Bitmasking In C

In this article, we are going to learn about Bitmask in C which is a powerful way to basically work with bits and functions upon boolean logic. It can be used to store data compactly and with much more efficiency in certain cases.

What is a Bit?

A bit is the smallest unit of data which can either store a 0 or 1 inside it. All the data in the computer is stored using these bits. These 2 possible values can also be represented as boolean values that are True or False. Using this we can apply boolean logic to manipulate data stored on the computer.

Bitmasking in C

Bitmasking is a technique that involves bit manipulation. It is basically like putting a mask over certain bits and hiding the other un-useful bits, so as to make the program much more efficient and optimize the memory.

A bitmask is a sequence of bits that can also be known as a bitset or bit field and is used to perform bitwise operations on the given data.

There are basically 6 bitwise operators in C that can be used to manipulate bits which are as follows:

• & (Bitwise AND Operator)
• | (Bitwise OR Operator)
• ^ (Bitwise XOR Operator)
• ~ (Bitwise NOT Operator)
• >> (RIght Shift Operator)
• << (Left Shift Operator)

Using these operators, we perform different bit masking techniques according to the requirements. Let’s discuss these techniques and how to implement them.

Bitmasking Techniques

1. Setting a Bit

In this technique, we set a particular bit to 1 without touching any of the other bits. For this, we use the bitwise OR ( | ) operator and the left shift (<<) operator.

Basically, we take the integer 1 and using the left shift operator, shift the binary representation of 1 (that is 1 only) to n places where (n+1) is the place of bit which we want to set. Then using the bitwise OR operator we turn the given number’s (n+1)^th bit to 1.

Syntax

number | (1 << bit_position_to_set)

Example:

C

#include <stdio.h>

int main()
{

    int x = 13;

    printf("Ans: %d", 13 | (1 << 5));

    return 0;
}

Output

Ans: 45

2. Clearing a Bit

In this operation, we set a specific bit to 0 (as opposed to 1 in the previous case) without touching any of the other bits. We use the bitwise AND operator (&), bitwise NOT operator (~), and the left shift operator (<<) to achieve the task.

Basically, we again take 1 and shift it the the specified position. Then, we perform the NOT operation on this to convert that into a 0 and other bits of the value (1<<n) to 1. Then we do the AND operation to clear the specified bit and obtain the result.

Syntax:

number & ~(1 << bit_position_to_clear)

Example:

C

#include<stdio.h>

int main() {
    int x = 13;

    printf("Ans: %d", 13 & ~(1 << 2) );

    return 0;
}

Output

Ans: 9

3. Flipping a Bit

In this operation, we flip a specific bit that is if the bit is 0 then turn it to 1 else turn it to 0. This operation requires the use of bitwise XOR (^) operator along with the left shift (<<) operator.

Basically, as in previous operations, we shift 1 to the specified number of positions and then perform the XOR operation to flip the bit of the given number.

Syntax:

number ^ (1 << bit_position_to_flip)

Example:

C

#include<stdio.h>

int main() {
    int x = 13;

    printf("Ans: %d", 13 ^ (1 << 3) );

    return 0;
}

Output

Ans: 5

4. Checking a Bit

In this operation, we check if a particular bit is 1 or not using the bitwise AND operator (&) along with the left shift operator (<<).

We shift 1 using the left shift operator to the specified position and then perform the bitwise AND operation on that so as to check if that specific bit is 0 or 1. If the bit is 0 then the result would be 0 else the result would be 2^(bit_position).

Syntax:

number & (1 << bit_position_to_check)

Example:

C

#include<stdio.h>

int main() {

    printf("Ans: %d \n", 13 & (1 << 3) );
    printf("Ans: %d \n", 13 & (1 << 4) );

    return 0;
}

Output

Ans: 8 
Ans: 0 

Application of Bitmasking in C

Following are some main applications of bit masking in the C programming language.

• It is used in data compression to decrease its size.
• It is used in cryptography to encrypt the data.
• It is used in the optimization of a lot of algorithms.