Find most significant set bit of a number

Given a number, find the greatest number less than the given a number which is the power of two or find the most significant bit number .

Examples:

Input: 10
Output: 8
Explanation:
Greatest number which is a Power of 2 less than 10 is 8
Binary representation of 10 is 1010
The most significant bit corresponds to decimal number 8.

Input: 18
Output: 16 

A simple solution is to one by one divide n by 2 until it becomes 0 and increment a count while doing this. This count actually represents the position of MSB. 

C++

// Simple CPP program to find MSB number
// for given POSITIVE n.
#include <iostream>
using namespace std;

int setBitNumber(int n)
{
    if (n == 0)
        return 0;

    int msb = 0;
    n = n / 2;
    while (n != 0) {
        n = n / 2;
        msb++;
    }

    return (1 << msb);
}

// Driver code
int main()
{
    int n = 0;
    cout << setBitNumber(n);
    n = ~(int)0; // test for possible overflow
    cout << "\n" << (unsigned int)setBitNumber(n);

    return 0;
}

C

#include <stdio.h>

int setBitNumber(int n)
{
    if (n == 0)
        return 0;

    int msb = 0;
    n = n / 2;
    while (n != 0) {
        n = n / 2;
        msb++;
    }

    return (1 << msb);
}
int main() {

    int n = 0;
    printf("%d \n",setBitNumber(n));
    n = ~(int)0; // test for possible overflow
    int ns = (unsigned int)setBitNumber(n);
    printf("%d",ns);

    return 0;
}

Java

// Simple Java program to find
// MSB number for given n.
import java.io.*;

class GFG {
    static int setBitNumber(int n)
    {
        if (n == 0)
            return 0;

        int msb = 0;
        n = n / 2;

        while (n != 0) {
            n = n / 2;
            msb++;
        }

        return (1 << msb);
    }

    // Driver code
    public static void main(String[] args)
    {
        int n = 0;
        System.out.println(setBitNumber(n));
    }
}

// This code is contributed by ajit

Python3

# Simple Python3 program
# to find MSB number
# for given n.
def setBitNumber(n):
    if (n == 0):
        return 0;

    msb = 0;
    n = int(n / 2);

    while (n > 0):
        n = int(n / 2);
        msb += 1;

    return (1 << msb);

# Driver code
n = 0;
print(setBitNumber(n));

# This code is contributed
# by mits

C#

// Simple C# program to find
// MSB number for given n.
using System;

class GFG {
    static int setBitNumber(int n)
    {
        if (n == 0)
            return 0;

        int msb = 0;
        n = n / 2;

        while (n != 0) {
            n = n / 2;
            msb++;
        }

        return (1 << msb);
    }

    // Driver code
    static public void Main()
    {
        int n = 0;
        Console.WriteLine(setBitNumber(n));
    }
}

// This code is contributed
// by akt_mit

PHP

<?php
// Simple PhP program
// to find MSB number
// for given n.

function setBitNumber($n)
{
    if ($n == 0)
        return 0;

    $msb = 0;
        $n = $n / 2;

    while ($n != 0)
    {
        $n = $n / 2;
        $msb++;
    }

    return (1 << $msb);
}

// Driver code
$n = 0;
echo setBitNumber($n);

// This code is contributed
// by akt_mit
?>

Javascript

<script>

// Javascript  program
// to find MSB number
// for given n.

function setBitNumber(n)
{
    if (n == 0)
        return 0;

    let msb = 0;
        n = n / 2;

    while (n != 0)
    {
        n = $n / 2;
        msb++;
    }

    return (1 << msb);
}

// Driver code
let n = 0;
document.write (setBitNumber(n));

// This code is contributed by Bobby

</script>

Output

0
1

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

An efficient solution for a fixed size integer (say 32 bits) is to one by one set bits, then add 1 so that only the bit after MSB is set. Finally right shift by 1 and return the answer. This solution does not require any condition checking.

