5 Best Ways to Check Whether the Bit at A Given Position is Set or Unset in Python

Rate this post

πŸ’‘ Problem Formulation: Understanding whether a bit at a specific position in a binary number is set (1) or unset (0) is a fundamental task in programming, particularly in systems, game development, and areas dealing with low-level data processing. If we have a number, say 42, and we want to check if the bit at position 3 is set, we’re inquiring if the binary representation of 42 (which is 101010) has a ‘1’ at that position (counting from the right, starting with position 0). The desired output would indicate the result as True or False.

Method 1: Using the Bitwise AND Operator

The bitwise AND operator can be used to check the status of a bit at a given position by comparing the number with a bitmask. The bitmask is created by shifting the number 1 to the left by the bit position. If the result of the AND operation is nonzero, then the bit is set.

Here’s an example:

def is_bit_set(num, pos):
    mask = 1 << pos
    return (num & mask) != 0

# Check if the 3rd bit of 42 is set.
print(is_bit_set(42, 3))

Output:

True

This snippet defines a function is_bit_set() that takes a number num and a bit position pos. It creates a mask by shifting 1 to the left by pos positions. It then uses the bitwise AND operator & with the mask and compares the result against zero to return True if the bit is set and False otherwise.

Method 2: Using the Right Shift Operator

Alternatively, instead of shifting the mask left and keeping the number static, we can shift the number right by the position and check the least significant bit. The bit at the given position will now be the least significant bit, and using the bitwise AND operator with 1 will reveal if it is set.

Here’s an example:

def is_bit_set(num, pos):
    return (num >> pos) & 1

# Check if the 3rd bit of 42 is set.
print(is_bit_set(42, 3))

Output:

True

In this method, the function shifts num right by pos bits using the right shift operator >>. The check (num >> pos) & 1 tests whether the least significant bit is set. If it is, the result is 1 (which is truthy), and the function returns True.

Method 3: Using Bit String Representation

This method involves converting the number into its binary representation as a string and then checking the character at the string index corresponding to the bit’s position. Since string indices start from the left with 0, and we count bit positions from the right beginning with 0, the position needs to be adjusted accordingly.

Here’s an example:

def is_bit_set(num, pos):
    # Convert number to binary string and remove the '0b' prefix.
    binary = bin(num)[2:]
    # Adjust the position to match the string index.
    pos_from_left = len(binary) - pos - 1
    return binary[pos_from_left] == '1'

# Check if the 3rd bit of 42 is set.
print(is_bit_set(42, 3))

Output:

True

The function is_bit_set() converts the number to a binary string with bin(), strips the ‘0b’ prefix, and calculates the adjusted position. It then assesses the character at this position to return True if it is ‘1’, or False if it is ‘0’.

Method 4: Using Integer Division and Modulo Operation

This method leverages the modulo operation to extract the remainder of the number after dividing by the power of two corresponding to bit position plus one. A subsequent division by the power of two corresponding to the bit position yields either a 0 or 1.

Here’s an example:

def is_bit_set(num, pos):
    return (num % (1 << (pos + 1))) // (1 << pos)

# Check if the 3rd bit of 42 is set.
print(is_bit_set(42, 3))

Output:

1

In the is_bit_set() function, num modulo (1 << (pos + 1)) gives a number containing only the lower pos + 1 bits of num. Integer division by (1 << pos) removes all but the bit at the given position, revealing whether it is set or not.

Bonus One-Liner Method 5: Using a Python One-Liner

This concise approach uses a combination of bit shifting and modulus in a single line, well-suited for in-lining in other code or for short scripts.

Here’s an example:

# Check if the 3rd bit of 42 is set using a single line.
is_bit_set = lambda num, pos: (num >> pos) & 1

print(is_bit_set(42, 3))

Output:

True

The lambda function in this snippet takes num and pos and returns (num >> pos) & 1. It’s directly analogous to the right shift method discussed earlier but presented in a compact form. It’s handy when the function is simple enough not to warrant a traditional function definition.

Summary/Discussion

  • Method 1: Bitwise AND Operator. Strengths: Intuitive understanding of masks. Weaknesses: Requires a bit of extra computation for the mask.
  • Method 2: Right Shift Operator. Strengths: Efficient and concise. Weaknesses: The shift semantics might be less intuitive for new developers.
  • Method 3: Bit String Representation. Strengths: Visual and straightforward if already working with string representations. Weaknesses: Inefficient for large numbers due to string manipulation.
  • Method 4: Integer Division and Modulo Operation. Strengths: Teaches fundamentals of bits mathematically. Weaknesses: More complex and less readable.
  • Method 5: Python One-Liner. Strengths: Extremely concise, perfect for simple scripts. Weaknesses: Could lack clarity in more extensive codebases.