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# What is the unordered_map::erase() function in C++?

Harsh Jain

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In this shot, we will learn how to use the unordered_map::erase() function in C++.

### Introduction

The unordered_map::erase() function is available in the <unordered_map> header file in C++.

The unordered_map::erase() removes some particular key-value pairs from the map. It has three variations:

1. Specifying the key: It erases the key-value pair of the specified key from the unordered map.
2. Specifying the iterator: It erases the key-value pair present at that iterator.
3. Specifying the range: It erases the key-value pairs present in the specified range of the two iterators (including the starting iterator but excluding the last).

### Syntax

The syntax of the three variations of the unordered_map::erase() function is given below:

iterator erase(iterator position);
size_type erase(K key);
iterator erase (iterator first, iterator last);


### Parameter

The three variations of the unordered_map::erase() function accepts the following parameters:

• Specifying the key: In this case, the function accepts only one parameter, i.e., the key whose equivalent key-value pair needs to be erased from the map.

• Specifying the iterator: In this case, the function accepts only one parameter, i.e., the iterator, which points to the key-value pair to be erased.

• Specifying the range: In this case, the function accepts two parameters:

• First: The starting iterator of the range from where the key-value pairs are to be erased.
• Last: The ending iterator of the range till the key-value pairs are to be erased.

### Return value

The three variations of the unordered_map::erase() function returns in the following manner:

• Specifying the key: It returns the number of elements erased in this case. In other words, it would return $1$ if the specified key exists (as unordered maps have only one unique key) and $0$ otherwise.

• Specifying the iterator and specifying the range: In this case, it returns an iterator that immediately points to the position following the last elements erased in the unordered map.

### Code

Let’s have a look at the code below:

#include <iostream>#include <unordered_map>using namespace std;int main(){  unordered_map<int,string> umap ={                            {12, "unordered"},                            {16, "map"},                            {89, "in"},                            {66, "C++"}                          };  cout << "After erasing by iterator : \n";  umap.erase(umap.begin());  for (auto p : umap)    cout << p.first << "->" << p.second << endl;  cout << "After erasing by key : \n";  umap.erase(16);  for (auto p : umap)    cout << p.first << "->" << p.second << endl;  cout << "After erasing by range : \n";  auto i = umap.begin();  i++;   umap.erase(i, umap.end());    for (auto p : umap)    cout << p.first << "->" << p.second << endl;  return 0;}
Use unordered_map::erase() function in C++

### Explanation

• In lines 1 and 2, we import the required header files.

• In line 5, we make a main() function.

• From lines 7 to 12, we initialize an unordered map with integer type keys and string type values.

• From lines 14 to 18, we erase elements using an iterator and display the remaining elements.

• From lines 20 to 24, we erase the elements using the key and display the remaining elements.

• From lines 26 to 32, we erase elements using a range and display the remaining elements.

This is how we use the unordered_map::erase() function in C++ to remove some particular key-value pairs from the map.

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Harsh Jain