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

How to extract ‘k’ bits from a given position ‘p’ in a number? Examples:

Input : number = 171
             k = 5 
             p = 2
Output : The extracted number is 21
171 is represented as 10101011 in binary,
so, you should get only 10101 i.e. 21.

Input : number = 72
            k = 5 
            p = 1
Output : The extracted number is 8
72 is represented as 1001000 in binary,
so, you should get only 01000 i.e 8.

We have existing solution for this problem please refer Extract ‘k’ bits from a given position in a number link. We can solve this problem quickly in python using slicing. Approach is simple,

1. Convert given number into it’s binary using bin() function and remove first two characters ‘0b’ from it, because bin function appends ‘0b’ as prefix in output binary string.
2. We need to start extracting k bits from starting position p from right, that means end index of extracted sub-string will be end = (len(binary) – p) and start index will be start = end – k + 1 of original binary string.
3. Convert extracted sub-string into decimal again.

Python3

# Function to extract ‘k’ bits from a given
# position in a number

def extractKBits(num,k,p):

     # convert number into binary first
     binary = bin(num)

     # remove first two characters
     binary = binary[2:]

     end = len(binary) - p
     start = end - k + 1

     # extract k  bit sub-string
     kBitSubStr = binary[start : end+1]

     # convert extracted sub-string into decimal again
     print (int(kBitSubStr,2))

# Driver program
if __name__ == "__main__":
    num = 171
    k = 5
    p = 2
    extractKBits(num,k,p)

Output:

21

Time Complexity: O(logn)
Space Complexity: O(logn)

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