### C program of Brian Kernighan’s Algorithm to count number of 1 in binary representation of a number :

In this example, we will learn how to find the *count of 1* in the binary representation of a number. For example, the binary value of *3* is *0011* and total number of *1* in this representation is 2. Different ways are there to calculate the total count. *Brian Kernighan’s Algorithm* is one of them. The complexity of this algorithm is *O(log(n))*.

### How Brian Kernighan’s Algorithm works ?

To understand the algorithm, first take a look into the binary representation from *0 to 10* :

```
0000 = 0
0001 = 1
0010 = 2
0011 = 3
0100 = 4
0101 = 5
0110 = 6
0111 = 7
1000 = 8
1001 = 9
1010 = 10
```

Let’s try to find how these bits are related :
If we subtract *1* from a number , *all ‘0’s (if available) to the right of rightmost ‘1’ changes to ‘1’ and the ‘1’ changes to ‘0’*. For example, *10 - 1 = 9*. *10* is *1010* . Change all *0* of the rightmost *1* to *1* = *1011*. Now change the *1* to *0* = *1001* which is the binary representation of *9*.
Brian Kernighan’s Algorithm is based on this relation. To count the number of bits in a number, first we will *subtract 1* from that number and will do *bitwise &* with itself. What will happen ? The *rightmost \*1_ will become *0* . For example, for *2* and *1*, it will be *0010 & 0001*. We will do the same process till the final result becomes *0* or *0000*. The total number of time required to reach *0* is the count of *1* in that binary number. _

### C program :

```
#include <stdio.h>
int main()
{
//1
int count = 0;
int number;
//2
printf("Enter a number : \n");
scanf("%d", &number);
//3
while (number)
{
//4
number &= (number - 1);
count++;
}
//5
printf("Total number of 1 is = %d \n", count);
return 0;
}
```

### Explanation :

*The commented numbers in the above program denotes the steps number below :*

- Create one integer variable
*count*to store the count of*1*. Create one more integer variable*number*to store the value of user input value. - Ask the user to enter the number and store it in variable
*number*. - Start one
*while*loop. The loop will run till the value of*number*becomes*0*. - Do
*bitwise &*of*number*and*number - 1*and set the value to*number*. Increment the value of count. Do the same process again and again till*number*becomes*0*. - After the loop is completed, print the value of
*count*i.e.*total number of 1*in binary representation of*number*.

### Sample Output :

```
Enter a number :
4
Total number of 1 is = 1
Enter a number :
5
Total number of 1 is = 2
Enter a number :
7
Total number of 1 is = 3
```