C++

// CPP program to find MSB number for ANY given n.
#include <iostream>
#include <limits.h>
using namespace std;

int setBitNumber(int n)
{
    // Below steps set bits after
    // MSB (including MSB)

    // Suppose n is 273 (binary
    // is 100010001). It does following
    // 100010001 | 010001000 = 110011001
    n |= n >> 1;

    // This makes sure 4 bits
    // (From MSB and including MSB)
    // are set. It does following
    // 110011001 | 001100110 = 111111111
    n |= n >> 2;
    n |= n >> 4;
    n |= n >> 8;
    n |= n >> 16;

    // The naive approach would increment n by 1,
    // so only the MSB+1 bit will be set,
    // So now n theoretically becomes 1000000000.
    // All the would remain is a single bit right shift:
    //    n = n + 1;
    //    return (n >> 1);
    //
    // ... however, this could overflow the type.
    // To avoid overflow, we must retain the value
    // of the bit that could overflow:
    //     n & (1 << ((sizeof(n) * CHAR_BIT)-1))
    // and OR its value with the naive approach:
    //     ((n + 1) >> 1)
    n = ((n + 1) >> 1) |
        (n & (1 << ((sizeof(n) * CHAR_BIT)-1)));
    return n;
}

// Driver code
int main()
{
    int n = 273;
    cout << setBitNumber(n);
    n = ~(int)0; // test for possible overflow
    cout << "\n" << (unsigned int)setBitNumber(n);
    return 0;
}

C

#include <stdio.h>
#include <limits.h>
int setBitNumber(int n)
{
    // Below steps set bits after
    // MSB (including MSB)

    // Suppose n is 273 (binary
    // is 100010001). It does following
    // 100010001 | 010001000 = 110011001
    n |= n >> 1;

    // This makes sure 4 bits
    // (From MSB and including MSB)
    // are set. It does following
    // 110011001 | 001100110 = 111111111
    n |= n >> 2;
    n |= n >> 4;
    n |= n >> 8;
    n |= n >> 16;

  // The naive approach would increment n by 1,
    // so only the MSB+1 bit will be set,
    // So now n theoretically becomes 1000000000.
    // All the would remain is a single bit right shift:
    //    n = n + 1;
    //    return (n >> 1);
    //
    // ... however, this could overflow the type.
    // To avoid overflow, we must retain the value
    // of the bit that could overflow:
    //     n & (1 << ((sizeof(n) * CHAR_BIT)-1))
    // and OR its value with the naive approach:
    //     ((n + 1) >> 1)
    n = ((n + 1) >> 1) |
        (n & (1 << ((sizeof(n) * CHAR_BIT)-1)));
    return n;
}

int main() {
   int n = 273;
    printf("%d\n",setBitNumber(n));
    return 0;
}

Java

// Java program to find MSB
// number for given n.

class GFG {

    static int setBitNumber(int n)
    {

        // Below steps set bits after
        // MSB (including MSB)

        // Suppose n is 273 (binary
        // is 100010001). It does following
        // 100010001 | 010001000 = 110011001
        n |= n >> 1;

        // This makes sure 4 bits
        // (From MSB and including MSB)
        // are set. It does following
        // 110011001 | 001100110 = 111111111
        n |= n >> 2;

        n |= n >> 4;
        n |= n >> 8;
        n |= n >> 16;

        // Increment n by 1 so that
        // there is only one set bit
        // which is just before original
        // MSB. n now becomes 1000000000
        n = n + 1;

        // Return original MSB after shifting.
        // n now becomes 100000000
        return (n >> 1);
    }

    // Driver code
    public static void main(String arg[])
    {
        int n = 273;
        System.out.print(setBitNumber(n));
    }
}

// This code is contributed by Anant Agarwal.

Python3

# Python program to find
# MSB number for given n.

def setBitNumber(n):

    # Below steps set bits after
    # MSB (including MSB)

    # Suppose n is 273 (binary
    # is 100010001). It does following
    # 100010001 | 010001000 = 110011001
    n |= n>>1

    # This makes sure 4 bits
    # (From MSB and including MSB)
    # are set. It does following
    # 110011001 | 001100110 = 111111111
    n |= n>>2

    n |= n>>4
    n |= n>>8
    n |= n>>16

    # Increment n by 1 so that
    # there is only one set bit
    # which is just before original
    # MSB. n now becomes 1000000000
    n = n + 1

    # Return original MSB after shifting.
    # n now becomes 100000000
    return (n >> 1)

# Driver code

n = 273
print(setBitNumber(n))

# This code is contributed
# by Anant Agarwal.

C#

// C# program to find MSB number for given n.
using System;

class GFG {

    static int setBitNumber(int n)
    {

        // Below steps set bits after
        // MSB (including MSB)

