Mode of frequencies of given array elements

Difficulty Level : Expert
Last Updated : 17 Sep, 2020

Given an array arr[], the task is to find the mode of frequencies of elements of the given array.

Examples:

Input: arr[] = {6, 10, 3, 10, 8, 3, 6, 4}, N = 8
Output: 2
Explanation:
Here (3, 10 and 6) have frequency 2, while (4 and 8) have frequency 1.
Three numbers have frequency 2, while 2 numbers have frequency 1.
Thus, the mode of the frequencies is 2.
Input: arr[] = {5, 9, 2, 9, 7, 2, 5, 3, 1}, N = 9
Output:1

Approach: The idea is to find the frequency of the frequencies of all array elements. Finally, compute the mode of the frequencies.

Below is the implementation of the above approach:

C++

// C++ program of the
// above approach
  
#include <bits/stdc++.h>
using namespace std;
  
// Function to find the mode of the
// frequency of the array
int countFreq(int arr[], int n)
{
  
    // Stores the frequencies
    // of array elements
    unordered_map<int, int> mp1;
  
    // Traverse through array
    // elements and
    // count frequencies
    for (int i = 0; i < n; ++i) {
        mp1[arr[i]]++;
    }
  
    // Stores the frequencies's of
    // frequencies of array elements
    unordered_map<int, int> mp2;
  
    for (auto it : mp1) {
        mp2[it.second]++;
    }
  
    // Stores teh minimum value
    int M = INT_MIN;
  
    // Find the Mode in 2nd map
    for (auto it : mp2) {
        M = max(M, it.second);
    }
  
    // search for this Mode
    for (auto it : mp2) {
        // When mode is find then return
        // to main function.
        if (M == it.second) {
            return it.first;
        }
    }
  
    // If mode is not found
    return 0;
}
  
// Driver Code
int main()
{
    int arr[] = { 6, 10, 3, 10, 8, 3, 6, 4 };
    int n = sizeof(arr) / sizeof(arr[0]);
    cout << countFreq(arr, n);
    return 0;
}

Java

// Java program of the
// above approach
import java.util.*;
  
class GFG{
  
// Function to find the mode of the
// frequency of the array
static int countFreq(int arr[], int n)
{
      
    // Stores the frequencies
    // of array elements
    HashMap<Integer,
            Integer> mp1 = new HashMap<Integer,
                                       Integer>();
  
    // Traverse through array
    // elements and
    // count frequencies
    for(int i = 0; i < n; ++i)
    {
        if (mp1.containsKey(arr[i]))
        {
            mp1.put(arr[i], mp1.get(arr[i]) + 1);
        }
        else
        {
            mp1.put(arr[i], 1);
        }
    }
  
    // Stores the frequencies's of
    // frequencies of array elements
    HashMap<Integer,
            Integer> mp2 = new HashMap<Integer,
                                       Integer>();
  
    for(Map.Entry<Integer,
                  Integer> it : mp1.entrySet())
    {
        if (mp2.containsKey(it.getValue()))
        {
            mp2.put(it.getValue(),
            mp2.get(it.getValue()) + 1);
        }
        else
        {
            mp2.put(it.getValue(), 1);
        }
    }
  
    // Stores teh minimum value
    int M = Integer.MIN_VALUE;
  
    // Find the Mode in 2nd map
    for(Map.Entry<Integer,
                  Integer> it : mp2.entrySet())
    {
        M = Math.max(M, it.getValue());
    }
  
    // Search for this Mode
    for(Map.Entry<Integer,
                  Integer> it : mp2.entrySet())
    {
          
        // When mode is find then return
        // to main function.
        if (M == it.getValue()) 
        {
            return it.getKey();
        }
    }
  
    // If mode is not found
    return 0;
}
  
// Driver Code
public static void main(String[] args)
{
    int arr[] = { 6, 10, 3, 10, 8, 3, 6, 4 };
    int n = arr.length;
      
    System.out.print(countFreq(arr, n));
}
}
  
// This code is contributed by Amit Katiyar

Python3

# Python3 program of the
# above approach
from collections import defaultdict
import sys
  
# Function to find the mode of the
# frequency of the array
def countFreq(arr, n):
  
  # Stores the frequencies
  # of array elements
  mp1 = defaultdict (int)
  
  # Traverse through array
  # elements and
  # count frequencies
  for i in range (n):
    mp1[arr[i]] += 1
  
    # Stores the frequencies's of
    # frequencies of array elements
    mp2 = defaultdict (int)
  
    for it in mp1:
      mp2[mp1[it]] += 1
  
      # Stores teh minimum value
      M = -sys.maxsize - 1
  
      # Find the Mode in 2nd map
      for it in mp2:
        M = max(M, mp2[it])
  
        # search for this Mode
        for it in mp2:
            
          # When mode is find then return
          # to main function.
            
          if (M == mp2[it]):
            return it
  
          # If mode is not found
          return 0
  
# Driver Code
if __name__ == "__main__":
  
  arr = [6, 10, 3, 10,
         8, 3, 6, 4]
  n = len(arr)
  print (countFreq(arr, n))
      
# This code is contributed by Chitranayal

C#

// C# program of the
// above approach
using System;
using System.Collections.Generic;
  
class GFG{
  
// Function to find the mode of the
// frequency of the array
static int countFreq(int []arr, int n)
{
      
    // Stores the frequencies
    // of array elements
    Dictionary<int,
               int> mp1 = new Dictionary<int,
                                         int>();
  
    // Traverse through array
    // elements and
    // count frequencies
    for(int i = 0; i < n; ++i)
    {
        if (mp1.ContainsKey(arr[i]))
        {
            mp1[arr[i]] = mp1[arr[i]] + 1;
        }
        else
        {
            mp1.Add(arr[i], 1);
        }
    }
  
    // Stores the frequencies's of
    // frequencies of array elements
    Dictionary<int,
               int> mp2 = new Dictionary<int,
                                         int>();
  
    foreach(KeyValuePair<int, int> it in mp1)
    {
        if (mp2.ContainsKey(it.Value))
        {
            mp2[it.Value] =
            mp2[it.Value] + 1;
        }
        else
        {
            mp2.Add(it.Value, 1);
        }
    }
  
    // Stores teh minimum value
    int M = int.MinValue;
  
    // Find the Mode in 2nd map
    foreach(KeyValuePair<int, int> it in mp2)
    {
        M = Math.Max(M, it.Value);
    }
  
    // Search for this Mode
    foreach(KeyValuePair<int, int> it in mp2)
    {
          
        // When mode is find then return
        // to main function.
        if (M == it.Value) 
        {
            return it.Key;
        }
    }
  
    // If mode is not found
    return 0;
}
  
// Driver Code
public static void Main(String[] args)
{
    int []arr = { 6, 10, 3, 10, 8, 3, 6, 4 };
    int n = arr.Length;
      
    Console.Write(countFreq(arr, n));
}
}
  
// This code is contributed by Amit Katiyar

Output:

Time Complexity: O(N)
Auxiliary Space: O(N)

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