Product of All Composite Numbers in an Array in C++

Given an array arr[n] of n number of integers, the task is to find the product of all composite numbers in an array.

Composite numbers are the whole numbers which are made by multiplying 2 other whole numbers. For example 6 is a composite number which can be made by multiplying 2 and 3 which are whole numbers. Also we can say they are not prime.

Input 

arr[] = {1, 2, 4, 5, 6, 7}

Output 

24

Explanation − the composite numbers in the array are 4 and 6 and their product is 24.

Input 

arr[] = {10, 2, 4, 5, 6, 11}

Output 

240

Explanation − the composite numbers in the array are 10, 4, 6 and their Product is 240.

Approach used below is as follows to solve the problem

• Iterate every element of an array.

• Find for the non-prime or composite numbers, i.e. divisible by some other number except 1.

• Multiply all the composite numbers.

• Return the result.

Algorithm

Start
Step 1→ Declare function to find the product of consecutive numbers in array
   int product_arr(int arr[], int size)
      declare int max = *max_element(arr, arr + size)
      set vector<bool> prime(max + 1, true)
      set prime[0] = true
      set prime[1] = true
      Loop For int i = 2 and i * i <= max and i++
         IF (prime[i] == true)
            Loop For int j = i * 2 and j <= max and j += i
               Set prime[j] = false
            End
         End
      End
      Set int product = 1
      Loop For int i = 0 and i < size and i++
         IF (!prime[arr[i]])
            Set product *= arr[i]
         End
      End
      return product
Stop

Example

 Live Demo

#include <bits/stdc++.h>
using namespace std;
//function to find product of consecutive numbers in an array
int product_arr(int arr[], int size){
   int max = *max_element(arr, arr + size);
   vector<bool> prime(max + 1, true);
   prime[0] = true;
   prime[1] = true;
   for (int i = 2; i * i <= max; i++){
      if (prime[i] == true){
         for (int j = i * 2; j <= max; j += i)
            prime[j] = false;
      }
   }
   int product = 1;
   for (int i = 0; i < size; i++)
      if (!prime[arr[i]]){
         product *= arr[i];
      }
      return product;
}
int main(){
   int arr[] = { 2, 4, 6, 8, 10};
   int size = sizeof(arr) / sizeof(arr[0]);
   cout<<"product of consecutive numbers in an array: "<<product_arr(arr, size);
   return 0;
}

Output

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

product of consecutive numbers in an array: 1920