two's complement of numbers in python

问题

I am writing code that will have negative and positive numbers all 16 bits long with the MSB being the sign aka two's complement. This means the smallest number I can have is -32768 which is 1000 0000 0000 0000 in two's complement form. The largest number I can have is 32767 which is 0111 1111 1111 1111.

The issue I am having is python is representing the negative numbers with the same binary notation as positive numbers just putting a minus sign out the front i.e. -16384 is displayed as -0100 0000 0000 0000 what I want to be displayed for a number like -16384 is 1100 0000 0000 0000.

I am not quite sure how this can be coded. This is the code i have. Essentially if the number is between 180 and 359 its going to be negative. I need to display this as a twos compliment value. I dont have any code on how to display it because i really have no idea how to do it.

def calculatebearingActive(i):

    numTracks = trackQty_Active
    bearing = (((i)*360.0)/numTracks)
    if 0< bearing <=179:
        FC = (bearing/360.0)
        FC_scaled = FC/(2**(-16))
        return int(FC_scaled)

    elif 180<= bearing <=359:
        FC = -1*(360-bearing)/(360.0)
        FC_scaled = FC/(2**(-16))
        return int(FC_scaled)

    elif bearing ==360:
        FC = 0
        return FC

回答1:

If you're doing something like

format(num, '016b')

to convert your numbers to a two's complement string representation, you'll want to actually take the two's complement of a negative number before stringifying it:

format(num if num >= 0 else (1 << 16) + num, '016b')

or take it mod 65536:

format(num % (1 << 16), '016b')

回答2:

The two's complement of a value is the one's complement plus one.

You can write your own conversion function based on that:

def to_binary(value):
    result = ''
    if value < 0:
        result = '-'
        value = ~value + 1
    result += bin(value)
    return result

The result looks like this:

>>> to_binary(10)
'0b1010'
>>> to_binary(-10)
'-0b1010'

Edit: To display the bits without the minus in front you can use this function:

def to_twoscomplement(bits, value):
    if value < 0:
        value = ( 1<<bits ) + value
    formatstring = '{:0%ib}' % bits
    return formatstring.format(value)

>>> to_twoscomplement(16, 3)
'0000000000000011'
>>> to_twoscomplement(16, -3)
'1111111111111101'

回答3:

If you actually want to store the numbers using 16 bits, you can use struct.

import struct

>>> struct.pack('h', 32767)
'\xff\x7f'
>>> struct.pack('h', -32767)
'\x01\x80'

You can unpack using unpack

>>> a = struct.pack('h', 32767)
>>> struct.unpack('H', a)
32767

回答4:

Since you haven't given any code examples, I can't be sure what's going on. Based on the numbers in your example, I don't think you're using bin(yourint) because you're output doesn't contain 0b. Maybe you're already slicing that off in your examples.

If you are storing your binary data as strings, you could do something like:

    def handle_negatives(binary_string):
        If binary_string < 0:
            binary_string = '1' + str(binary_string)[1:]
        Return binary_string

来源：https://stackoverflow.com/questions/21871829/twos-complement-of-numbers-in-python

标签

python

twos-complement