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

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 11 if the specified key exists (as unordered maps have only one unique key) and 00 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;
}

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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