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Set the rightmost unset bit
• Difficulty Level : Easy
• Last Updated : 17 May, 2021

Given a non-negative number n. The problem is to set the rightmost unset bit in the binary representation of n. If there are no unset bits, then just leave the number as it is.

Examples:

```Input : 21
Output : 23
(21)10 = (10101)2
Rightmost unset bit is at position 2(from right) as
highlighted in the binary representation of 21.
(23)10 = (10111)2
The bit at position 2 has been set.

Input : 15
Output : 15```

Approach: Following are the steps:

1. If n = 0, return 1.
2. If all bits of n are set, return n. Refer to this post.
3. Else perform bitwise not on the given number(operation equivalent to 1’s complement). Let it be num = ~n.
4. Get the position of rightmost set bit of num. Let the position be pos.
5. Return (1 << (pos – 1)) | n.

## C++

 `// C++ implementation to set the rightmost unset bit``#include ``using` `namespace` `std;`` ` `// function to find the position ``// of rightmost set bit``int` `getPosOfRightmostSetBit(``int` `n)``{``    ``return` `log2(n&-n)+1;``}`` ` `int` `setRightmostUnsetBit(``int` `n)``{``    ``// if n = 0, return 1``    ``if` `(n == 0)``        ``return` `1;``     ` `    ``// if all bits of 'n' are set``    ``if` `((n & (n + 1)) == 0)    ``        ``return` `n;``     ` `    ``// position of rightmost unset bit in 'n'``    ``// passing ~n as argument``    ``int` `pos = getPosOfRightmostSetBit(~n);    ``     ` `    ``// set the bit at position 'pos'``    ``return` `((1 << (pos - 1)) | n);``}`` ` `// Driver program to test above``int` `main()``{``    ``int` `n = 21;``    ``cout << setRightmostUnsetBit(n);``    ``return` `0;``} `

## Java

 `// Java implementation to set ``// the rightmost unset bit`` ` `class` `GFG {``     ` `// function to find the position``// of rightmost set bit``static` `int` `getPosOfRightmostSetBit(``int` `n) ``{``    ``return` `(``int``)((Math.log10(n & -n)) / (Math.log10(``2``))) + ``1``;``}`` ` `static` `int` `setRightmostUnsetBit(``int` `n) ``{``    ``// if n = 0, return 1``    ``if` `(n == ``0``)``    ``return` `1``;`` ` `    ``// if all bits of 'n' are set``    ``if` `((n & (n + ``1``)) == ``0``)``    ``return` `n;`` ` `    ``// position of rightmost unset bit in 'n'``    ``// passing ~n as argument``    ``int` `pos = getPosOfRightmostSetBit(~n);`` ` `    ``// set the bit at position 'pos'``    ``return` `((``1` `<< (pos - ``1``)) | n);``}`` ` `// Driver code``public` `static` `void` `main(String arg[]) {``    ``int` `n = ``21``;``    ``System.out.print(setRightmostUnsetBit(n));``}``}`` ` `// This code is contributed by Anant Agarwal.`

## Python3

 `# Python3 implementation to``# set the rightmost unset bit``import` `math`` ` `# function to find the position ``# of rightmost set bit``def` `getPosOfRightmostSetBit(n):`` ` `    ``return` `int``(math.log2(n&``-``n)``+``1``)`` ` `  ` `def` `setRightmostUnsetBit(n):`` ` `    ``# if n = 0, return 1``    ``if` `(n ``=``=` `0``):``        ``return` `1``      ` `    ``# if all bits of 'n' are set``    ``if` `((n & (n ``+` `1``)) ``=``=` `0``):    ``        ``return` `n``      ` `    ``# position of rightmost unset bit in 'n'``    ``# passing ~n as argument``    ``pos ``=` `getPosOfRightmostSetBit(~n)    ``      ` `    ``# set the bit at position 'pos'``    ``return` `((``1` `<< (pos ``-` `1``)) | n)`` ` `# Driver code`` ` `n ``=` `21``print``(setRightmostUnsetBit(n))`` ` `# This code is contributed``# by Anant Agarwal.`

