When to Use Hash Sets

Hash sets are particularly useful in scenarios where you need fast insertion, deletion, and search operations, and where the order of elements is not important. Here are some common use cases where hash sets shine:

Uniqueness Checking

Hash sets are ideal for checking the uniqueness of elements. Since hash sets only store unique elements, you can efficiently check if an element already exists in the set. This is useful in scenarios like removing duplicates from a collection or tracking unique occurrences of elements.

Example of using a hash set for uniqueness checking:

1#include <unordered_set>
2#include <vector>
3
4std::vector<int> removeDuplicates(
5  const std::vector<int>& input) {
6  std::unordered_set<int> uniqueElements(
7    input.begin(), input.end());
8  return std::vector<int>(
9    uniqueElements.begin(), uniqueElements.end());
10}

Fast Membership Testing

Hash sets provide constant-time average-case complexity for checking if an element exists in the set. This is useful when you need to frequently check the presence of elements and want to avoid the linear search time complexity of other data structures like arrays or linked lists.

Example of using a hash set for fast membership testing of words within a dictionary:

1#include <string>
2#include <unordered_set>
3
4bool isWordPresent(
5  const std::unordered_set<std::string>& dict,
6  const std::string& word) {
7  return dict.count(word) > 0;
8}

Implementing Caches

Hash sets can be used to implement caches, where you want to store and quickly retrieve previously computed or frequently accessed data. By using a hash set, you can efficiently check if the data is already in the cache and retrieve it in constant time on average.

Example of using a hash set as a cache:

1#include <string>
2#include <unordered_set>
3
4class Cache {
5 private:
6  std::unordered_set<std::string> data;
7  int capacity;
8
9 public:
10  Cache(int size) : capacity(size) {}
11
12  bool contains(const std::string& key) {
13    return data.count(key) > 0; }
14
15  void insert(const std::string& key) {
16    if (data.size() >= capacity) {
17      // Evict older entries if cache is full
18      // (not shown for simplicity)
19    }
20    data.insert(key);
21  }
22};

Intersection and Union Operations

Hash sets can efficiently perform set operations like intersection and union. By leveraging the fast search capabilities of hash sets, you can quickly find common or unique elements between multiple sets.

Example of using hash sets for intersection and union operations:

1#include <unordered_set>
2
3std::unordered_set<int> setIntersection(
4  const std::unordered_set<int>& set1,
5  const std::unordered_set<int>& set2
6) {
7  std::unordered_set<int> result;
8  for (int element : set1) {
9    if (set2.count(element) > 0) {
10      result.insert(element);
11    }
12  }
13  return result;
14}
15
16std::unordered_set<int> setUnion(
17  const std::unordered_set<int>& set1,
18  const std::unordered_set<int>& set2
19) {
20  std::unordered_set<int> result(set1);
21  result.insert(set2.begin(), set2.end());
22  return result;
23}

These are just a few examples of when hash sets can be particularly useful. In general, hash sets are a go-to choice when you need fast insertion, deletion, and search operations, and when the order of elements is not important. However, it's important to consider the specific requirements of your problem and evaluate the trade-offs with other data structures before making a decision.