Python Slicing | Extract ‘k’ bits from a given position

Python Slicing | Extract ‘k’ bits from a given position

In Python, if you want to extract k bits from a given position in a number, you can use bitwise operations. The most relevant operations for this task are the bitwise shift (<< and >>) and bitwise AND (&).

Here's a step-by-step guide to extracting k bits from a given position:

  1. Create a mask with k bits set to 1. This can be done by shifting the number 1 to the left k times and then subtracting 1. For example, for k=3, (1 << 3) - 1 will give 111 in binary, which is 7 in decimal.

  2. Shift the number to the right so that the bit at the specified position becomes the least significant bit.

  3. Perform a bitwise AND operation between the shifted number and the mask. This will set all bits other than the last k bits to 0.

Here's a function that demonstrates the process:

def extract_k_bits(num, pos, k): """ Extract k bits from a given position in num. Parameters: - num: the number from which to extract bits - pos: the position of the first bit to be extracted (0-indexed from the right) - k: the number of bits to extract Returns: - The extracted bits as an integer """ # Create a mask with k bits set to 1 mask = (1 << k) - 1 # Right shift num so that the bit at position pos becomes the least significant bit shifted_num = num >> pos # Return the result of a bitwise AND operation between shifted_num and mask return shifted_num & mask # Test the function num = 171 # 10101011 in binary pos = 2 k = 4 print(bin(extract_k_bits(num, pos, k))) # This should print 0b1010

In this example, we are extracting 4 bits starting from the 2nd position (0-indexed from the right) of the number 171, which in binary is 10101011. The output will be 1010, which is 10 in decimal.

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