Different ways to initialize unordered_map in C++

Reading time: 20 minutes | Coding time: 5 minutes

In this article we are going to learn about Unordered_map in C++ and specifically how to initialize them in our code in 4 different ways.

An unordered_map can be initialized in different ways like:

• simple initialization using assignment operator and subscript operator
• initializing using a Initializer List
• initializing using pair of arrays
• initializing from another map

What is unordered_map?

In most simple words unordered_map is a dictionary like data structure.It is available under the c++ STL library.Being a map, it is a sequence of key-value pairs, each single value is mapped to an unique key.You can access any value through its subscript operator[], providing the key inside the operator.This gives fast access to its elements.
The unordered_maps are implemented using hash tables

insert, delete and search operations in an unordered_map takes only o(1) time.

this makes this container one of the most useful data structures to use, specially when we want least time complexity in our code.The secret behind the fast access quality of unordered_maps is because it does not sort its element in any particular order with respect to either their key or mapped values, instead sets them into buckets depending on their hash values.So generally unordered_maps perform better than maps.

some functions you need to know.

1. begin(): Returns an iterator pointing to the first element in the container in the unordered_map container
2. end(): Returns an iterator pointing to the position past the last element in the container in the unordered_map container
3. at(): This function in C++ unordered_map returns the reference to the value with the element as key k.

initializing an unordered_map

An unordered_map can be initialized in different ways like:

• simple initialization using assignment operator and subscript operator
• initializing using a Initializer List
• initializing using pair of arrays
• initializing from another map

1. simple initialization using assignment operator and subscript operator

The key idea of such initialization is as follows:

unordered_map<string, int> demo; 
demo["abc"] = 1; 

Complete example:

#include <iostream> 
#include <unordered_map> 
using namespace std; 

int main() 
{ 
//this map contains keys as strings and values as integers
unordered_map<string, int> demo; 


demo["abc"] = 1; 
demo["pqr"] = 2; 
demo["xyz"] = 3; 

//We can iterate over all elements of unordered_map using Iterator
unordered_map<string, int>:: iterator iter; 
for (iter = umap.begin(); iter != umap.end(); itr++) 
    { 
        // iter is like a pointer to demo<string, int>, itr->first stores the key
        //iter->second stores the value part
        cout << iter->first << "  " << iter->second << endl; 
     } 
return 0;
} 

output

abc 1
pqr 2
xyz 3

2. initializing using a Initializer List

A list can be used to initialize an unordered_map such as:

unordered_map<int, string> m =
    {
        {1, "one"},
        {2, "two"},
        {3, "three"}
    };

Complete example:

#include <iostream>
#include <unordered_map>
using namespace std;
int main()
{
    unordered_map<int, string> m =
    {
        {1, "one"},
        {2, "two"},
        {3, "three"}
    };
    unordered_map<string, int>:: iterator iter;
    for (iter = m.begin(); iter != m.end(); itr++) 
    { 
        
        cout << iter->first << "  " << iter->second << endl; 
     } 
    return 0;
}

output

{3 -> three}
{2 -> two}
{1 -> one}

3. initializing using pair of arrays

Initializing the unordered_map using a pair of arrays:

unordered_map<int, string> m(arr, arr + n);

Complete C++ example:

#include <iostream>
#include <unordered_map>
using namespace std;

int main()
{   //following is the initialisation of an paired container or array.
	pair<int, string> arr[] =
	{
		make_pair(1, "one"),
		make_pair(2, "two"),
		make_pair(3, "three")
	};

	int n = sizeof(arr) / sizeof(arr[0]);
    //now we copy the pair values to our unordered_map
	unordered_map<int, string> m(arr, arr + n);

	
    unordered_map<string, int>:: iterator iter;
	 for (iter = m.begin(); iter != m.end(); itr++) 
    { 
        
        cout << iter->first << "  " << iter->second << endl; 
     } 

	return 0;
}

output

{3 -> three}
{2 -> two}
{1 -> one}

4. initializing from another map

we can simply copy contents from another map using a copy constructor.

unordered_map<int, string> m(map);

Complete C++ example:

#include <iostream>
#include <unordered_map>
using namespace std;
int main()
{
	unordered_map<int, string> map =
	{
		make_pair(1, "one"),
		make_pair(2, "two"),
		make_pair(3, "three")
	};

	unordered_map<int, string> m(map);

	 unordered_map<string, int>:: iterator iter;
	 for (iter = m.begin(); iter != m.end(); itr++) 
    { 
        
        cout << iter->first << "  " << iter->second << endl; 
     } 
	return 0;
}

output

{3 -> three}
{2 -> two}
{1 -> one}

So these were some of the ways of initializing an unordered_map in c++, i hope you liked it. Happy coding