        // Suppose n is 273 (binary
        // is 100010001). It does following
        // 100010001 | 010001000 = 110011001
        n |= n >> 1;

        // This makes sure 4 bits
        // (From MSB and including MSB)
        // are set. It does following
        // 110011001 | 001100110 = 111111111
        n |= n >> 2;

        n |= n >> 4;
        n |= n >> 8;
        n |= n >> 16;

        // Increment n by 1 so that
        // there is only one set bit
        // which is just before original
        // MSB. n now becomes 1000000000
        n = n + 1;

        // Return original MSB after shifting.
        // n now becomes 100000000
        return (n >> 1);
    }

    // Driver code
    public static void Main()
    {
        int n = 273;
        Console.WriteLine(setBitNumber(n));
    }
}

// This code is contributed by Sam007.

PHP

<?php
// PHP program to find
// MSB number for given n.

function setBitNumber($n)
{
    // Below steps set bits
    // after MSB (including MSB)

    // Suppose n is 273 (binary
    // is 100010001). It does
    // following 100010001 |
    // 010001000 = 110011001
    $n |= $n >> 1;

    // This makes sure 4 bits
    // (From MSB and including
    // MSB) are set. It does
    // following 110011001 |
    // 001100110 = 111111111
    $n |= $n >> 2;

    $n |= $n >> 4;
    $n |= $n >> 8;
    $n |= $n >> 16;

    // Increment n by 1 so
    // that there is only
    // one set bit which is
    // just before original
    // MSB. n now becomes
    // 1000000000
    $n = $n + 1;

    // Return original MSB
    // after shifting. n
    // now becomes 100000000
    return ($n >> 1);
}

// Driver code
$n = 273;
echo setBitNumber($n);

// This code is contributed
// by akt_mit
?>

Javascript

<script>

// Javascript program to find MSB
// number for given n.
function setBitNumber(n)
{

    // Below steps set bits after
    // MSB (including MSB)

    // Suppose n is 273 (binary
    // is 100010001). It does following
    // 100010001 | 010001000 = 110011001
    n |= n >> 1;

    // This makes sure 4 bits
    // (From MSB and including MSB)
    // are set. It does following
    // 110011001 | 001100110 = 111111111
    n |= n >> 2;

    n |= n >> 4;
    n |= n >> 8;
    n |= n >> 16;

    // Increment n by 1 so that
    // there is only one set bit
    // which is just before original
    // MSB. n now becomes 1000000000
    n = n + 1;

    // Return original MSB after shifting.
    // n now becomes 100000000
    return (n >> 1);
}

// Driver code
let n = 273;
document.write(setBitNumber(n));

// This code is contributed by suresh07

</script>

Output

256
2147483648

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

Using __builtin_clz(x) (GCC builtin function)

Say for a fixed integer (32 bits), count the number of leading zeroes by using the built-in function and subtract it from 31 to get the position of MSB from left, then return the MSB using left shift operation on 1.

An efficient solution for a fixed size integer (say 32 bits) is to one by one set bits, then add 1 so that only the bit after MSB is set. Finally right shift by 1 and return the answer. This solution does not require any condition checking.

C++

// CPP program to find MSB
// number for a given POSITIVE n.
#include <bits/stdc++.h>
using namespace std;

int setBitNumber(int n)
{

    // calculate the  number
    // of leading zeroes
    int k = __builtin_clz(n);

    // To return the value
    // of the number with set
    // bit at (31 - k)-th position
    // assuming 32 bits are used
    return 1 << (31 - k);
}

// Driver code
int main()
{
    int n = 273;
    cout << setBitNumber(n);
    n = ~(int)0; // test for possible overflow
    cout << "\n" << (unsigned int)setBitNumber(n);
    return 0;
}

Java

import java.lang.*;

public class Main {
    public static int setBitNumber(int n) {
        // calculate the  number
        // of leading zeroes
        int k = Integer.numberOfLeadingZeros(n);

        // To return the value
        // of the number with set
        // bit at (31 - k)-th position
        // assuming 32 bits are used
        return 1 << (31 - k);
    }

    // Driver code
    public static void main(String[] args) {
        int n = 273;
        System.out.println(setBitNumber(n));

        n = ~(int)0; // test for possible overflow
        System.out.println((int)setBitNumber(n));
    }
}
// This code is Contributed by 'Shiv1o43g'

