# C program to count leading zeros in a binary number

Write a C program to input any number from user and find total number of leading zeros of the given number. How to find total leading zeros of a given number (in binary representation) using bitwise operator in C programming. Logic to count total leading zeros of a given number using C programming.

Example

Input

```Input any number: 22
```

Output

```Output leading zeros: 27 (using 4 byte signed integer)
```

## Logic to count leading zeros in a binary number

Leading zeros in a binary number is equal to zeros preceding the highest order set bit of the number. Step by step descriptive logic to count leading zeros in a binary number.

1. Input number from user. Store it in some variable say, num.
2. Find total bits required to store an integer in memory say, `INT_SIZE = sizeof(int) * 8`.
3. Initialize a variable and set its MSB to 1, say `msb = 1 << (INT_SIZE - 1);`.
4. Initialize a variable to store leading zeros count, say `count = 0;`.
5. Run a loop from 0 to INT_SIZE. The loop structure should look like `for(i=0; i<INT_SIZE; i++)`.
6. Inside the loop, left shift num, i times and check its MSB is set or not. If its MSB is set i.e. `if((num << i) & msb)` then terminate the loop; otherwise increment count by 1.

## Program to count leading zeros in a binary number

``````/**
* C program to count leading zeros in a binary number using bitwise operator
*/

#include <stdio.h>
#define INT_SIZE sizeof(int) * 8

int main()
{
int num, count, msb, i;

/* Input number from user */
printf("Enter any number: ");
scanf("%d", &num);

// Equivalent to
// 10000000 00000000 00000000 00000000
msb = 1 << (INT_SIZE - 1);

count = 0;

/* Iterate over each bit */
for(i=0; i<INT_SIZE; i++)
{
/* If leading set bit is found */
if((num << i) & msb)
{
/* Terminate the loop */
break;
}

count++;
}

printf("Total number of leading zeros in %d is %d", num, count);

return 0;
}``````

Below is another short and effective approach to solve the program.

## Program to count leading zeros in a binary number using while loop

``````/**
* C program to count leading zeros in a binary number using bitwise operator
*/

#include <stdio.h>
#include <limits.h> // Used for INT_MAX

int main()
{
int num, count;

/* Input number from user */
printf("Enter any number: ");
scanf("%d", &num);

count = 0;
while(!(num & (~INT_MAX)))
{
count++;
num <<= 1;
}

printf("Total number of leading zeros = %d", count);

return 0;
}``````

`INT_MAX` is a constant defined in `limits.h` header file. It is used to get maximum range of integer.

The statement `num <<= 1;` is a shorthand assignment operator equivalent to `num = num << 1;`

Output

```Enter any number: 22
Total number of leading zeros in 22 is 27```

Important Note: Different compilers may yield different output. Since, data types size is machine dependent.

Happy coding 😉 