C - structures and lists - c

Question

I want to save the cartesian product of two arrays into one multidimentional array.
arr[number][0] - number from the first array
arr[number][1] - number from the second array
number = sizeof(array1)*sizeof(array2)
int main() {
int arr1[4] = {1, 4, 2, 3};
int arr2[4] = {4, 1, 3, 3};
int **outcomes = getProduct(arr1, 4, arr2, 4);
int i;
for(i = 0; i < 16; i++)
printf("%d", outcomes[i][0]);
getchar();
}
int* getProduct(int arr1[], int size1, int arr2[], int size2) {
int **outcomes = (int *)malloc(sizeof(int)*16), count = 0, i, j;
for(i = 0; i < size1; i++) {
for(j = 0; j < size2; j++) {
outcomes[count][0] = arr1[i];
outcomes[count][1] = arr2[j];
count++;
}
}
return outcomes;
};
However, this code strangly breaks while being in the nested loop inside getProduct() function. The logic is ok, I thing the problem might be with pointers.
Why do I need to initialize multidimentional arrays with two pointers?
Why isn't this code working?
Do u have any advices for me on how can I improve this code? ( I didn't code in C too much so I don't know which structures a "good" code uses).

Answer 1

The function getProduct would fail because you are assigning a pointer to an int( ie int*) to a pointer to an int pointer (ie int**).
Why do I need to initialize multidimentional arrays with two pointers?
Because
int** (pointer to int pointer) - with this, we can access an element of any row and any column which is not possible with a single pointer(or single dimensional array)
Similarly int*** can be used for three dimensional array or in other words pointer to pointer to an int pointer
To improve the code
I think your cartesian product function should look something like this
int** getProduct(int arr1[], int size1, int arr2[], int size2) {
int **outcome = malloc(sizeof(int*)*size1);
int i,j;
for(i = 0;i<size1;i++)
outcome[i] = malloc(sizeof(int)*size2);
for(i = 0; i < size1; i++) {
for(j = 0; j < size2; j++) {
outcome[i][j] = arr1[i] * arr2[j];
}
}
return outcome;
}
And your main() like this
int main() {
int arr1[4] = {1, 4, 2, 3};
int arr2[4] = {4, 1, 3, 3};
int **outcomes = getProduct(arr1, 4, arr2, 4);
int i,j;
for(i = 0; i < 4; i++){
for(j = 0;j <4; j++){
printf("%d ", outcomes[i][j]);
}
printf("\n");
}
getchar();
}