Reverse bits of a positive integer number in Python

Given an positive integer and size of bits, reverse all bits of it and return the number with reversed bits.

Examples:

Input : n = 1, bitSize=32
Output : 2147483648  
On a machine with size of 
bit as 32. Reverse of 0....001 is
100....0.

Input : n = 2147483648, bitSize=32
Output : 1  



We can solve this problem quickly in Python. Approach is very simple,

  1. Convert integer number into it’s binary representation using bin(num) function.
  2. bin() function appends 0b as a prefix in binary representation of number, skip first two characters of binary representation and reverse remaining part of string.
  3. As we know in memory any binary representation of a number is filled with leading zeros after last set bit from left that means we need to append bitSize – len(reversedBits) number of zeros after reversing remaining string.
  4. Now convert binary representation into integer number using int(string,base) method.

How int() method wroks ?

int(string,base) method takes a string and base to identify that string is referring to what number system ( binary=2, heaxadecimal=16, octal=8 etc. ) and converts string into decimal number system accordingly. For example ; int(‘1010’,2) = 10.

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# Function to reverse bits of positive 
# integer number
  
def reverseBits(num,bitSize):
  
     # convert number into binary representation
     # output will be like bin(10) = '0b10101'
     binary = bin(num)
  
     # skip first two characters of binary
     # representation string and reverse
     # remaining string and then append zeros
     # after it. binary[-1:1:-1]  --> start
     # from last character and reverse it until
     # second last character from left
     reverse = binary[-1:1:-1]
     reverse = reverse + (bitSize - len(reverse))*'0'
  
     # converts reversed binary string into integer
     print int(reverse,2)
  
# Driver program
if __name__ == "__main__":
    num = 1
    bitSize = 32
    reverseBits(num,bitSize)

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Output:

2147483648


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