C program to test if a number is a power of 2

In this example, you will learn about C program to test if a number is a power of 2 in two ways.

There are various ways to check if a given number is a power of 2. First check below which numbers are the power of two or not.

Numbers that are power of 2:
2, 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048 ...
22 = 4
25 = 32
210 = 1024

We will solve this problem in two different ways:

• Using function
• Using bitwise operation

Let’s do it.

C program to test if a number is a power of 2 using simple function

#include <stdio.h>

//function prototype for checking power of two
int checkPowerofTwo(int n);

int main()
{
   int num;

   printf("Enter the number you want to test: ");
   scanf("%d", &num);

   if (checkPowerofTwo(num) == 1)
      printf("\n%d is a power of 2\n", num);
   else
      printf("\n%d is not a power of 2\n", num);

   return 0;
}

//function body
int checkPowerofTwo(int x)
{
   //checks whether a number is zero or not
   if (x == 0)
      return 0;

   //true till x is not equal to 1
   while( x != 1)
   {
      //checks whether a number is divisible by 2
      if(x % 2 != 0)
         return 0;
         x /= 2;
   }
   return 1;
}

Output

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Explanation

In the above program, we have created user-defined function checkPowerofTwo() to accomplish the task.

Logic of the program is simple i.e. keep dividing the number by 2 iteratively.

Check whether x % 2 is not equal to zero and x not equal to 1 which indicates x is not power of 2 otherwise if x becomes 1 then it is the power of 2.

C program to check if a number is a power of 2 using bitwise operation.

Logic of the program

If a number is a power of 2, then the bit’s of the previous number will be a complement of the number.

For example:

Bit's of 8 = 1000
Bit's of 7 = 0111

Both are the complement of each other.

So let’s implement this logic.

//C program to check whether a number is power of 2 or not

#include <stdio.h>

int main()
{
   int num;

   printf("Enter the number you want to test: ");
   scanf("%d", &num);

   //condition to check whether number is power of two or not
   //performing bitwise operation
   if((num != 0) && ((num &(num - 1)) == 0))
      printf("\n%d is a power of 2", num);

   else
      printf("\n%d is not a power of 2\n", num);

 return 0;
}

Output

Enter the number you want to test: 9
9 is not a power of 2

Enter the number you want to test: 512
512 is a power of 2

Explanation

In the above program, if condition is used to check whether a number is not equal to zero and it’s previous number is complement or not .