std::unordered_map::operator[]
T& operator[]( const Key& key );
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(1) | (since C++11) |
T& operator[]( Key&& key );
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(2) | (since C++11) |
Inserts a new element to the container using key
as the key and a default constructed mapped value and returns a reference to the newly constructed mapped value. If an element with key key
already exists, no insertion is performed and a reference to its mapped value is returned.
If an insertion occurs and results in a rehashing of the container, all iterators are invalidated. Otherwise iterators are not affected. References are not invalidated. Rehashing occurs only if the new number of elements is higher than max_load_factor()*bucket_count().
Contents |
[edit] Parameters
key | - | the key of the element to find |
[edit] Return value
Reference to the mapped value of the new element if no element with key key
existed. Otherwise a reference to the mapped value of the existing element is returned.
[edit] Complexity
Average case: constant, worst case: linear in size.
[edit] Example
This example demonstrates how to modify existing values and insert new values using operator[]
:
#include <iostream> #include <unordered_map> int main() { std::unordered_map<char, int> letter_counts {{'a', 27}, {'b', 3}, {'c', 1}}; std::cout << "initially:\n"; for (const auto &pair : letter_counts) { std::cout << pair.first << ": " << pair.second << '\n'; } letter_counts['b'] = 42; // update an existing value letter_counts['x'] = 9; // insert a new value std::cout << "after modifications:\n"; for (const auto &pair : letter_counts) { std::cout << pair.first << ": " << pair.second << '\n'; } }
Output:
initially: a: 27 b: 3 c: 1 after modifications: a: 27 b: 42 c: 1 x: 9
The following example counts the occurrences of each word in a vector of strings:
#include <string> #include <iostream> #include <vector> #include <unordered_map> int main() { std::vector<std::string> words = { "this", "sentence", "is", "not", "a", "sentence", "this", "sentence", "is", "a", "hoax" }; std::unordered_map<std::string, size_t> word_map; for (const auto &w : words) { ++word_map[w]; } for (const auto &pair : word_map) { std::cout << pair.second << " occurrences of word '" << pair.first << "'\n"; } }
Output:
1 occurrences of word 'hoax' 2 occurrences of word 'this' 2 occurrences of word 'a' 2 occurrences of word 'is' 1 occurrences of word 'not' 3 occurrences of word 'sentence'
[edit] See also
access specified element with bounds checking (public member function) |