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Count pairs from a given array whose product lies in a given range

  • Difficulty Level : Medium
  • Last Updated : 17 Aug, 2021

Given an array arr[] of size N, and integers L and R, the task is to count the number of pairs [arri , arrj] such that i < j and the product of arr[i] * arr[j] lies in the given range [L, R], i.e. L ≤ arr[i] * arr[j] ≤ R.

Examples:

Input: arr[ ] = {4, 1, 2, 5}, L = 4, R = 9
Output: 3
Explanation: Valid pairs are {4, 1}, {1, 5} and {4, 2}.

Input: arr[ ] = {1, 2, 5, 10, 5}, L = 2, R = 15   
Output: 6
Explanation: Valid pairs are {1, 2}, {1, 5}, {1, 10}, {1, 5}, {2, 5}, {2, 5}.

Naive Approach: The simplest approach to solve the problem is to generate all possible pairs from the array and for each pair, check if its product lies in the range [L, R] or not.

Time Complexity: O(N2)
Auxiliary Space: O(1)

Efficient Approach: This problem can be solved by Sorting and Binary Search technique. Follow the steps below to solve this problem:

  • Sort the array arr[].
  • Initialize a variable, say ans as 0, to store the number of pairs whose product lies in the range [L, R].
  • Iterate over  the range [0, N-1] using a variable i and perform the following steps:
    • Find upper bound of an element such that the element is less than equal to R / arr[i].
    • Find lower bound of an element such that the element is greater than or equal to L / arr[i].
    • Add upper bound – lower bound to ans
  • After completing the above steps, print ans.

Below is the implementation of the above approach.



C++




// C++ program for above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to count pairs from an array
// whose product lies in the range [l, r]
void countPairs(int arr[], int l,
                int r, int n)
{
    // Sort the array arr[]
    sort(arr, arr + n);
 
    // Stores the final answer
    int ans = 0;
 
    for (int i = 0; i < n; i++) {
 
        // Upper Bound for arr[j] such
        // that arr[j] <= r/arr[i]
        auto itr1 = upper_bound(
                        arr + i + 1,
                        arr + n, r / arr[i])
                    - arr;
 
        // Lower Bound for arr[j] such
        // that arr[j] >= l/arr[i]
        auto itr2 = lower_bound(
                        arr + i + 1, arr + n,
                        ceil(double(l) / double(arr[i])))
                    - arr;
 
        ans += itr1 - itr2;
    }
 
    // Print the answer
    cout << ans << endl;
}
 
// Driver Code
int main()
{
    // Given Input
    int arr[] = { 2,2 };
    int l = 5, r = 9;
 
    int n = sizeof(arr) / sizeof(arr[0]);
 
    // Function Call
    countPairs(arr, l, r, n);
 
    return 0;
}

Java




// Java program for above approach
import java.util.Arrays;
 
class GFG{
     
// Function to count pairs from an array
// whose product lies in the range [l, r]
public static void countPairs(int[] arr, int l,
                              int r, int n)
{
     
    // Sort the array arr[]
    Arrays.sort(arr);
 
    // Stores the final answer
    int ans = 0;
 
    for(int i = 0; i < n; i++)
    {
         
        // Upper Bound for arr[j] such
        // that arr[j] <= r/arr[i]
        int itr1 = upper_bound(arr, 0, arr.length - 1,
                               l / arr[i]);
 
        // Lower Bound for arr[j] such
        // that arr[j] >= l/arr[i]
        int itr2 = lower_bound(arr, 0, arr.length - 1,
                               l / arr[i]);
        ans += itr1 - itr2;
    }
 
    // Print the answer
    System.out.println(ans);
}
 
public static int lower_bound(int[] arr, int low,
                              int high, int X)
{
     
    // Base Case
    if (low > high)
    {
        return low;
    }
 
    // Find the middle index
    int mid = low + (high - low) / 2;
 
    // If arr[mid] is greater than
    // or equal to X then search
    // in left subarray
    if (arr[mid] >= X)
    {
        return lower_bound(arr, low, mid - 1, X);
    }
 
    // If arr[mid] is less than X
    // then search in right subarray
    return lower_bound(arr, mid + 1, high, X);
}
 
public static int upper_bound(int[] arr, int low,
                              int high, int X)
{
     
    // Base Case
    if (low > high)
        return low;
 
    // Find the middle index
    int mid = low + (high - low) / 2;
 
    // If arr[mid] is less than
    // or equal to X search in
    // right subarray
    if (arr[mid] <= X)
    {
        return upper_bound(arr, mid + 1, high, X);
    }
 
    // If arr[mid] is greater than X
    // then search in left subarray
    return upper_bound(arr, low, mid - 1, X);
}
 
// Driver Code
public static void main(String args[])
{
     
    // Given Input
    int[] arr = { 4, 1, 2, 5 };
    int l = 4, r = 9;
    int n = arr.length;
 
    // Function Call
    countPairs(arr, l, r, n);
}
}
 
// This code is contributed by gfgking.

