Converting an integer to signed 2's complement binary string

Question

Right now, as far as I know, all means of conversion from int to binary bit string is for unsigned conversions (bin, format, etc.). Is there a way to quickly convert a given integer into its corresponding 2's complement bitstring (using minimal bits)?

For example, I'd want this function f to output:

f(-4) = '100'
f(5) = '0101'
f(-13) = '10011'

Right now, my implementation is this code here:

def f(x):
    """Convert decimal to two's complement binary string"""
    if x < 0:
        bs = bin(x)[3:]
        bs_pad = zero_pad(bs, roundup(tc_bits(x)))
        return bin((int(invert(bs_pad),2) + 1))#negate and add 1
    else: #Positive- sign bit 0.
        bs = bin(x)[2:]
        return "0b" + zero_pad(bs, roundup(tc_bits(x)))

which basically traces each step of the conversion process- zero-padding, negation, adding 1, then converting back to binary (it actually also ensures the bit width is a multiple of four). This was super tedious to write and I'm wondering if Python supports a faster/more code-concise way.

Yes that's my mistake, edited

km142646
– km142646

2020-08-06 00:12:20 +00:00
Commented Aug 6, 2020 at 0:12 — km142646
– km142646, Commented Aug 6, 2020 at 0:12

Tim Peters · Accepted Answer · 2020-08-05 23:51:20Z

3

Nothing built in, but this is more concise:

def f(n):
    nbits = n.bit_length() + 1
    return f"{n & ((1 << nbits) - 1):0{nbits}b}"

Then, e.g.,

>>> f(0)
'0'
>>> f(1)
'01'
>>> f(2)
'010'
>>> f(3)
'011'
>>> f(-1)
'11'
>>> f(-2)
'110'
>>> f(-3)
'101'

answered Aug 5, 2020 at 23:51

Tim Peters

71.4k14 gold badges133 silver badges140 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Marcnu · Accepted Answer · 2023-07-20 08:57:41Z

0

I'm not sure why you don't include the front 1s. Here is a solution with those 1s, slow but easier to understand than bit operations

def f(val, byte_len=1):
    int_complement = val + (256**byte_len)*(val<0)
    bin_complement = format(int_complement, 'b')
    return bin_complement 
f(-1)
'11111111'
f(-4)
'11111100'
f(5)
'101'
f(-13)
'11110011'
f(-13, byte_len=2)
'1111111111110011'

# Bonus: the reverse way too:
def twos_complement(val, byte_len=1):
    return val + (256**byte_len)*(val<0)

def reverse_twos_complement(val, byte_len=1):
    return val - (256**byte_len)*(val>(256**byte_len/2)-1)

edited Jul 20, 2023 at 8:57

answered Jul 20, 2023 at 8:18

Marcnu

213 bronze badges

Collectives™ on Stack Overflow

Converting an integer to signed 2's complement binary string

2 Answers 2

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related