simple parity bit scheme

Question

I am trying to make a simple paritybit method in PyCharm, and I've written this code:

import bitarray as ba       # Library with bit strings

data = ba.bitarray(endian = 'little')       # String for data
data.frombytes('Secret message!'.encode())  # String for encoded EDC bits

def parity(bitarray):
    print("Original:\t\t\t{}".format(bitarray))
    #even = True                                        # Two protocols agree on either even or odd
    pbit = '0'                                          # Initiate parity bit as 0
    count = bitarray.count('1')                         # Count the number of ones
    print("Counted {} 1's in bitarray".format(count))
    if (count % 2) != 0:
        pbit = '1'                                      # If the bit array have an odd number of 1's we add parity bit 1 to make the number of 1's even
    print("Parity bit is {}".format(pbit))
    bitarray.append(pbit)
    print("Parity bitarray: \t{}".format(bitarray))
    return bitarray

parity(data)

And I get this output:

Original:        bitarray('110010101010011011000110010011101010011000101110000001001011011010100110110011101100111010000110111001101010011010000100')
Counted 58 1's in bitarray
Parity bit is 0
Parity bitarray: bitarray('1100101010100110110001100100111010100110001011100000010010110110101001101100111011001110100001101110011010100110100001001')

As you can see, the number of 1's is even, and the parity bit is 0, but my code has added a 1 at the end of the bitarray istead of a 0. I have tried to fix the problem, but I haven't managed to find a solution.

Still figuring what is the core issue, but the first ```if``` is redundant and ```even``` is never used — mushishi, Jun 06 '20 at 11:28
Unable to reproduce. Initially I get `'str' object has no attribute 'append'` so your sample code is not complete, but after making `data` a list it works, and a `0` gets printed as 'parity' and then appended. It also works correctly after changing any `0` to a `1` (or reverse) in the data. — Jongware, Jun 06 '20 at 11:29
I forgot to include the type of object I use as input. I have edited the sample code now. — Mampenda, Jun 06 '20 at 11:35

score 3 · Accepted Answer · answered Jun 06 '20 at 12:00

The issue is with the assignment of '0' and/or '1' to the parity bit pbit.

It seems that these strings, are both interpreted as binary ones. If you instead use the integer values 0 and 1 for pbit your code works:

def parity(bitarray):
    print("Original:\t\t\t{}".format(bitarray))
    pbit = 0 # Updated line
    count = bitarray.count('1')
    print("Counted {} 1's in bitarray".format(count))
    if (count % 2) != 0:
        pbit = 1 # Updated line
    print("Parity bit is {}".format(pbit))
    bitarray.append(pbit)
    print("Parity bitarray: \t{}".format(bitarray))
    return bitarray

This may be a bug in the bitarray module as it seems that strings seem to work in other places, such as in the .count() method, but I don't know enough about bitarray to say for certain.

Also, especially given that the pbit needs to be an integer, it would be easier to just assign it once, using the expression pbit = count % 2 directly, i.e.:

def parity(bitarray):
    count = bitarray.count('1')
    pbit = count % 2
    bitarray.append(pbit)
    return bitarray

simple parity bit scheme

1 Answers1