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unordered_map clear in C++ STL
  • Difficulty Level : Basic
  • Last Updated : 18 Dec, 2018

unordered_map::clear() function is used to remove all elements from the container. When this function is applied to unordered_map its size becomes zero.
Syntax:

 unordered_map_name.clear()

Parameters: This function does not accepts any parameter
Return type: This function return nothing.

Examples:

Input: ump = { {1, 2}, {3, 4}, {5, 6}, {7, 8}}
ump.clear();
Output: ump = { };




// CPP program to illustrate
// Implementation of unordered_map clear() function
#include <bits/stdc++.h>
using namespace std;
  
int main()
{
    // Take any two unordered_map
    unordered_map<int, int> ump1, ump2;
  
    // Inserting values
    ump1[1] = 2;
    ump1[3] = 4;
    ump1[5] = 6;
    ump1[7] = 8;
  
    // Print the size of container
    cout << "Unordered_map size before calling clear function: \n";
    cout << "ump1 size = " << ump1.size() << endl;
    cout << "ump2 size = " << ump2.size() << endl;
  
    // Deleting the  elements
    ump1.clear();
    ump2.clear();
  
    // Print the size of container
    cout << "Unordered_map size after calling clear function: \n";
    cout << "ump1 size = " << ump1.size() << endl;
    cout << "ump2 size = " << ump2.size() << endl;
  
    return 0;
}
Output:
Unordered_map size before calling clear function: 
ump1 size = 4
ump2 size = 0
Unordered_map size after calling clear function: 
ump1 size = 0
ump2 size = 0

What is the application?
clear is used when we wish to delete old elements and start from fresh, especially in loops. We can achieve same functionality by creating a new map, but clearing same map is better performance wise as we do not have to create new object.

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