[Tutor] Negative Signed Binary Conversion

[Eric Brunson]

So far I've gotten five copies of this.  It could be mailman doing 
something wrong, but if you're resending because you don't see your 
reply, please be patient, this list is not always instantaneous.

I think you're missing the gist of what I'm saying.  Calculate the 
binary of the absolute.  I'm not going to debug your code for you, I'm 
just offering a suggestion to simplify your logic.

Here's the way I might do it:

def toBinary( num ):
    bits = []
    negative = num < 0
    n = abs( num )
    while not bits or n:
        n, bit = divmod(n,2)
        bits.append( str(bit) )

    bits.reverse()
    binary = "".join( bits )
    if negative:
        return( twoscomplement( binary ) )
    else:
        return( binary )


For what it's worth...

ddm2 at sfu.ca wrote:
> For Two's Complement, I would have to re-assign the 1's to 0's, and vice
> versa, then add one in binary. That's my second step, and I'm pretty sure I
> can get it to work IF I could get this negative signed binary part to work.
>
> On Mon, 22 Oct 2007 12:09:27 -0600 brunson at brunson.com wrote:
>   
>> Why not find the sign, calculate the binary of the absolute value, then 
>> make the result negative (twos complement) if necessary?
>>
>> Just a thought.
>>
>> ddm2 at sfu.ca wrote:
>>     
>>> Hi,
>>> I'm trying to get this binary converter working, but I can't seem to 
>>>       
>> get the
>>     
>>> negatives to work properly. If the integer is too low, the 0's that are
>>> added for the amount of bits gets out of whack. I've tried to solve the
>>> problem by adding another 'count' meter by which I can then tell if
>>>       
> there
>   
>>> are too many 0's, but it hasn't worked at all.
>>> CODE:
>>> def conversion(n):
>>> 	b = ''
>>> 	while n > 0:
>>> 	r = n%2
>>> 	n = n/2
>>> 	r = str(r)	
>>> 	b = r+b 	
>>> 	return b	
>>>
>>> def positive(n,bits,count):
>>> 	append_zeros = bits - count
>>> 	if append_zeros > 0:
>>> 	return "0" + (append_zeros-1)*"0" + conversion(n)
>>>
>>> def negative(n,bits,count,new_count):
>>> 	n = abs(n)
>>> 	append_zeros = bits - count
>>> 	while new_count > bits and new_count < bits-count:
>>> 	append_zeros = append_zeros - 2
>>> 	continue
>>> 	if append_zeros > 0:
>>> 	return "1" + (append_zeros-6)*"0" + conversion(n)
>>>
>>> def signed_mag():
>>> 	print ""
>>> 	print "--------------------"
>>> 	print "Signed Binary"
>>> 	print "--------------------"
>>> 	print ""
>>> 	n = int(raw_input("Please enter a signed integer: "))
>>> 	bits = int(raw_input("Please enter the number of bits: "))
>>> 	count = conversion(n).count("0") + conversion(n).count("1")
>>> 	new_count = 0
>>> 	new_count = negative(n,bits,count,new_count).count("0") \
>>> 		+ negative(n,bits,count,new_count).count("1")
>>> 	if bits - 1 < count:
>>> 	print "Out of range."
>>> 	else:
>>> 	if n < 0:
>>> 	    print n,"is encoded as", negative(n,bits,count,new_count), \
>>> 		  "in Signed Binary."		
>>> 	elif n > 0:
>>> 	    print n,"is encoded as", positive(n,bits,count), \
>>> 		  "in Signed Binary."
>>> 	else:
>>> 	    print "That is not an valid signed interger."
>>>
>>> Any help will be greatly appreciated. Thanks in advance.
>>>
>>> Cheers,
>>> Devon
>>> _______________________________________________
>>> Tutor maillist  -  Tutor at python.org
>>> http://mail.python.org/mailman/listinfo/tutor
>>> 	
>>>       
>>