Convert signed binary number into integer in Python

Question

I found out that in python, if you use bin:

>>> bin(4)
'0b100'  
>>> bin(-4)
'-0b100'

but if you try this:
```
-4 >> 8 => -1
```

in fact, when right shift more than 3 bits, the answer is always -1. I know this is implementation stuff, but I came across this problem:

In the following array, find out the number which appears only once, while others appears always three times

[-2,-2,1,1,-3,1,-3,-3,-4,-2]

This is the code

class Solution(object):
    def singleNumber(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        if nums == None:
            return None
        ans = 0
        for i in range(32):
            numOfI = 0
            for num in nums:
                if (( num >> i) & 1) == 1 :
                    numOfI += 1
                    numOfI %=3
            print i,numOfI
            if numOfI != 0:
                ans = ans + (numOfI << i)
        return ans

the answer should be -4, but instead I get 11111111111111111111111111111100; how can I convert it into -4?

Julien Palard · Accepted Answer · 2016-06-02 20:24:42Z

3

Bit manipulation on Python are easily done with int.from_bytes and int.to_bytes.

On the other hand, solving the "find the value appearing x times" is easily solved using the Counter class:

>>> from collections import Counter
>>> values = [-2,-2,1,1,-3,1,-3,-3,-4,-2]
>>> [value for value, count in Counter(values).items() if count == 1]
[-4]

Using from_bytes and to_bytes is clear and readable, so you don't want to try to do the conversion yourself.

Using Counter is also more readable than your cryptic implementation, yet my one-liner may not be the most readable imaginable version, it's still a single line and far more readeable.

answered Jun 2, 2016 at 20:24

Julien Palard

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