lab_1 (sets)

class Sets {
    public static void main(String... args) {
        String[] a = {"1", "2", "3", "4", "5"};
        String[] b = {"1", "3", "6", "7"};
        print_set(a, "a");
        print_set(b, "b");
        print_set(union(a, b), "union");
        print_set(intersection(a, b), "intersection");
        print_set(difference(a, b), "difference a/b");
        print_set(difference(b, a), "difference b/a");
        print_set(sim_difference(a, b), "sim_difference");
    }

// start of the interface:

    public static String[] extend(String[] set) { // returns extended set with length set.length*2+1
        String[] new_set = new String[set.length*2+1];
        for (int i = 0; i < new_set.length; i++){
            if (i < set.length)
                new_set[i] = set[i];
        }
        return new_set;
    }

    public static String[] reduce(String[] set) { // returns reduced set with length set.length/2
        String[] new_set = new String[set.length/2];
        for (int i = 0; i < new_set.length; i++){
            new_set[i] = set[i];
        }
        return new_set;
    }

    public static String[] add(String[] set, String elem) { // returns "set" with added element "elem"
        int end = -1;
        for (int i = 0; i < set.length; i++) {
            if (elem == set[i])
                return set;
            if (set[i] != null)
                end = i;
        }
        if ((set.length == 0) || (end >= set.length/2))
            set = extend(set);
        set[end+1] = elem;
        return set;
    }

    public static String[] remove(String[] set, String elem) { // returns "set" with removed element "elem"
        int end = 0;
        for (int i = 0; i < set.length; i++){
            if (set[i] == elem) {
                for (int j = i; j < set.length-1; j++)
                    set[j] = set[j+1];
                set[set.length-1] = null;
            }
            if (set[i] != null)
                end = i;
        }
        if (end < set.length/2)
            set = reduce(set);
        return set;
    }

    public static void print_set(String[] set, String name) { //prints the set as: "name": set[0] set[1] ... set[set.length - 1]
        String s = name + ": ";
        for (int i = 0; i < set.length; i++)
            if (set[i] != null)
                s = s + set[i] + " ";
    System.out.println(s);
    }

    public static String[] union(String[] a, String[] b) { // returns the union-set of sets "a" and "b"
        String[] result = a;
        for (int i = 0; i < b.length; i++)
            result = add(result, b[i]);
        return result;
    }

    public static String[] intersection(String[] a, String[] b) { // returns the intersection-set of sets "a" and "b"
        String[] result = new String[0];
        for (int i = 0; i < a.length; i++)
            for (int j = 0; j < b.length; j++)
                if (a[i] == b[j])
                    result = add(result, a[i]);

        return result;
    }

    public static String[] difference(String[] a, String[] b) { // returns the difference-set of sets "a" and "b"
        String[] result = new String[0];
        for (int i = 0; i < a.length; i++)
            result = add(result, a[i]);
        for (int i = 0; i < b.length; i++)
            result = remove(result, b[i]);
        return result;
    }

    public static String[] sim_difference(String[] a, String[] b) { // returns the simmetric-difference-set of sets "a" and "b"
        return union(difference(a, b), difference(b, a));
    }
}