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

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# Program to find whether// a no is power of twousing 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)
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# program to find whether// a no is power of twousing 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
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 –> 01111So, 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# program to efficiently// check for power for 2using 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
No
Yes
 
Please refer complete article on Program to find whether a no is power of two for more details! 
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