unordered_map in C++ STL
unordered_map is an associated container that stores elements formed by the combination of a key value and a mapped value. The key value is used to uniquely identify the element and the mapped value is the content associated with the key. Both key and value can be of any type predefined or user-defined. In simple terms, an unordered_map is like a data structure of dictionary type that stores elements in itself. It contains successive pairs (key, value), which allows fast retrieval of an individual element based on its unique key.
Internally unordered_map is implemented using Hash Table, the key provided to map is hashed into indices of a hash table which is why the performance of data structure depends on the hash function a lot but on average, the cost of search, insert, and delete from the hash table is O(1).
Note: In the worst case, its time complexity can go from O(1) to O(n), especially for big prime numbers. In this situation, it is highly advisable to use a map instead to avoid getting a TLE(Time Limit Exceeded) error.
Below is the C++ program to demonstrate an unordered map:
Contribute 30 Practice 20 GeeksforGeeks 10
Explanation: The specific thing that this output justifies is that the value of the outcome of unordered_map is produced in a random key-to-value manner whereas the map displays value and key in an ordered manner.
unordered_map vs unordered_set
|Unordered_map contains elements only in the form of (key-value) pairs.||Unordered_set does not necessarily contain elements in the form of key-value pairs, these are mainly used to see the presence/absence of a set.|
|Operator ‘’ to extract the corresponding value of a key that is present in the map.||The searching for an element is done using a find() function. So no need for an operator.|
Note: For example, consider the problem of counting the frequencies of individual words. We can’t use unordered_set (or set) as we can’t store counts while we can use unordered_map.
|The unordered_map key can be stored in any order.||The map is an ordered sequence of unique keys|
|Unordered_Map implements an unbalanced tree structure due to which it is not possible to maintain order between the elements||Map implements a balanced tree structure which is why it is possible to maintain order between the elements (by specific tree traversal)|
|The time complexity of unordered_map operations is O(1) on average.||The time complexity of map operations is O(log n)|
Methods on unordered_map
A lot of functions are available that work on unordered_map. The most useful of them are:
- operator =
- operator 
- size for capacity
- begin and end for the iterator.
- find and count for lookup.
- insert and erase for modification.
The below table shows the complete list of the methods of an unordered_map:
|at()||This function in C++ unordered_map returns the reference to the value with the element as key k|
|begin()||Returns an iterator pointing to the first element in the container in the unordered_map container|
|end()||Returns an iterator pointing to the position past the last element in the container in the unordered_map container|
|bucket()||Returns the bucket number where the element with the key k is located in the map|
|bucket_count||Bucket_count is used to count the total no. of buckets in the unordered_map. No parameter is required to pass into this function|
|bucket_size||Returns the number of elements in each bucket of the unordered_map|
|count()||Count the number of elements present in an unordered_map with a given key|
|equal_range||Return the bounds of a range that includes all the elements in the container with a key that compares equal to k|
|find()||Returns iterator to the element|
|empty()||Checks whether the container is empty in the unordered_map container|
|erase()||Erase elements in the container in the unordered_map container|
The C++11 library also provides functions to see internally used bucket count, bucket size, and also used hash function and various hash policies but they are less useful in real applications. We can iterate over all elements of unordered_map using Iterator.
Found PI lambda not found All Elements : e 2.718 loge 1 log10 2.302 Two 2 One 1 Three 3 PI 3.14 root2 1.414 root3 1.732
Find frequencies of individual words
Given a string of words, the task is to find the frequencies of the individual words:
Input: str = “geeks for geeks geeks quiz practice qa for”;
Output: Frequencies of individual words are
Below is the C++ program to implement the above approach:
(practice, 1) (for, 2) (qa, 1) (quiz, 1) (geeks, 3)
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This article is contributed by Utkarsh Trivedi. Please write comments if you find anything incorrect, or if you want to share more information about the topic discussed above.