Checking if a Key Exists without Inserting

To check if a key exists in a std::unordered_set without inserting it if it doesn't, you can use the find() method. The find() method searches for an element with a specific key and returns an iterator to it if found, or an iterator to end() if not found.

Here's an example:

1#include <iostream>
2#include <unordered_set>
3
4int main() {
5  std::unordered_set<int> Set{1, 2, 3, 4, 5};
6
7  int Key = 3;
8  auto it = Set.find(Key);  
9
10  if (it != Set.end()) {
11    std::cout << "Key " << Key
12      << " exists in the set\n";
13  } else {
14    std::cout << "Key " << Key
15      << " does not exist in the set\n";
16  }
17
18  Key = 7;
19  it = Set.find(Key);  
20
21  if (it != Set.end()) {
22    std::cout << "Key " << Key
23      << " exists in the set\n";
24  } else {
25    std::cout << "Key " << Key
26      << " does not exist in the set\n";
27  }
28}

1Key 3 exists in the set
2Key 7 does not exist in the set

In this example:

• We create an unordered set Set with some initial elements
• We use find() to search for the key 3 in the set. Since 3 exists, find() returns an iterator to the element.
• We compare the returned iterator with Set.end() to determine if the key was found. If the iterator is not equal to Set.end(), it means the key exists in the set.
• We then search for the key 7, which doesn't exist in the set. In this case, find() returns an iterator equal to Set.end(), indicating that the key was not found.

Using find() allows you to check for the existence of a key without modifying the set. If the key is not found, no insertion takes place.

Note that if you want to insert the key if it doesn't exist, you can use the insert() method instead. The insert() method returns a pair containing an iterator to the inserted element (or the existing element if the key already exists) and a boolean value indicating whether the insertion took place.