# 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 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 .