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Count pairs made up of an element divisible by the other from an array consisting of powers of 2
  • Last Updated : 11 Feb, 2021

Given an array arr[] consisting of N powers of 2, the task is to count the number of pairs (arr[i], arr[j]) such that i < j and arr[j] is divisible by arr[i].

Examples:

Input: arr[] = {4, 16, 8, 64}
Output: 5
Explanation:
The pairs satisfying the given condition are {4, 16}, {4, 8}, {4, 64}, {16, 64}, {8, 64}.

Input: arr[] = {2, 4, 8, 16}
Output: 6
Explanation:
The pairs satisfying the given condition are {2, 4}, {2, 8}, {2, 16}, {4, 8}, {4, 16}, {8, 16}.

Naive Approach: The simplest approach is to generate all pairs of the given array arr[] and for each pair, check if arr[j] % arr[i] is 0 or not. If found to be true, increment count by 1. Finally, print the value of count after checking for all pairs. 
Time Complexity: O(N2)
Auxiliary Space: O(1)

Efficient Approach: To optimize the above approach, the idea is based on the observation that any power of 2 has only one set bit in its binary representation. For any such element arr[j], all the powers of 2 which have their set bits at a position less than or equal to the position of set bit of arr[j], will satisfy the given condition. Follow the steps below to solve the problem:



  • Initialize an auxiliary array setBits of size equal to 31, and initialize count as 0 to store the number of required pairs.
  • Traverse the array arr[] using the variable i and perform the following operations:
  • After completing the above steps, print the value of count as the result.

Below is the implementation of the above approach:

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
 
// Function to count the number of
// pairs as per the given conditions
void numberOfPairs(int arr[], int N)
{
    // Initialize array set_bits as 0
    int set_bits[31] = { 0 };
 
    // Store the total number of
    // required pairs
    int count = 0;
 
    // Traverse the array arr[]
    for (int i = 0; i < N; i++) {
 
        // Store arr[i] in x
        int x = arr[i];
 
        // Store the position of the
        // leftmost set bit in arr[i]
        int bitpos = -1;
 
        while (x > 0) {
 
            // Increase bit position
            bitpos++;
 
            // Divide by 2 to shift bits
            // in right at each step
            x /= 2;
        }
 
        // Count of pairs for index i
        // till its set bit position
        for (int j = 0;
             j <= bitpos; j++) {
            count += set_bits[j];
        }
 
        // Increasing count of set bit
        // position of current elelement
        set_bits[bitpos]++;
    }
 
    // Print the answer
    cout << count;
}
 
// Driver Code
int main()
{
    int arr[] = { 4, 16, 8, 64 };
    int N = sizeof(arr) / sizeof(arr[0]);
 
    // Function Call
    numberOfPairs(arr, N);
 
    return 0;
}

Java




// Java program for the above approach
import java.util.*;
class GFG
{
 
// Function to count the number of
// pairs as per the given conditions
static void numberOfPairs(int arr[], int N)
{
   
    // Initialize array set_bits as 0
    int []set_bits = new int[31];
    Arrays.fill(set_bits, 0);
 
    // Store the total number of
    // required pairs
    int count = 0;
 
    // Traverse the array arr[]
    for (int i = 0; i < N; i++)
    {
 
        // Store arr[i] in x
        int x = arr[i];
 
        // Store the position of the
        // leftmost set bit in arr[i]
        int bitpos = -1;
        while (x > 0)
        {
 
            // Increase bit position
            bitpos++;
 
            // Divide by 2 to shift bits
            // in right at each step
            x /= 2;
        }
 
        // Count of pairs for index i
        // till its set bit position
        for (int j = 0;
             j <= bitpos; j++)
        {
            count += set_bits[j];
        }
 
        // Increasing count of set bit
        // position of current elelement
        set_bits[bitpos]++;
    }
 
    // Print the answer
    System.out.println(count);
}
 
// Driver Code
public static void main(String args[])
{
    int arr[] = { 4, 16, 8, 64 };
    int N = arr.length;
 
    // Function Call
    numberOfPairs(arr, N);
}
}
 
// This code is contributed by SURENDRA_GANGWAR.

Python3




# Python3 program for the above approach
 
# Function to count the number of
# pairs as per the given conditions
def numberOfPairs(arr, N):
   
    # Initialize array set_bits as 0
    set_bits = [0]*31
 
    # Store the total number of
    # required pairs
    count = 0
 
    # Traverse the array arr[]
    for i in range(N):
 
        # Store arr[i] in x
        x = arr[i]
 
        # Store the position of the
        # leftmost set bit in arr[i]
        bitpos = -1
 
        while (x > 0):
 
            # Increase bit position
            bitpos += 1
 
            # Divide by 2 to shift bits
            # in right at each step
            x //= 2
 
        # Count of pairs for index i
        # till its set bit position
        for j in range(bitpos + 1):
            count += set_bits[j]
 
        # Increasing count of set bit
        # position of current elelement
        set_bits[bitpos] += 1
 
    # Prthe answer
    print (count)
 
# Driver Code
if __name__ == '__main__':
    arr = [4, 16, 8, 64]
    N = len(arr)
 
    # Function Call
    numberOfPairs(arr, N)
 
    # This code is contributed by mohit kumar 29.

C#




// C# program for the above approach
using System;
class GFG
{
 
// Function to count the number of
// pairs as per the given conditions
static void numberOfPairs(int[] arr, int N)
{
   
    // Initialize array set_bits as 0
    int []set_bits = new int[31];
    for (int i = 0; i < N; i++)
    {
        set_bits[i] = 0;
    }
 
    // Store the total number of
    // required pairs
    int count = 0;
 
    // Traverse the array arr[]
    for (int i = 0; i < N; i++)
    {
 
        // Store arr[i] in x
        int x = arr[i];
 
        // Store the position of the
        // leftmost set bit in arr[i]
        int bitpos = -1;
        while (x > 0)
        {
 
            // Increase bit position
            bitpos++;
 
            // Divide by 2 to shift bits
            // in right at each step
            x /= 2;
        }
 
        // Count of pairs for index i
        // till its set bit position
        for (int j = 0;
             j <= bitpos; j++)
        {
            count += set_bits[j];
        }
 
        // Increasing count of set bit
        // position of current elelement
        set_bits[bitpos]++;
    }
 
    // Print the answer
    Console.Write(count);
}
 
// Driver Code
static public void Main()
{
    int[] arr = { 4, 16, 8, 64 };
    int N = arr.Length;
 
    // Function Call
    numberOfPairs(arr, N);
}
}
 
// This code is contributed by splevel62.
Output: 
5

 

Time Complexity: O(N*log M), where M is the largest element in the array.
Auxiliary Space: O(1)

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