to solve this problem from geeksforgeeks: https://practice.geeksforgeeks.org/problems/rotate-array-by-n-elements/0 I am trying to output values from a 2d array by collecting all the input from the user first then outputting the values, but I am getting random integers instead. The values appear outside the for loop just fine which is what I am not understanding. The algorithm I use to rotate the values works the way I want it to.

Here is my code:

int * rotate_array(int *array, int max_size, int rotate_by_D);

int main()
{
    //First accept and store all inputs
    int T, N, D;
    scanf("%d", &T);
    int arr_len[T];
    int *results[T];
    for (int i = 0; i < T; i++)
    {
        scanf("%d %d", &N, &D);
        int arr[N];
        arr_len[i] = N;
        //store each length of the array
        for (int j = 0; j < N; j++)
        {
            int user_input;
            scanf("%d", &user_input);
            arr[j] = user_input;
        }
        results[i] = rotate_array(arr, N, D);
    }
    //I can get the value I anticipate here 
    printf("%d\n", results[0][0]);
    //the code below is where i am trying to output the new values to the users
    //But I see random numbers instead
    for (int di = 0; di < T; di++)
    {
        for (int dj = 0; dj < arr_len[di]; dj++)
        {
            printf("%d ", results[di][dj]);
        }
        printf("\n");
    }
    return 0;
}

I will also display the algorithm I have used just in case it may be useful in solving my problem

int * rotate_array(int *array, int max_size, int rotate_by_D)
{
    int original_value[max_size];
    for (int k = 0; k < max_size; k++)
    {
        original_value[k] = array[k];
        if ((k + rotate_by_D) >= max_size)
        {
            int new_pos = (k + rotate_by_D) % max_size;
            array[k] = original_value[new_pos];
        }
        else
        {
            array[k] = array[k + rotate_by_D];
        }
    }
    return array;

Here is the output I receive when I run my code:

1 (The number of test cases)
3 2 (The array size and how much to rotate them by respectively)
1 2 3 (the array of values)
output
3 (This is from outside the loop)
-494200848 32766 221572309 (This what I get inside of the loop)
I expect the value to be: 3 1 2

Thank you in advance :)

Upvotes: 1

Views: 37

Answers (1)

chux
chux

Reputation: 153498

Right after results[i] = rotate_array(arr, N, D);, arr goes away and so results[i] is no longer valid.

{
    ...
    int arr[N];
    ...
    results[i] = rotate_array(arr, N, D); 
    ... // arr[] is no longer valid at the close of this block. 
        // Since rotate_array() returns `arr`, results[i] like-wise becomes invalid. 
}

Alternative, allocate data

    // int arr[N];
    int *arr = malloc(sizeof *arr * N);
    ...
    results[i] = rotate_array(arr, N, D);

And later free it

    for (int dj = 0; dj < arr_len[di]; dj++) {
        printf("%d ", results[di][dj]);
    }
    printf("\n");
    free(results[di]); // add

Detail: int *results[T]; is not a 2D array. results is an array of pointers.

"A pointer is not an array. An array is not a pointer."

Upvotes: 1

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