Counting frequencies of array elements in C++

We are given an array of integer elements which contains duplicate values and the task is to calculate the frequencies of the distinct elements present in an array and print the result.

Input − int arr[] = {1, 1, 2, 3, 4, 1, 2, 3}

Output −

frequency of 1 is: 3
frequency of 2 is: 2
frequency of 3 is: 2
Frequency of 4 is: 1

Input − int arr[] = {2, 3, 4, 1, 5}

Output −

frequency of 1 is: 1
frequency of 2 is: 1
frequency of 3 is: 1
Frequency of 4 is: 1
Frequency of 5 is: 1

Approach used in the below program is as follows

There can be multiple solutions for this and those can be simpler in coding terms or simpler in complexity terms. So let’s first look at the simpler approach in coding terms

• Create an array of integer type variables

• Calculate the size of an array using size() function.

• Create a boolean array let’s say, check of array size

• Start loop FOR from i to 0 and i less than size

• Inside the loop, set check[i] = 0

• Start loop FOR from i to 0 and i less than size

• Inside the loop, check IF check[i] = 1 then continue

• Declare variable count and initialise it with 1 that will print the frequency count

• Start loop FOR j from i+1 till the size

• Inside the loop, check if arr[i] = arr[j] then set check[j] to 1 and increment the count by 1

• Print the value of count.

Another solution of it can be −

• Create an array of integer type variables

• Calculate the size of an array using size() function.

• Create a variable of type unordered_map let’s say um

• Start loop FOR from i to 0 and till size

• Inside the loop, set um[arr[i]]++

• Start another loop for from auto x till um

• Inside the loop, print the frequency.

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
int frequency(int arr[], int size){
   bool check[size];
   for(int i=0;i<size;i++){
      check[i] = 0;
   }
   for(int i=0; i<size; i++){
      if(check[i]== 1){
         continue;
      }
      int count = 1;
      for(int j = i+1; j<size; j++){
         if (arr[i] == arr[j]){
            check[j] = 1;
            count++;
         }
      }
      cout<<"frequency of "<<arr[i]<<" is: " << count << endl;
   }
}
int main(){
   int arr[] = {1, 2, 3, 1, 2, 3};
   //calculate the size of an array
   int size = sizeof(arr) / sizeof(arr[0]);
   //call function to calculate the frequency
   frequency(arr, size);
   return 0;
}

Output

If we run the above code it will generate the following output −

frequency of 1 is: 2
frequency of 2 is: 2
frequency of 3 is: 2

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
void frequency(int arr[], int size){
   unordered_map<int, int< um;
   for (int i = 0; i < size; i++){
      um[arr[i]]++;
   }
   for (auto x : um){
      cout<<"frequency of "<<x.first<<" is: "<< x.second<< endl;
   }
}
int main(){
   int arr[] = {1, 2, 3, 1, 2, 3 };
   int size = sizeof(arr) / sizeof(arr[0]);
   frequency(arr, size);
   return 0;
}

Output

If we run the above code it will generate the following output −

frequency of 3 is: 2
frequency of 1 is: 2
frequency of 2 is: 2