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

• Last Updated : 19 Jul, 2021

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.

C#

 // C# Program to find whether // a no is power of two using System;   class GFG {       /* Function to check if    x is power of 2*/     static bool isPowerOfTwo(int n)     {         return (int)(Math.Ceiling((Math.Log(n) / Math.Log(2))))               == (int)(Math.Floor(((Math.Log(n) / Math.Log(2)))));     }       // Driver Code     public static void Main()     {         if (isPowerOfTwo(31))             Console.WriteLine("Yes");         else             Console.WriteLine("No");           if (isPowerOfTwo(64))             Console.WriteLine("Yes");         else             Console.WriteLine("No");     } }   // This code is contributed // by Akanksha Rai(Abby_akku)

Output:

No
Yes

Time Complexity: O(log2n)

Auxiliary Space: O(1)

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.

C#

 // C# program to find whether // a no is power of two using System;   class GFG {       // Function to check if     // x is power of 2     static bool 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()     {         Console.WriteLine(isPowerOfTwo(31) ? "Yes" : "No");         Console.WriteLine(isPowerOfTwo(64) ? "Yes" : "No");     } }   // This code is contributed by Sam007

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.

C#

 // C# program to efficiently // check for power for 2 using System;   class GFG {     // Method to check if x is power of 2     static bool 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()     {         Console.WriteLine(isPowerOfTwo(31) ? "Yes" : "No");         Console.WriteLine(isPowerOfTwo(64) ? "Yes" : "No");     } }   // This code is contributed by Sam007

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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