CODEWITHSUNDEEP
pythonbinaryintegertwos-complement
langlauf.io
langlauf.io

Reputation: 3221

convert python integer to its signed binary representation

Given a positive integer such as 171 and a "register" size, e.g. 8.

I want the integer which is represented by the binary representation of 171, i.e. '0b10101011' interpreted as twos complement.

In the present case, the 171 should become -85. It is negative because given the "register" size 8, the MSB is 1.

I hope I managed to explain my problem. How can I do this conversion?

What I tried:

size = 8
value = 171

b = bin(value)

if b[len(b)-size] == '1':
    print "signed"
    # What to do next?

Upvotes: 0

Views: 4469

Answers (2)

Vincent
Vincent

Reputation: 13425

You don't need binary conversion to achieve that:

>>> size = 8
>>> value = 171
>>> unsigned = value % 2**size 
>>> signed = unsigned - 2**size if unsigned >= 2**(size-1) else unsigned
>>> signed
-85

Upvotes: 3

Jeffrey Nichols
Jeffrey Nichols

Reputation: 185

There are probably a hundred different ways to do this. Here are a couple.

If the size is a multiple of 8, then something like this will do the job:

x = int.from_bytes(value.to_bytes(size // 8, byteorder='big'), byteorder='big', signed=True)

If the size is not a multiple of 8, then you can do something like this:

mask = 1 << (size - 1)
x = (value ^ mask) - mask

Both assume the value isn't too big to fit into the "register".

Upvotes: 2

Related Questions