# Java Program to find whether a no is power of two

Given a positive integer, write a function to find if it is a power of two or not.

Examples :

```Input : n = 4
Output : Yes
22 = 4

Input : n = 7
Output : No

Input : n = 32
Output : Yes
25 = 32
```

1. A simple method for this is to simply take the log of the number on base 2 and if you get an integer then number is power of 2.

## Java

 `// Java Program to find whether a ` `// no is power of two ` `class` `GFG { ` `    ``/* Function to check if x is power of 2*/` `    ``static` `boolean` `isPowerOfTwo(``int` `n) ` `    ``{ ` `        ``return` `(``int``)(Math.ceil((Math.log(n) / Math.log(``2``))))  ` `            ``== (``int``)(Math.floor(((Math.log(n) / Math.log(``2``))))); ` `    ``} ` ` `  `    ``// Driver Code ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``if` `(isPowerOfTwo(``31``)) ` `            ``System.out.println(``"Yes"``); ` `        ``else` `            ``System.out.println(``"No"``); ` ` `  `        ``if` `(isPowerOfTwo(``64``)) ` `            ``System.out.println(``"Yes"``); ` `        ``else` `            ``System.out.println(``"No"``); ` `    ``} ` `} ` ` `  `// This code is contributed by mits `

Output:

```No
Yes
```

2. Another solution is to keep dividing the number by two, i.e, do n = n/2 iteratively. In any iteration, if n%2 becomes non-zero and n is not 1 then n is not a power of 2. If n becomes 1 then it is a power of 2.

## Java

 `// Java program to find whether ` `// a no is power of two ` `import` `java.io.*; ` ` `  `class` `GFG { ` ` `  `    ``// Function to check if ` `    ``// x is power of 2 ` `    ``static` `boolean` `isPowerOfTwo(``int` `n) ` `    ``{ ` `        ``if` `(n == ``0``) ` `            ``return` `false``; ` ` `  `        ``while` `(n != ``1``) { ` `            ``if` `(n % ``2` `!= ``0``) ` `                ``return` `false``; ` `            ``n = n / ``2``; ` `        ``} ` `        ``return` `true``; ` `    ``} ` ` `  `    ``// Driver program ` `    ``public` `static` `void` `main(String args[]) ` `    ``{ ` `        ``if` `(isPowerOfTwo(``31``)) ` `            ``System.out.println(``"Yes"``); ` `        ``else` `            ``System.out.println(``"No"``); ` ` `  `        ``if` `(isPowerOfTwo(``64``)) ` `            ``System.out.println(``"Yes"``); ` `        ``else` `            ``System.out.println(``"No"``); ` `    ``} ` `} ` ` `  `// This code is contributed by Nikita tiwari. `

Output:

```No
Yes
```

3. All power of two numbers have only one bit set. So count the no. of set bits and if you get 1 then number is a power of 2. Please see Count set bits in an integer for counting set bits.

4. If we subtract a power of 2 numbers by 1 then all unset bits after the only set bit become set; and the set bit become unset.

For example for 4 ( 100) and 16(10000), we get following after subtracting 1
3 –> 011
15 –> 01111

So, if a number n is a power of 2 then bitwise & of n and n-1 will be zero. We can say n is a power of 2 or not based on value of n&(n-1). The expression n&(n-1) will not work when n is 0. To handle this case also, our expression will become n& (!n&(n-1)) (thanks to https://www.geeksforgeeks.org/program-to-find-whether-a-no-is-power-of-two/Mohammad for adding this case).
Below is the implementation of this method.

## Java

 `// Java program to efficiently ` `// check for power for 2 ` ` `  `class` `Test { ` `    ``/* Method to check if x is power of 2*/` `    ``static` `boolean` `isPowerOfTwo(``int` `x) ` `    ``{ ` `        ``/* First x in the below expression is  ` `        ``for the case when x is 0 */` `        ``return` `x != ``0` `&& ((x & (x - ``1``)) == ``0``); ` `    ``} ` ` `  `    ``// Driver method ` `    ``public` `static` `void` `main(String[] args) ` `    ``{ ` `        ``System.out.println(isPowerOfTwo(``31``) ? ``"Yes"` `: ``"No"``); ` `        ``System.out.println(isPowerOfTwo(``64``) ? ``"Yes"` `: ``"No"``); ` `    ``} ` `} ` `// This program is contributed by Gaurav Miglani `

Output:

```No
Yes
```

Please refer complete article on Program to find whether a no is power of two for more details!

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