How to Modify a Key in a C++ Map or Set

How to Modify a Key in a C++ Map or Set

C++ maps and sets allow you to store data with a key-value pair, making it easy to access the value associated with a specific key. However, if you need to change the value associated with a specific key, you may have difficulty doing so. Fortunately, C++ provides an easy way to modify keys in maps and sets.

Modifying Keys in Sets

If you have a set that contains elements of type T, then you can use the find() and erase() methods to modify the key of an element. To find an element, you use find() to locate the iterator pointing to the desired element by passing in the value of the key you are looking for. Then, you can erase that element and insert the new one with the modified key.

set<T> s;

// Find element with key 'oldKey'
auto it = s.find(oldKey);
if (it != s.end()) {
    // Erase old element
    s.erase(it);

    // Insert new element with modified key
    s.insert(newKey);
}

Modifying Keys in Maps

Modifying a key in a map is similar to a set, but slightly different. To find an element with a specific key, you use the find() method, like before. However, you can’t just erase and insert the modified key, because a map holds a pair<K, V>, so you have to construct a new pair with the modified key and the same value.

map<K, V> m;

// Find element with key 'oldKey'
auto it = m.find(oldKey);
if (it != m.end()) {
    // Create new pair with modified key
    auto modifiedPair = make_pair(newKey, it->second);

    // Erase old pair
    m.erase(it);

    // Insert new modified pair
    m.insert(modifiedPair);
}

By using the find(), erase(), and insert() methods of sets and maps, you can easily modify the key of an element in C++.