## C#

 `// C# implementation to set ``// the rightmost unset bit``using` `System;`` ` `class` `GFG{``     ` `// Function to find the position``// of rightmost set bit``static` `int` `getPosOfRightmostSetBit(``int` `n) ``{``    ``return` `(``int``)((Math.Log10(n & -n)) / ``                 ``(Math.Log10(2))) + 1;``}`` ` `static` `int` `setRightmostUnsetBit(``int` `n) ``{``     ` `    ``// If n = 0, return 1``    ``if` `(n == 0)``        ``return` `1;`` ` `    ``// If all bits of 'n' are set``    ``if` `((n & (n + 1)) == 0)``        ``return` `n;`` ` `    ``// Position of rightmost unset bit in 'n'``    ``// passing ~n as argument``    ``int` `pos = getPosOfRightmostSetBit(~n);`` ` `    ``// Set the bit at position 'pos'``    ``return` `((1 << (pos - 1)) | n);``}`` ` `// Driver code``public` `static` `void` `Main(String []arg) ``{``    ``int` `n = 21;``     ` `    ``Console.Write(setRightmostUnsetBit(n));``}``}`` ` `// This code is contributed by shivanisinghss2110`

## Javascript

 ``

Output:

`23`

Alternate Implementation
The idea is to use Integer.toBinaryString()

## Java

 `import` `java.io.*;``import` `java.util.Scanner;`` ` `public` `class` `SetMostRightUnsetBit {``    ``public` `static` `void` `main(String args[])``    ``{``        ``int` `a = ``21``;``        ``setMostRightUnset(a);``    ``}`` ` `    ``private` `static` `void` `setMostRightUnset(``int` `a)``    ``{``        ``// will get a number with all set bits except the ``        ``// first set bit``        ``int` `x = a ^ (a - ``1``);``        ``System.out.println(Integer.toBinaryString(x));`` ` `        ``// We reduce it to the number with single 1's on ``        ``// the position of first set bit in given number``        ``x = x & a; ``        ``System.out.println(Integer.toBinaryString(x));`` ` `        ``// Move x on right by one shift to make OR ``        ``// operation and make first rightest unset bit 1``        ``x = x >> ``1``; `` ` `        ``int` `b = a | x;`` ` `        ``System.out.println(``"before setiing bit "` `+ ``                               ``Integer.toBinaryString(a));``        ``System.out.println(``"after setting bit "` `+ ``                               ``Integer.toBinaryString(b));``    ``}``}`

## Python3

 `def` `setMostRightUnset(a):``     ` `    ``# Will get a number with all set ``    ``# bits except the first set bit``    ``x ``=` `a ^ (a ``-` `1``)``    ``print``(``bin``(x)[``2``:]) `` ` `    ``# We reduce it to the number with ``    ``# single 1's on the position of ``    ``# first set bit in given number``    ``x ``=` `x & a``    ``print``(``bin``(x)[``2``:]) `` ` `    ``# Move x on right by one shift to ``    ``# make OR operation and make first ``    ``# rightest unset bit 1``    ``x ``=` `x >> ``1`` ` `    ``b ``=` `a | x`` ` `    ``print``(``"before setiing bit "``, ``bin``(a)[``2``:]) ``    ``print``(``"after setting bit "``, ``bin``(b)[``2``:]) ``     ` `# Driver Code``if` `__name__ ``=``=` `'__main__'``:``     ` `    ``a ``=` `21``     ` `    ``setMostRightUnset(a)`` ` `# This code is contributed by mohit kumar 29`

## C#

 `using` `System;`` ` `class` `SetMostRightUnsetBit{``     ` `// Driver Code``public` `static` `void` `Main(String []args)``{``    ``int` `a = 21;``    ``setMostRightUnset(a);``}`` ` `private` `static` `void` `setMostRightUnset(``int` `a)``{``     ` `    ``// will get a number with all set bits ``    ``// except the first set bit``    ``int` `x = a ^ (a - 1);``    ``Console.WriteLine(Convert.ToString(x));`` ` `    ``// We reduce it to the number with single 1's on ``    ``// the position of first set bit in given number``    ``x = x & a; ``    ``Console.WriteLine(Convert.ToString(x));`` ` `    ``// Move x on right by one shift to make OR ``    ``// operation and make first rightest unset bit 1``    ``x = x >> 1; `` ` `    ``int` `b = a | x;`` ` `    ``Console.WriteLine(``"before setiing bit "` `+``    ``Convert.ToString(a, 2));``    ``Console.WriteLine(``"after setting bit "` `+``    ``Convert.ToString(b, 2));``}``}`` ` `// This code is contributed by shivanisinghss2110`

## Javascript

 ``

Output:

```1
1
before setiing bit 10101
after setting bit 10101```

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