Highest power of 2 less than or equal to given Integer

Difficulty Level : Basic
Last Updated : 15 Apr, 2021

Given an integer N, the task is to find the highest power of 2 that is smaller than or equal to N.

Examples:

Input: N = 9
Output: 8
Explanation:
Highest power of 2 less than 9 is 8.
Input: N = -20
Output: -32
Explanation:
Highest power of 2 less than -20 is -32.
Input: N = -84
Output: -128

Approach: The idea is to use logarithm to solve the above problem. For any given number N, it can be either positive or negative. The following task can be performed for each case:

If the input is positive: floor(log₂(N)) can be calculated.
If the input is negative: ceil(log₂(N)) can be calculated and a -ve sign can be added to the value.

Below is the implementation of the above approach:

C++

// C++ implementation of the above approach
 
#include <bits/stdc++.h>
using namespace std;
 
// Function to return the lowest power
// of 2 close to given positive number
int powOfPositive(int n)
{
    // Floor function is used to determine
    // the value close to the number
    int pos = floor(log2(n));
    return pow(2, pos);
}
 
// Function to return the lowest power
// of 2 close to given negative number
int powOfNegative(int n)
{
    // Ceil function is used for negative numbers
    // as -1 > -4. It would be opposite
    // to positive numbers where 1 < 4
    int pos = ceil(log2(n));
    return (-1 * pow(2, pos));
}
 
// Function to find the highest power of 2
void highestPowerOf2(int n)
{
 
    // To check if the given number
    // is positive or negative
    if (n > 0) {
        cout << powOfPositive(n);
    }
    else {
        // If the number is negative,
        // then the ceil of the positive number
        // is calculated and
        // negative sign is added
        n = -n;
        cout << powOfNegative(n);
    }
}
 
// Driver code
int main()
{
    int n = -24;
    highestPowerOf2(n);
 
    return 0;
}

Java

// Java implementation of the above approach
class GFG
{
     
    // Function to return the lowest power
    // of 2 close to given positive number
    static int powOfPositive(int n)
    {
        // Floor function is used to determine
        // the value close to the number
        int pos = (int)Math.floor((Math.log(n)/Math.log(2)));
        return (int)Math.pow(2, pos);
    }
     
    // Function to return the lowest power
    // of 2 close to given negative number
    static int powOfNegative(int n)
    {
        // Ceil function is used for negative numbers
        // as -1 > -4. It would be opposite
        // to positive numbers where 1 < 4
        int pos = (int)Math.ceil((Math.log(n)/Math.log(2)));
        return (int)(-1 * Math.pow(2, pos));
    }
     
    // Function to find the highest power of 2
    static void highestPowerOf2(int n)
    {
     
        // To check if the given number
        // is positive or negative
        if (n > 0)
        {
            System.out.println(powOfPositive(n));
        }
        else
        {
            // If the number is negative,
            // then the ceil of the positive number
            // is calculated and
            // negative sign is added
            n = -n;
            System.out.println(powOfNegative(n));
        }
    }
     
    // Driver code
    public static void main (String[] args)
    {
        int n = -24;
        highestPowerOf2(n);
    }
}
 
// This code is contributed by AnkitRai01

C#

// C# implementation of the above approach
using System;
 
class GFG
{
     
    // Function to return the lowest power
    // of 2 close to given positive number
    static int powOfPositive(int n)
    {
        // Floor function is used to determine
        // the value close to the number
        int pos = (int)Math.Floor((Math.Log(n)/Math.Log(2)));
        return (int)Math.Pow(2, pos);
    }
     
    // Function to return the lowest power
    // of 2 close to given negative number
    static int powOfNegative(int n)
    {
        // Ceil function is used for negative numbers
        // as -1 > -4. It would be opposite
        // to positive numbers where 1 < 4
        int pos = (int)Math.Ceiling((Math.Log(n)/Math.Log(2)));
        return (int)(-1 * Math.Pow(2, pos));
    }
     
    // Function to find the highest power of 2
    static void highestPowerOf2(int n)
    {
     
        // To check if the given number
        // is positive or negative
        if (n > 0)
        {
            Console.WriteLine(powOfPositive(n));
        }
        else
        {
            // If the number is negative,
            // then the ceil of the positive number
            // is calculated and
            // negative sign is added
            n = -n;
            Console.WriteLine(powOfNegative(n));
        }
    }
     
    // Driver code
    public static void Main()
    {
        int n = -24;
        highestPowerOf2(n);
    }
}
 
// This code is contributed by AnkitRai01

Python3

# Python3 implementation of the above approach
from math import floor,ceil,log2
 
# Function to return the lowest power
# of 2 close to given positive number
def powOfPositive(n) :
 
    # Floor function is used to determine
    # the value close to the number
    pos = floor(log2(n));
    return 2**pos;
 
# Function to return the lowest power
# of 2 close to given negative number
def powOfNegative(n) :
 
    # Ceil function is used for negative numbers
    # as -1 > -4. It would be opposite
    # to positive numbers where 1 < 4
    pos = ceil(log2(n));
     
    return (-1 * pow(2, pos));
 
# Function to find the highest power of 2
def highestPowerOf2(n) :
 
    # To check if the given number
    # is positive or negative
    if (n > 0) :
        print(powOfPositive(n));
 
    else :
         
        # If the number is negative,
        # then the ceil of the positive number
        # is calculated and
        # negative sign is added
        n = -n;
        print(powOfNegative(n));
 
# Driver code
if __name__ == "__main__" :
 
    n = -24;
    highestPowerOf2(n);
 
# This code is contributed by AnkitRai01

Javascript

<script>
 
// Javascript implementation of the above approach
 
// Function to return the lowest power
// of 2 close to given positive number
function powOfPositive(n)
{
     
    // Floor function is used to determine
    // the value close to the number
    let pos = Math.floor(Math.log2(n));
    return Math.pow(2, pos);
}
 
// Function to return the lowest power
// of 2 close to given negative number
function powOfNegative(n)
{
     
    // Ceil function is used for negative numbers
    // as -1 > -4. It would be opposite
    // to positive numbers where 1 < 4
    let pos = Math.ceil(Math.log2(n));
    return (-1 * Math.pow(2, pos));
}
 
// Function to find the highest power of 2
function highestPowerOf2(n)
{
 
    // To check if the given number
    // is positive or negative
    if (n > 0)
    {
        document.write(powOfPositive(n));
    }
    else
    {
         
        // If the number is negative,
        // then the ceil of the positive number
        // is calculated and
        // negative sign is added
        n = -n;
        document.write(powOfNegative(n));
    }
}
 
// Driver code
let n = -24;
 
highestPowerOf2(n);
 
// This code is contributed by mohit kumar 29
 
</script>

Output:

-32

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