Extract bits in C

In C programming, extracting a bit means retrieving the value (0 or 1) of a bit or a group of bits present at a specific position in the binary representation of a number

In this article, we will learn how to extract a bit or multiple bits at given positions in a binary number. We will also explore how to extract a range of bits simultaneously.

How to Extract a Bit in C?

In C, we can extract a given bit using the AND operator (&) combined with a bit mask and then right shifting the result. Let's see how it is possible with the help of the truth table of AND:

From the above table, we can infer that:

• By ANDing a bit with 1, we can extract the bit's value.
• By ANDing a bit with 0, the result is always 0.

Extracting a Single Bit from Specific Position

To extract a specific bit in a number, you can use a bit mask with the AND operator and then right shift the result to get the bit's value.

Approach

1. Create a mask with only k^th bit set to 1.
2. Apply the mask using &.
3. Right-Shift the resultant number k times, where k is the position of the bit you wanted to extract.

Example:

Input:
binary_number: 01100111
bit to extract: 5th

Output:
extracted_bit: 0

Program to Illustrate How to Extract a Specific Bit in C

// C Program to Extract a given bit of a binary number
#include <stdio.h>

int main() {
  
    // Binary: 01100111
    unsigned int num = 103;

    // Extracting the 5th bit (0-based index)
    unsigned int bit_position = 4;

    // Create a mask with only the 5th bit set to 1
    unsigned int mask = 1 << bit_position;

    // Extract the bit using AND and right shift
    unsigned int extracted_bit
        = (num & mask) >> bit_position;

    // Print the result
    printf("Extracted Bit: %u\n", extracted_bit);

    return 0;
}

Extracted Bit: 0

Time Complexity: O(1)
Space Complexity: O(1)

Extracting Multiple Bits in C

We can also extract multiple bits by creating a mask with the desired bits set to 1 and then right shifting the result to get the bits' values.

Approach

1. Create a mask with n bits set to 1 in the range k and l, where k and l is the range you want to extract bits from.
2. Apply the mask using &.
3. Right-Shift the resultant number k number of times, where k is the starting position of the bits you want to extract.

Example:

Input:
binary_number: 01100111
bit to extract: 5th to 7th

Output:
extracted_bits: 110

Program to Illustrate How to Extract Multiple Bits in C

// C Program to Extract multiple bits of a binary number
#include <stdio.h>

int main()
{

    // Binary: 01100111
    unsigned int num = 103;

  	// defining the range
    unsigned int start = 4, end = 6;

    // Create a mask
    unsigned int mask = 0;
    for (int i = start; i <= end; i++) {
        mask = mask | (1 << i);
    }

    // Extract the bits using AND and right shift
    unsigned int extracted_bits = (num & mask) >> 4;

    // Print the result
    printf("Extracted Bits: %u\n", extracted_bits);

    return 0;
}

Extracted Bits: 6

Time Complexity: O(1), Bitwise operations and shifts are constant-time operations.
Auxiliary Space: O(1)

Conclusion

By following these methods, you can efficiently extract specific bits, multiple bits, or a range of bits from a binary number in C, allowing for precise control and manipulation of binary data.