unordered_map insert in C++ STL

The std::unordered_map::insert() in C++ STL is a built-in function used to insert a key-value pair in unordered_map container. As unordered maps only store unique elements, this function does not insert elements with duplicate keys. In this article, we will learn about std::unordered_map::insert() in C++.

Example:

// C++ program to illustrate the use of // unordered_map::insert() #include <bits/stdc++.h> using namespace std; int main() {  unordered_map<int, string> um;  // Insert single elements in random order  um.insert({1, "one"});    // Insert multiple elements  um.insert({{2, "two"}, {4, "four"}});  for (auto i : um)  cout << i.first << ": " << i.second << '\n';  return 0; } 

4: four 2: two 1: one 

unordered_map::insert() Syntax

um.insert({k, v}) // For single element
um.insert(pos, {k, v}) // For single element near pos
um.insert({ {k1, v1}, {k2, v2}, ….}); // For multiple elements
um.insert(first, last); // For range

We can use these overloads for different ways to insert elements in std::map() in C++:

Insert a Single Element

unordered_map::insert() method can be used to insert the single key value pair in std::unordered_map container.

Syntax

um.insert({k, v});

Parameters

• {k, v}: Key-value pair to be inserted.

Return Value

• Returns a pair, where pair::first is an iterator to either inserted element or the element with same key in the map.
• The pair::second tells whether the insertion was successful or not.

Example

// C++ program to insert a single element using // unordered_map::insert method #include <bits/stdc++.h> using namespace std; int main() {  unordered_map<int, string> um;  // Insert single elements one by one  um.insert({1, "one"});  um.insert({4, "four"});  um.insert({2, "two"});  // Trying to insert duplicate key  um.insert({2, "TWO"});  for (auto i : um)  cout << i.first << '\t' << i.second << '\n';  return 0; } 

2	two 4	four 1	one 

Time Complexity: O(1) average, O(n) worst, where n is the number of elements in unordered_map
Auxiliary Space: O(1)

Insert Element Near Given Position

We can also use the unordered_map::insert() function to insert the key-value pair near the given position. The std::unordered_map are stored according to their hash codes. We cannot force the insertion at any particular position, so the given position only gives a hint to unordered_map::insert() function.

Syntax

um.insert(pos, {k, v});

Parameters

• {k, v}: Key-value pair to be inserted.
• pos: Iterator to the position near which the new element is to be inserted.

Return Value

• Returns an iterator pointing to either the inserted element or the element with same key in the map.

Example

// C++ program to insert a pair near some given // position using unordered_map::insert() #include <bits/stdc++.h> using namespace std; int main() {  unordered_map<int, string> um;  um.insert({1, "one"});  um.insert({4, "four"});  // Finding the position to insert  auto it = um.find(4);  // Inserting {2, "two"} starting the search  // from position where 4 is present  um.insert(it, {2, "two"});  for (auto i : um)  cout << i.first << '\t' << i.second << '\n';  return 0; } 

2	two 4	four 1	one 

Time Complexity: O(1) average, O(n) worst, where n is the number of elements in unordered_map
Auxiliary Space: O(1)

Insert Multiple Elements

We can also use the std::unordered_map::insert() method to insert multiple elements at once using initializer list.

Syntax

um.insert({ {k1, v1}, {k2, v2}, …});

Parameters

• {k1, v1}, {k2, v2}, …: First pair, second pair and so on inside { } braces.

Return Value

• This function does not return anything.

Example

// C++ program to insert multiple key-value pairs // using initializer list with unordered_map::insert() #include <bits/stdc++.h> using namespace std; int main() {  unordered_map<int, string> um;  // Insert multiple key-value pairs using  // initializer list  um.insert({{1, "one"}, {2, "two"}, {4, "four"}});  // Display the elements of the unordered_map  for (auto i : um)  cout << i.first << '\t' << i.second << '\n';  return 0; } 

4	four 2	two 1	one 

Time Complexity: O(k) average, O(n * k) worst, where n is the number of elements in unordered_map.
Auxiliary Space: O(k), where k is the number of elements to be inserted.

Insert Elements from Given Range

The unordered_map::insert() function can also be used to insert elements from the given range. This range can by any STL container or an array.

Syntax

um.insert(first, last);

Parameters

• first: Iterator to the first element of the range.
• last: Iterator to the element just after the last element of the range.

Return Value

• This function does not return anything.

Example

// C++ program to insert multiple elements from // given range using unordered_map::insert() #include <bits/stdc++.h> using namespace std; int main() {  unordered_map<int, string> um;  // Vector of pairs to insert into unordered_map  vector<pair<int, string>> v = {{1, "one"},  {2, "two"}, {4, "four"}};  // Insert elements from the defined range  um.insert(v.begin(), v.end());  for (auto i : um)  cout << i.first << '\t' << i.second << '\n';  return 0; } 

4	four 2	two 1	one 

Time Complexity: O(k) average, O(n * k) worst, where n is the number of elements in unordered_map.
Auxiliary Space: O(k), where k is the number of elements to be inserted.