unordered_map in C++ STL

What is an unordered_map in STL?

unordered_map in STL are associative containers where each element consists of a key value and a mapped value. Two mapped values cannot have the same key value. The elements can be in any order.

Syntax:

unordered_map<object_type,object_type> variable_name;

Example:

unordered_map<int,int> mpp;
unordered_map<string,int> mpp;

Functions in Unordered map:

• insert() – to insert an element in the map.

unordered_map<int,int> mp;
mp.insert({1,10});
mp.insert({2,20});

• begin() – return an iterator pointing to the first element in the map.

mp.begin();

• end() – returns an iterator to the theoretical element after the last element.

mp.end();

• clear() – deletes all the elements in the map.

mp.clear();

• find() – to search for an element in the map.

unordered_map<int,int> mp;
mp.insert({1,10});
mp.insert({2,20});
if(mp.find(2)!=mp.end())
cout<<"true"<<endl;

• erase() – to delete a single element or elements between a particular range.

mp.erase(key);
mp.erase(iterator position);
mp.erase(iterator position 1, iterator position 2);

• size() – returns the number of elements on the map.

mp.size();

• empty() – to check if the map is empty or not.

mp.empty();

Code:

#include<bits/stdc++.h>

using namespace std;

int main() {
  unordered_map < int, int > mp;
   for (int i = 1; i <= 5; i++) {
    mp.insert({ i , i * 10});
  }

  cout << "Elements present in the map: " << endl;
  cout << "Key\tElement" << endl;
  for (auto it = mp.begin(); it != mp.end(); it++) {
    cout << it -> first << "\t" << it -> second << endl;
  }

  int n = 2;
  if (mp.find(2) != mp.end())
    cout << n << " is present in map" << endl;

  mp.erase(mp.begin());
  cout << "Elements after deleting the first element: " << endl;
  cout << "Key\tElement" << endl;
  for (auto it = mp.begin(); it != mp.end(); it++) {
    cout << it -> first << "\t" << it -> second << endl;
  }

  cout << "The size of the map is: " << mp.size() << endl;

  if (mp.empty() == false)
    cout << "The map is not empty " << endl;
  else
    cout << "The map is empty" << endl;
  mp.clear();
  cout << "Size of the set after clearing all the elements: " << mp.size();
}

Other functions:

• cbegin() – it refers to the first element of the unordered_map.
• cend() – it refers to the theoretical element after the last element of the unordered_map.
• rbegin() – it points to the last element of the unordered_map.
• rend() – it points to the theoretical element before the first element of the unordered_map.
• emplace() – to insert an element in the unordered_map.
• max_size() – the maximum elements a unordered_map can hold.

Special thanks to Subhrajit Das for contributing to this article on takeUforward. If you also wish to share your knowledge with the takeUforward fam, please check out this article