C#

using System;

public class MainClass {
    public static int SetBitNumber(int n)
    {
        // calculate the  number
        // of leading zeroes
        int k = 32 - Convert.ToString(n, 2).Length;

        // To return the value
        // of the number with set
        // bit at (31 - k)-th position
        // assuming 32 bits are used
        return 1 << (31 - k);
    }

    // Driver code
    public static void Main()
    {
        int n = 273;
        Console.WriteLine(SetBitNumber(n));

        n = ~(int)0; // test for possible overflow
        Console.WriteLine((uint)SetBitNumber(n));
    }
}
// This code is contributed by user_dtewbxkn77n

Javascript

function setBitNumber(n) {
  // calculate the number of leading zeroes
  let k = 31 - Math.floor(Math.log2(n));

  // To return the value of the number with set
  // bit at (31 - k)-th position
  return 1 << k;
}

// Driver code
let n = 273;
console.log(setBitNumber(n)); // expected output: 256

n = ~(0); // test for possible overflow
console.log(setBitNumber(n)); // expected output: 2147483648

Output

256
2147483648

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

Another Approach: Given a number n. First, find the position of the most significant set bit and then compute the value of the number with a set bit at k-th position. 

Thanks Rohit Narayan for suggesting this method. 

C++

// CPP program to find MSB
// number for given POSITIVE n.
#include <bits/stdc++.h>
using namespace std;

int setBitNumber(int n)
{
  //this will be the answer going to return
  //This will work for 64-bit if using with long long
  //while in-built functions overflow
    int ans = 1;
    while (n) {
        ans *= 2;
        n /= 2;
    }
      ans/=2;
    return ans;
}

// Driver code
int main()
{
    int n = 273;
    cout << setBitNumber(n);

    return 0;
}

C

#include <stdio.h>
#include <math.h>

int setBitNumber(int n)
{

    if (n == 0)
        return 0;

    int msb = 0;
    n = n / 2;
    while (n != 0) {
        n = n / 2;
        msb++;
    }

    return (1 << msb);
}
int main() {
    int n = 273;
    printf("%d",setBitNumber(n));

    return 0;
}

Java

// Java program to find MSB
// number for given n.

class GFG {

    static int setBitNumber(int n)
    {

        // To find the position of the
        // most significant set bit
        int k = (int)(Math.log(n) / Math.log(2));

        // To return the value of the number
        // with set bit at k-th position
        return 1 << k;
    }

    // Driver code
    public static void main(String arg[])
    {
        int n = 273;
        System.out.print(setBitNumber(n));
    }
}

Python3

# Python program to find
# MSB number for given n.
import math

def setBitNumber(n):

    # To find the position of
    # the most significant
    # set bit
    k = int(math.log(n, 2))

    # To return the value
    # of the number with set
    # bit at k-th position
    return 1 << k

# Driver code
n = 273
print(setBitNumber(n))

C#

// C# program to find MSB
// number for given n.
using System;

public class GFG {

    static int setBitNumber(int n)
    {

        // To find the position of the
        // most significant set bit
        int k = (int)(Math.Log(n) / Math.Log(2));

        // To return the  value of the number
        // with set bit at k-th position
        return 1 << k;
    }

    // Driver code
    static public void Main()
    {
        int n = 273;
        Console.WriteLine(setBitNumber(n));
    }
}

PHP

<?php
// PHP program to find MSB
// number for given n.

function setBitNumber($n)
{

    // To find the position
    // of the most significant
    // set bit
    $k =(int)(log($n, 2));

    // To return the value
    // of the number with set
    // bit at k-th position
    return 1 << $k;
}

// Driver code
    $n = 273;
    echo setBitNumber($n);

// This code is contributed
// by jit_t.
?>

Javascript

<script>

    // Javascript program to find
    // MSB number for given n.

    function setBitNumber(n)
    {

        // To find the position of the
        // most significant set bit
        let k = parseInt(Math.log(n) / Math.log(2), 10);

        // To return the value of the number
        // with set bit at k-th position
        return 1 << k;
    }

    let n = 273;
      document.write(setBitNumber(n));

</script>

Output

256

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

This article is contributed by Devanshu Agarwal. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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