Set Operations with an Array (Intersection, Difference)

This lesson covers computing an intersection and difference in C using arrays, so you understand the basics of set operations. It's for anyone searching "C set operations implementation."

C has no dedicated Set type, but comparing two arrays with a nested for loop lets you find an "intersection" (values common to both) or a "difference" (values in only one).

The example computes the intersection of two arrays, a and b, by finding matching values in the inner for loop. Cutting the search short the moment a match is found is also a good detail for an efficient implementation.

A common early mistake is not accounting for this nested loop's computational cost. Processing time increases sharply as array size grows, so a more efficient implementation would need a technique like using a sorted array.

In real projects, being able to implement logic like this efficiently, within limited memory, matters in C programming, whenever you need to find something like "data common to both groups."

📖 Reference code

#include <stdio.h>

int main() {
    int a[] = {1, 2, 3, 4};
    int b[] = {3, 4, 5, 6};
    int na = 4, nb = 4;

    printf("Intersection: ");
    for (int i = 0; i < na; i++) {
        for (int j = 0; j < nb; j++) {
            if (a[i] == b[j]) printf("%d ", a[i]);
        }
    }

    printf("\nDifference: ");
    for (int i = 0; i < na; i++) {
        int found = 0;
        for (int j = 0; j < nb; j++) {
            if (a[i] == b[j]) found = 1;
        }
        if (!found) printf("%d ", a[i]);
    }
    return 0;
}

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