Understanding Python sequence

Question

I am doing a hackerrank example called Flipping bits where given a list of 32 bit unsigned integers. Flip all the bits (1->0 and 0->1) and return the result as an unsigned integer. The correct code is:

def flippingBits(n):
    seq = format(n, '032b')
    return int(''.join(['0' if bit == '1' else '1' for bit in seq]), 2)

I dont understand the last line, what does the ''. part do? and why is there a ,2 at the end?

I have understood most of the code but need help in understanding the last part.

Does this answer your question? [Why is it string.join(list) instead of list.join(string)?](https://stackoverflow.com/questions/493819/why-is-it-string-joinlist-instead-of-list-joinstring) — mozway, Jan 17 '23 at 07:36

score 0 · Answer 1 · answered Jan 17 '23 at 07:40

what does the ''. part do

'' represents an empty string which will be used as separator to join collection elements into string (some examples can be found here)

and why is there a ,2 at the end?

from int docs:

class int(x=0)
class int(x, base=10)
Return an integer object constructed from a number or string x

In this case it will parse the string provided in binary format (i.e. with base 2) into int.

abdou_dev · Answer 2 · 2023-01-17T09:20:32.837

I hope the below explanation helps:

def flippingBits(n):
  seq = format(n, '032b')  # change the format from base 10 to base 2 with 32bit size unsigned integer into string
  return int(''.join(['0' if bit == '1' else '1' for bit in seq]), 2)
# ['0' if bit == '1' else '1' for bit in seq]  => means: build a list of characters from "seq" string
# in which whenever there is 1 convert it to 0, and to 1 otherwise; then
# ''.join(char_list) => means: build string by joining characters in char_list
# without space between them ('' means empty delimiter); then
# int(num_string, 2) => convert num_string from string to integer in a base 2

Notice that you can do the bit flipping by using bit-wise operations without converting to string back and forth.

def flippingBits(n):
  inverted_n = ~n    # flip all bits from 0 to 1, and 1 to 0
  return inverted_n+2**32  # because the number is a signed integer, the most significant bit should be flipped as well

Oops, *since bit size of an integer in python is 32 by default*... Are you really sure? It was indeed true in old Python 2 versions, but in current Python3.x versions, the integer type is a multiprecision type than can use as many bits as required. Your code works fine anyway for any positive integer below 2^32 which is OP's concern. — Serge Ballesta, Jan 17 '23 at 08:54
Thank you @SergeBallesta for pointing out this. The answer has been corrected. — abdou_dev, Jan 17 '23 at 09:26

Understanding Python sequence

2 Answers2