How do I exclude elements from an array?

Question

I have got 2 arrays a1 = {1,2,3,4,5,6,7.....} and a2 = {1,3,5,7} If I have to exclude the elements of a2 from a1 and print a1 what would be the best way?

Answer 1

When interpreted as "print from a1 if not present in a2":

#include <stdio.h>

int main(void) {
    int a1[] = {1, 2, 3, 4, 5, 6, 7, 8};
    int a2[] = {1, 3, 5, 7};
    printf("elements of a1 not present in a2:");
    for (int i = 0; i < sizeof a1 / sizeof *a1; i++) {
        int found = 0;
        for (int j = 0; j < sizeof a2 / sizeof *a2; j++) {
            if (a1[i] == a2[j]) { found = 1; break; }
        }
        if (!found) printf(" %d", a1[i]);
    }
    puts("");
    return 0;
}

See code running on ideone

Answer 2

Example function which removes all the elements having particular value from the array. This example is for type int but it is very easy to change any other type

size_t remove_element(int *arr, size_t size, int element)
{
    for(size_t index = 0; index < size; )
    {
        if(arr[index] == element)
        {
            memmove(arr + index, arr + index + 1, (size - index - 1) * sizeof(*arr));
            size --;
            continue;
        }
        index++;
    }
    return size;
}

int main()
{

    int h[] = {4,6,8,9,2,3,0,1,2,5,6,9,0};
    size_t size = sizeof(h) / sizeof(h[0]), newsize = remove_element(h, size, 0);

    printf("Size Before:%zu New size %zu\n", size, newsize);


    for(size_t index = 0; index < newsize; index++)
    {
        printf("h[%zu] = %d\n", index, h[index]);
    }

    return 0;
}

and to remove elements which are in another array

size_t remove_elements(int *arr, size_t arrsize, const int *elements, size_t elemsize)
{
    for(size_t index = 0; index < elemsize; index++)
    {
        arrsize = remove_element(arr, arrsize, elements[index])
    }
    return arrsize;
}

Try yourself: https://onlinegdb.com/ryRJOGNnN