Count pairs made up of an element divisible by the other from an array consisting of powers of 2

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 a 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:
  • Store the leftmost set bit of arr[i] in bitPos.
  • Iterate in the range[0, bitPos] using the variable j and increment count by setBits[j] in each step.
  • Increment setBits[bitPos] by 1.
• 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 element
        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 element
        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 element
        set_bits[bitpos] += 1

    # Print the 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 element
        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.

Javascript

<script>

// Javascript program for the above approach

// Function to count the number of
// pairs as per the given conditions
function numberOfPairs(arr, N)
{

    // Initialize array set_bits as 0
    let set_bits = [];
    for (let i = 0; i < 31; i++)
    {
        set_bits[i] = 0;
    }

    // Store the total number of
    // required pairs
    let count = 0;

    // Traverse the array arr[]
    for (let i = 0; i < N; i++)
    {

        // Store arr[i] in x
        let x = arr[i];

        // Store the position of the
        // leftmost set bit in arr[i]
        let bitpos = -1;
        while (x > 0)
        {

            // Increase bit position
            bitpos++;

            // Divide by 2 to shift bits
            // in right at each step
            x = Math.floor( x / 2 );
        }

        // Count of pairs for index i
        // till its set bit position
        for (let j = 0;
             j <= bitpos; j++)
        {
            count += set_bits[j];
        }

        // Increasing count of set bit
        // position of current element
        set_bits[bitpos]++;
    }

    // Print the answer
    document.write(count);
}

// Driver Code

      let arr = [ 4, 16, 8, 64 ];
    let N = arr.length;

    // Function Call
    numberOfPairs(arr, N);

</script>

Output:

5

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