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Introduction to C++ unordered_map

In C++, Unordered maps are considered associative containers, which helps store elements generated by the key-value and mapped value combination. This function permits the fast retrieval of separate elements that are based on their keys. Here, the key value is mainly used to find the item uniquely, and the mapped value is considered an object with the content linked to this key. There may be differences in the types of key-value and mapped value. Let us see more about the unordered map in the following sections. In this topic, we are going to learn about C++ unordered_map.

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Definition

classunordered_map;

The parameters are:

K, which is the key type.

T, which is then mapped value type

Hash, a type of unary function object that gets an object of key type as a parameter and returns a specific value of size t.

Pred, this is a binary predicate.

Alloc, which is the type of object of the allocator.

T may be replaced by any data type containing a user-defined type.

Member Types of C++ unordered_map

Below are the member types that can be used by member functions as arguments or return types.

Member types Description

key_type Key; Parameter 1 used for the template

mapped_type T; Parameter 2 used for the template

hasher

;Parameter 3 used for the template

key_equal

Parameter 4 used for the template

allocator_type Alloc; Parameter 5 used for the template

value_type

reference value_type&

const_reference constvalue_type&

difference_type ptrdiff_t

size_type size_t

pointer

iterator A forward iterator for the value_type value_type

local_iterator A forward iterator for the value_type

const_iterator A forward iterator for the constvalue_type value_type

const_pointer

const_local_iterator A forward iterator for the constvalue_type

Constructors

The following are the constructors of the c++ unordered map.

unordered_map::unordered_mapdefault constructor

An empty unordered_map will be constructed with a number of elements as zero.

unordered_map::unordered_mapcopy constructor

An unordered_map will be constructed with each element’s copy. Thus, these elements will be already on the existing map.

unordered_map::unordered_mapmove constructor

An unordered_map will be constructed with the content present in another map using the semantics move.

unordered_map::unordered_maprange constructor

An unordered_map will be constructed with items in the range from first to last.

unordered_map::unordered_mapinitializer_list constructor

An unordered_map will be constructed from the initializer list.

Methods on unordered_map How unordered_map function work in C++?

In unordered_map, the elements are not sorted initially based on any particular order with respect to key values or mapped values. Instead, it is but structured into buckets subject to the hash values to permit fast access to distinct items directly by their values of keys.

Moreover, the containers of the unordered maps are faster than the containers of the map to access distinct elements based on their key, even though they are usually less efficient for iteration based on range through their elements subset.

These unordered maps implement the operator [], also known as a direct access operator that permits the mapped value direct access using its key value.

Let us understand more about the unordered map using the sample code.

Define the unordered_map with some elements

{‘H’, 21} , {‘I’, 52} , {‘J’, 36} , {‘K’, 47} , {‘L’, 54} };

Define an Iterator itr

auto itr = mp.find('L');

print the values based on the requirement

Examples of C++ unordered_map

To understand more about the unordered map, let us work with some sample programs.

Example #1

C++ program to find a particular element in an unordered map.

Code:

//use the namespace as std using namespace std; int main(void) { {‘H’, 21} , {‘I’, 52} , {‘J’, 36} , {‘K’, 47} , {‘L’, 54} }; auto itr = mp.find(‘L’); return 0; }

Output:

First, use the namespace as std. Then, define the unordered_map with elements {‘H’, 21} ,{‘I’, 52} , {‘J’, 36} , {‘K’, 47} , {‘L’, 54. Once the elements are defined, use the iterator to find the element L and key that is linked to that particular character. On executing the code, the value linked to L will be printed.

Example #2

C++ program to print all the elements in an unordered map.

Code:

//use the namespace as std using namespace std; int main() { mp1[1] = ‘a’; mp1[2] = ‘b’; mp1[3] = ‘c’; mp1[4] = ‘d’; mp1[5] = ‘e’; mp1[6] = ‘f’; mp1[7] = ‘g’; mp1[8] = ‘h’; cout<< “Key valuet corresponding element” <<endl; for (crs = mp1.begin(); crs != mp1.end(); crs++) { <<endl; } }

Output:

In this program, also, first, use the namespace as std. Then, define the unordered_map with elements {and an iterator. Once the elements are defined, use the iterator to find all the elements and key that is linked to that particular character. It is done with the help of using begin() and end () functions. Finally, on executing the code, all the elements and corresponding values will be printed.

Conclusion

Unordered maps are the associative containers that help in storing elements generated by the key-value and mapped value combination. In this article, different aspects such as definition, constructors, methods, working, and examples of the unordered map are explained in detail.