unordered_map erase in C++ STL (original) (raw)

Last Updated : 11 Jul, 2025

in C++, **std::unordered_map::erase() is a built-in function used remove elements from the unordered_map container. It is a member function of std::unordered_map class defined inside <unordered_map> header file.

**Example:

C++ `

// C++ Program to demonstrate the use of // unordered_map::erase() #include <bits/stdc++.h> using namespace std;

int main() { unordered_map<int, string> um = {{1, "Geeks"}, {3, "GeeksforGeeks"}, {2, "Geeksfor"}};

um.erase(3);

for (auto i : um)
    cout << i.first << ": " << i.second << endl;
return 0;

}

`

Output

2: Geeksfor 1: Geeks

unordered_map::erase() Syntax

C++ provides 3 implementations of the **std::unordered_map::erase() method:

um.**erase(k); // _Erases by key
um.**erase(it); // _Erases by iterator
um.**erase(first, last); // Erases range of elements

These implementations can be used to do erase the elements in the following ways:

Table of Content

• Erase an Element by Key
• Erase an Element by Iterator
• Erase a Range of Elements

Erase an Element by Key

With the help of std::unordered_map::erase() function, we can erase the specified key from the **std::unordered_map container.

Syntax

um.**erase(k);

**Parameters

• **k: Key of the element to be removed.

**Return Value

• Returns the number of elements removed from the std::unordered_map container. Can only be 0 or 1.

Example

C++ `

// C++ program to remove element by key // suing unordered_map::erase #include <bits/stdc++.h> using namespace std;

int main() { unordered_map<int, string> um = {{1, "Geeks"}, {3, "GeeksforGeeks"}, {2, "Geeksfor"}};

// Remove the element with key 2
um.erase(2);

for (auto i: um)
    cout << i.first << ": " << i.second << '\n';
return 0;

}

`

Output

1: Geeks 3: GeeksforGeeks

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

Erase an Element by Iterator

We can also use unordered_map::erase() to delete an element at a specific position using an iterator. This method is useful when we are iterating the map and removing the elements based on some given conditions.

Syntax

um.**erase(it);

**Parameters

• **it: Iterator to the element to be removed.

**Return Value

• Returns an iterator point to the element that is just after the removed element.
• If there is no such element, returns iterator to unordered_map::end().

Example

C++ `

// C++ program to remove element by iterator // using unordered_map::erase() #include <bits/stdc++.h> using namespace std;

int main() { unordered_map<int, string> um = {{1, "Geeks"}, {3, "GeeksforGeeks"}, {2, "Geeksfor"}};

// Remove the second element
um.erase(next(um.begin(), 1));

for (auto i: um)
    cout << i.first << ": " << i.second << '\n';
return 0;

}

`

Output

2: Geeksfor 3: GeeksforGeeks

**Time Complexity: O(1)
**Auxiliary Space: O(1)

Erase a Range of Elements

We can also remove a sequence of elements in one operation using std::unordered_map::erase() method by defining the range using iterators.

Syntax

um.**erase(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

• Returns an iterator point to the element that is just after the removed range.
• If there is no such element, returns iterator to unordered_map::end().

Example

C++ `

// C++ program to remove range of elements // using unordered_map::erase() #include <bits/stdc++.h> using namespace std;

int main() { unordered_map<int, string> um = {{1, "Geeks"}, {3, "GeeksforGeeks"}, {2, "Geeksfor"}};

// Remove all elements between 2nd element to
  // third element
um.erase(next(um.begin(), 1), next(um.begin(),
                                   3));

for (auto i: um)
    cout << i.first << ": " << i.second << '\n';
return 0;

}

`

**Time Complexity: O(k) in average case, where k is number of elements between first and last, O(n) in worst case, where n is the number of elements in unordered_map.
**Auxiliary Space: O(1)