Question

I have a master list integer array which has around 500 numbers. And, i have a set of 100 randomized number which has picked from the master list to find the missing numbers. Now, I need to go through this randomized number list against the master list. What would be the best approach in C programming to go through it without hanging the program. If i go through in simple ‘for’ loop for 500 elements, it will hang as it needs to go through the entire list. Could someone direct me on this?

Thanks.

Answer 1

First, you should profile it. It’s only 500*100=50,000 operations at the max we’re talking about. An average modern computer is capable of finishing it off in under one-tenth of a second, unless you code it very inefficiently.

Assuming that you would like to optimize it anyway, you should sort the master array, and run a binary search on it for each element of the randomized array. This would reduce the number of operations from 50,000 to at most 900, because a binary search of 500 numbers requires at most 9 comparisons.

Here is an implementation that uses built-in sorting and binary search functions (qsort and bsearch) of the standard C library:

int less_int(const void* left, const void* right) {
    return *((const int*)left) - *((const int*)right);
}

int main(void) {
    size_t num_elements = 500;
    int* a = malloc(num_elements*sizeof(int));
    for(size_t i=0 ; i<num_elements ; i++) {
        a[i] = rand() % num_elements;
    }
    qsort(a, num_elements, sizeof(int), less_int);
    size_t num_rand = 100;
    int* r = malloc(num_rand*sizeof(int));
    for(size_t i=0 ; i < num_rand ; i++) {
        r[i] = rand() % num_rand;
    }

    for (size_t i = 0 ; i != num_rand ; i++) {
        int *p = (int*) bsearch (&r[i], a, num_elements, sizeof(int), less_int);
        if (p) {
            printf ("%d is in the array.\n", *p);
        } else {
            printf ("%d is not in the array.\n", r[i]);
        }
    }

    free(a);
    free(r);
    return 0;
}

Here is a link to this running program on ideone.