How to convert between floats and decimal twos complement numbers in Python?

Question

I want to convert decimal float numbers in python into twos complement decimal binary numbers. For example 1.5 in twos complement decimal 2.6 (8 bits) would be 0b011000.

Is there a module that can do this for me?

"decimal binary" suggests that you may not understand what you're doing. What's your input, and what's your desired output? (And are you trying to reimplement the number system?) — user2357112, Jul 10 '13 at 23:51
I edited it to provide an example. I thought it was pretty understandable. twos complement: http://en.wikipedia.org/wiki/Two's_complement binary decimal: http://en.wikipedia.org/wiki/Binary_number#Decimal — Chet, Jul 15 '13 at 04:15
Okay, I think I understand what you want to do, but the terms you're using to describe it don't mean anything like what you think they mean. — user2357112, Jul 15 '13 at 04:20
what dont you understand? "twos complement decimal binary numbers" or "decimal float numbers"? — Chet, Jul 15 '13 at 04:21
Decimal has nothing to do with anything you're talking about. — user2357112, Jul 15 '13 at 04:23

score 1 · Answer 1 · answered Jul 15 '13 at 04:37

What you're describing has nothing at all to do with decimal. You want to convert a float to a fixed-point binary representation. To do this, you would multiply the float by a scale factor, cast it to an integer, and use Python's built-in string formatting tools to get the string representation:

def float_to_binary(float_):
    # Turns the provided floating-point number into a fixed-point
    # binary representation with 2 bits for the integer component and
    # 6 bits for the fractional component.

    temp = float_ * 2**6  # Scale the number up.
    temp = int(temp)     # Turn it into an integer.
    # If you want -1 to display as 0b11000000, include this part:
    # if temp < 0:
    #     temp += 2**8

    # The 0 means "pad the number with zeros".
    # The 8 means to pad to a width of 8 characters.
    # The b means to use binary.
    return '{:08b}'.format(temp)

score 0 · Accepted Answer · answered Jul 10 '13 at 23:28

Well, I couldnt find anything so I wrote some code and tested it.Should work...

def floatToTwosComplementDecimal(intBits,decBits,number):
    if decBits == 0:
        mx = pow(2,intBits-1) - 1 # maximum number
    else:
        mx = pow(2,intBits-1) - pow(2,-1*decBits) # maximum number
    mn = -1*pow(2,intBits-1) # minimum number
    if number > mx:
        print "number:" + str(number) + " has been truncated to: " + str(mx)
        number = mx
    elif number < mn:
        print "number:" + str(number) + " has been truncated to: " + str(mn)
        number = mn
    n = []
    m = 0
    if number < 0:
        n.append(1)
        m = -1*pow(2,intBits-1)
    else:
        n.append(0)
        m = 0
    for i in reversed(range(intBits-1)):
        m1 = m + pow(2,i)
        if number < m1:
            n.append(0)
        else:
            n.append(1)
            m = m1
    for i in range(1,decBits+1):
        m1 = m + pow(2,-1*i)
        if number < m1:
            n.append(0)
        else:
            n.append(1)
            m = m1
    return string.join([str(i) for i in n], '')

def twosComplementDecimalToFloat(intBits,decBits,binString):
    n = 0.0
    if binString[0] == "1":
        n = -1*pow(2,intBits-1)
    for i in range(intBits-1):
        n += int(binString[intBits-1-i])*pow(2,i)
    for i in range(1,decBits+1):
        n += int(binString[intBits-1+i])*pow(2,-1*i)
    return n

score 0 · Answer 3 · answered Jul 16 '21 at 22:06

You can use the Binary fractions package. This package implements TwosComplement with binary integers and binary fractions. You can convert binary-fraction strings into their twos complement and vice-versa

Example:

>>> from binary_fractions import TwosComplement
>>> str(TwosComplement(-1.5)) # float --> TwosComplement
'10.1'
>>> str(TwosComplement(1.5)) # float --> TwosComplement
'01.1'
>>> TwosComplement.to_float("11111111111") # TwosComplement --> float
-1.0
>>> TwosComplement.to_float("11111111100") # TwosComplement --> float
-4.0
>>> str(TwosComplement(5)) # int --> TwosComplement
'0101'

To use this with Binary's instead of float's you can use the Binary class inside the same package.

PS: Shameless plug, I'm the author of this package.

How to convert between floats and decimal twos complement numbers in Python?

3 Answers3