Returning the X+1 item in a list of strings

Question

So far I have a function that creates gray code then I need to make a function that will return the gray code "plus one", basically the next in sequence. so if D = (0,0,1,1) I need to return (0,0,1,0)

I have

def gray(x):
    if x:
        return ['0' + x[0]] + gray(x[1:]) + ['1' + x[0]]
    else:
        return []

def graycode(n):
    if n:
        return gray(graycode(n-1))
    else:
        return ['']

then lastly,

def GrayFinal(D):
    z = ''.join(map(str,D))
    str(z)
    if z in graycode(len(D)):
        return graycode(len(D))[z+1]
    else:
        return ['']

I can't figure out how to return the Zth+1 entry

Answer 1

If I interpreted your question correctly, the heart of your question really has nothing to do with Gray codes, but rather with the more general question, " Given an element I wish to find in a Python list, how do I retrieve the next element after that? "

Unfortunately I can only come up with a rather brain-dead solution right now which is

def next_elem(elem, input_list):
    index = input_list.index(elem)
    return input_list[index + 1]

Note that this has absolutely no support whatsoever for error checking, which I assume in your case would be done in the body of the code. Throwing that into your code would result in the following:

def GrayFinal(D):
    z = ''.join(map(str,D))
    try:
        return next_elem(z, graycode(len(D)))
    except ValueError:
        # Doesn't look like z was ever in the Gray code generated
        return ['']
    except IndexError:
        # The next element is beyond the last element of the array!
        return next_elem('0' + z, graycode(len(D) + 1))

Answer 2

Here something I heard about:

>>> bin2gray = lambda x: (x >> 1) ^ x # create graycode
>>> for i in range(10):
    print(bin(bin2gray(i))[2:].zfill(4))


0000
0001
0011
0010
0110
0111
0101
0100
1100
1101

>>> def gray2bin(g):
    bits = list(map(int, bin(g)[2:]))
    n = [0]
    for bit in bits:
        if not n[-1]:
            n.append(bit)
        else:
            n.append(1 - bit)
    return sum([n[i] << (-1 - i) for i in range(-len(n), 0)])

>>> def inc(g):
    return bin2gray(gray2bin(g) + 1)

You will need to adapt the inc function if you want it to work with strings instead of ints.