Python3




# Python program for above approach
 
# Function to count pairs from an array
# whose product lies in the range [l, r]
def countPairs(arr, l, r, n):
 
    # Sort the array arr[]
    arr[::-1]
 
    # Stores the final answer
    ans = 0;
 
    for i in range(n):
 
        # Upper Bound for arr[j] such
        # that arr[j] <= r/arr[i]
        itr1 = upper_bound(arr, 0, len(arr) - 1, l // arr[i])
 
        # Lower Bound for arr[j] such
        # that arr[j] >= l/arr[i]
        itr2 = lower_bound(arr, 0, len(arr) - 1, l // arr[i]);
 
        ans += itr1 - itr2;
 
    # Print the answer
    print(ans);
 
def lower_bound(arr, low, high, X):
 
    # Base Case
    if (low > high):
        return low;
 
    # Find the middle index
    mid = low + (high - low) // 2;
 
    # If arr[mid] is greater than
    # or equal to X then search
    # in left subarray
    if (arr[mid] >= X):
        return lower_bound(arr, low, mid - 1, X);
 
    # If arr[mid] is less than X
    # then search in right subarray
    return lower_bound(arr, mid + 1, high, X);
 
def upper_bound(arr, low, high, X):
 
    # Base Case
    if (low > high):
        return low;
 
    # Find the middle index
    mid = low + (high - low) // 2;
 
    # If arr[mid] is less than
    # or equal to X search in
    # right subarray
    if (arr[mid] <= X):
        return upper_bound(arr, mid + 1, high, X);
 
    # If arr[mid] is greater than X
    # then search in left subarray
    return upper_bound(arr, low, mid - 1, X);
 
 
# Driver Code
 
# Given Input
arr = [4, 1, 2, 5];
l = 4;
r = 9;
 
n = len(arr)
 
# Function Call
countPairs(arr, l, r, n);
 
# This code is contributed by _Saurabh_Jaiswal.

C#




// C# program for the above approach
using System;
 
class GFG{
 
// Function to count pairs from an array
// whose product lies in the range [l, r]
public static void countPairs(int[] arr, int l,
                              int r, int n)
{
     
    // Sort the array arr[]
    Array.Sort(arr);
  
    // Stores the final answer
    int ans = 0;
  
    for(int i = 0; i < n; i++)
    {
         
        // Upper Bound for arr[j] such
        // that arr[j] <= r/arr[i]
        int itr1 = upper_bound(arr, 0, arr.Length - 1,
                               l / arr[i]);
  
        // Lower Bound for arr[j] such
        // that arr[j] >= l/arr[i]
        int itr2 = lower_bound(arr, 0, arr.Length - 1,
                               l / arr[i]);
        ans += itr1 - itr2;
    }
  
    // Print the answer
    Console.WriteLine(ans);
}
  
public static int lower_bound(int[] arr, int low,
                              int high, int X)
{
     
    // Base Case
    if (low > high)
    {
        return low;
    }
  
    // Find the middle index
    int mid = low + (high - low) / 2;
  
    // If arr[mid] is greater than
    // or equal to X then search
    // in left subarray
    if (arr[mid] >= X)
    {
        return lower_bound(arr, low, mid - 1, X);
    }
  
    // If arr[mid] is less than X
    // then search in right subarray
    return lower_bound(arr, mid + 1, high, X);
}
  
public static int upper_bound(int[] arr, int low,
                              int high, int X)
{
     
    // Base Case
    if (low > high)
        return low;
  
    // Find the middle index
    int mid = low + (high - low) / 2;
  
    // If arr[mid] is less than
    // or equal to X search in
    // right subarray
    if (arr[mid] <= X)
    {
        return upper_bound(arr, mid + 1, high, X);
    }
  
    // If arr[mid] is greater than X
    // then search in left subarray
    return upper_bound(arr, low, mid - 1, X);
}
 
// Driver code
public static void Main(string[] args)
{
     
    // Given Input
    int[] arr = { 4, 1, 2, 5 };
    int l = 4, r = 9;
    int n = arr.Length;
     
    // Function Call
    countPairs(arr, l, r, n);
}
}
 
// This code is contributed by sanjoy_62

Javascript




<script>
// Javascript program for above approach
 
// Function to count pairs from an array
// whose product lies in the range [l, r]
function countPairs(arr, l, r, n)
{
 
    // Sort the array arr[]
    arr.sort((a, b) => a - b);
 
    // Stores the final answer
    let ans = 0;
 
    for (let i = 0; i < n; i++)
    {
 
        // Upper Bound for arr[j] such
        // that arr[j] <= r/arr[i]
        let itr1 = upper_bound(arr, 0, arr.length - 1, Math.floor(l / arr[i]));
 
        // Lower Bound for arr[j] such
        // that arr[j] >= l/arr[i]
         let itr2 = lower_bound(arr, 0, arr.length - 1, Math.floor(l / arr[i]));
         ans += itr1 - itr2;
    }
 
    // Print the answer
    document.write(ans + "<br>");
}
 
 
 
function lower_bound(arr, low, high, X) {
 
    // Base Case
    if (low > high) {
        return low;
    }
 
    // Find the middle index
    let mid = Math.floor(low + (high - low) / 2);
 
    // If arr[mid] is greater than
    // or equal to X then search
    // in left subarray
    if (arr[mid] >= X) {
        return lower_bound(arr, low, mid - 1, X);
    }
 
    // If arr[mid] is less than X
    // then search in right subarray
    return lower_bound(arr, mid + 1, high, X);
}
 
 
function upper_bound(arr, low, high, X) {
 
    // Base Case
    if (low > high)
        return low;
 
    // Find the middle index
    let mid = Math.floor(low + (high - low) / 2);
 
    // If arr[mid] is less than
    // or equal to X search in
    // right subarray
    if (arr[mid] <= X) {
        return upper_bound(arr, mid + 1, high, X);
    }
 
    // If arr[mid] is greater than X
    // then search in left subarray
    return upper_bound(arr, low, mid - 1, X);
}
 
// Driver Code
 
// Given Input
let arr = [4, 1, 2, 5];
let l = 4, r = 9;
 
let n = arr.length
 
// Function Call
countPairs(arr, l, r, n);
 
// This code is contributed by gfgking.
</script>
Output: 
3

 

Time Complexity: O(NlogN)
Auxiliary Space: O(1)

 

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