Sets

# Constructing all four required set operations from first principles
class MySet:
    def __init__(self):
        self.items = []

    def contains(self, value):        # membership
        return value in self.items

    def add(self, value):
        if not self.contains(value):   # the check that enforces uniqueness
            self.items.append(value)

    def remove(self, value):
        if self.contains(value):
            self.items.remove(value)

    def is_subset_of(self, other):
        for value in self.items:
            if not other.contains(value):
                return False          # found one element NOT in other: not a subset
        return True              # every element checked and found: it is a subset

# Using Python's real set type and operators, a separate skill
subscribers = {"Amir", "Beth", "Chen"}
notified = {"Beth", "Chen", "Dara"}

print("Beth" in subscribers)          # membership test
print(subscribers & notified)            # intersection
print(subscribers | notified)            # union
print({"Beth", "Chen"} <= subscribers)  # subset test

// Constructing all four required set operations from first principles
class MySet {
    List<String> items = new ArrayList<>();

    boolean contains(String value) {        // membership
        return items.contains(value);
    }

    void add(String value) {
        if (!contains(value)) {   // the check that enforces uniqueness
            items.add(value);
        }
    }

    void remove(String value) {
        if (contains(value)) {
            items.remove(value);
        }
    }

    boolean isSubsetOf(MySet other) {
        for (String value : items) {
            if (!other.contains(value)) {
                return false;          // found one element NOT in other: not a subset
            }
        }
        return true;              // every element checked and found: it is a subset
    }
}

// Java's real Set type, a separate skill from building one by hand
Set<String> subscribers = new HashSet<>(Arrays.asList("Amir", "Beth", "Chen"));
Set<String> notified = new HashSet<>(Arrays.asList("Beth", "Chen", "Dara"));

System.out.println(subscribers.contains("Beth"));       // membership test
System.out.println(subscribers.containsAll(notified));  // superset test