How to Find the Mode of All Elements in a List in C++?

In C++, a list is a sequence container used to store data of similar type in non-contiguous memory locations. The mode of a list represents the element that occurs most frequently in the container. In this article, we will learn how to find the mode of a list in C++.

Example:

Input:
myList = {1, 2, 3, 4, 5, 2, 2, 3, 4, 4, 4};

Output: 
Mode of the list is :4

Finding the Mode of All Elements in a List in C++

To find the mode of all elements in a std::list in C++, we need to count the frequency of all elements in the list. To do that, we can use the std::unordered_map container where the key will represent the list element and its value will represent its frequency.

Approach

• Create an unordered_map of <int, int>.
• Iterate over the list and if the list element is present in the unordered_map, increment its value.
• If the list element is not present, add the list element as a key with value 1.
• Iterate again on the map and find the element with the highest frequency.
• Finally, print the element with the highest frequency which will denote the mode of the list elements.

C++ Program to Find the Mode of a List

The following program demonstrates how we can find the mode of a list in C++.

C++

// C++ program to find the mode of a list

#include <iostream>
#include <list>
#include <unordered_map>
using namespace std;

int main()
{
    // Initializing  a list of integers
    list<int> myList = { 1, 2, 3, 4, 5, 2, 2, 3, 4, 4, 4 };

    // Creating a map to store the frequency of each element
    unordered_map<int, int> mp;

    // Iterating over the list and updating the frequency of
    // elements
    for (int num : myList) {
        mp[num]++;
    }

    // Finding the maximum frequency from the map
    int maxCount = 0, mode = 0;
    for (auto it : mp) {
        if (it.second > maxCount) {
            maxCount = it.second;
            mode = it.first;
        }
    }

    // Printing the mode of the list
    cout << "Mode of the list is : " << mode << endl;

    return 0;
}

Output

Mode of the list is : 4

Time Complexity: O(N), where N is the size of the list.
Auxiliary Space: O(N)