Question

I would like to build an an encoder and decoder using text coding.

A string “AAABBBBCDDDDDDDDDDEEDDDD” as input, returning a string “A3B4C1D10E2D4”, where each alphabet symbol is followed by its frequency in the string. The decoder reverses the process.

Would like help getting started in python.

Answer 1

One possible solution for the cnoder would be to simply iterate over the string and count the character occurences, not very fancy but O(n).

def encode(s):
    last  = s[0]
    count = 0
    for c in s:
        if last != c:
            yield '%s%i' % (last, count)
            last = c
            count = 0
        count += 1
    yield '%s%i' % (last, count)

For the decoder you could use a regular expression which splits the string up nicely for you, no need to write your own parser.

import re

def decode(s):
    for c, n in re.findall(r'(\w)(\d+)', s):
        yield c * int(n)

given your test input

s = 'AAABBBBCDDDDDDDDDDEEDDDD'

encoded = ''.join(encode(s))
print encoded

decoded = ''.join(decode(encoded))
print decoded

results in

A3B4C1D10E2D4
AAABBBBCDDDDDDDDDDEEDDDD

One more note, there’s no real reason to use yield here, you could of course also build the strings in the en-/decode functions first, then return.