converting binary string into float

Question

I have an object that I am storing bits in.

class Bitset:
    def __init__(self, bitstring):
        self.bitlist = []
        for char in bitstring:
            self.bitlist.append(int(char))

    def flipBit(self, index):
        val = self.bitlist[index]
        val = (val + 1) % 2
        self.bitlist[index] = val
        self.newBitstring()

    def bitstring(self):
        newString = ''
        for val in self.bitlist:
            newString = newString + str(val)
        return newString

    def __len__(self):
        return len(self.bitlist)

    def __str__(self):
        return self.bitstring()

    def __repr__(self):
        return self.bitstring()

Is there anyway I can convert the bits into a float? Thanks.

I'm not sure what you mean by "Convert the bits into a float". Do you want to do the equivalent of a C typecast (i.e. have the system interpret the sequence of bits as if it were a float)? Also, if this is a homework problem, you should add the [homework] tag. — Carl F.
– Carl F., Commented Sep 13, 2011 at 1:22
Thank you for understanding. It is essentially a C typecast. This is not a homework problem though. I just have no idea how to do this. — nickles
– nickles, Commented Sep 13, 2011 at 1:24
How do you want to interpret them as a float? As a number between 0 and 1? Two's compliment? You need to be more specific? struct only works for floats in IEEE 754 binary format. — agf
– agf, Commented Sep 13, 2011 at 1:36
I would like to interpret them using the IEEE 754 format. When I try using struct to unpack it I am told that the string can only be 4 characters long. Doesn't the standard require 32 bits? — nickles
– nickles, Commented Sep 13, 2011 at 1:44
You are not storing bits. You are storing integers that happen to have the values 0 or 1. Forget this at your peril. — Russell Borogove
– Russell Borogove, Commented Sep 13, 2011 at 2:04

Carl F. · Accepted Answer · 2011-09-13 01:53:47Z

5

Here is a solution that works. as_float32 could be extended to as_float64 by replacing "I" with "L" and "f" with "d". See the struct documentation for an explanation.

def as_float32(self):
    """
    See: http://en.wikipedia.org/wiki/IEEE_754-2008
    """
    from struct import pack,unpack
    s = self.bitlist
    return unpack("f",pack("I", bits2int(s)))

# Where the bits2int function converts bits to an integer.  
def bits2int(bits):
    # You may want to change ::-1 if depending on which bit is assumed
    # to be most significant. 
    bits = [int(x) for x in bits[::-1]]

    x = 0
    for i in range(len(bits)):
        x += bits[i]*2**i
    return x

edited Sep 13, 2011 at 1:53

answered Sep 13, 2011 at 1:48

Carl F.

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1 Comment

Carl F. Over a year ago

I found a few errors when I adapted the code from my spyder window to something that should work in your object. They should be corrected now.

Scott Griffiths · Accepted Answer · 2011-11-21 12:05:59Z

5

There are libraries that can do all this for you if you don't want to reinvent the wheel. My bitstring library could help:

>>> from bitstring import BitArray
>>> a = BitArray(float=0.34, length=32)
>>> a.bin
'00111110101011100001010001111011'
>>> a.float
0.3400000035762787
>>> a.bin = '01001001011101000010010000000000'
>>> a.float
1000000.0
>>> a.bytes = 'helloall'
>>> a.float
7.819486525216155e+194

The float interpretation is only allowed when there are either 32 or 64 bits; take a look at the documentation for more details.

edited Nov 21, 2011 at 12:05

answered Sep 13, 2011 at 9:53

Scott Griffiths

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2 Comments

Prashant Over a year ago

Useful library to convert between IEEE754 and binary32 notation

noobcoder Over a year ago

it adds extra digits in the float representation. The original number is 0.34. How to keep it as 0.34?

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