Question

I am trying to get a counter that looks through a text and returns the frequency of a letter with regard to the previous pair of letters.
For example part of the output would be :

'th' : Counter ({'e':119, 'a':145 etc... })

I want it to iterate over all possible pairs in the lowercase characters.

Until now, I have been using the following code to get an output that only takes into account the previous letter:

def pairwise(iterable):
    it = iter(iterable)
    last = next(it)
    for curr in it:
        yield last, curr
        last = curr

valid = set('abcdefghijklmnopqrstuvwxyz ')

def valid_pair((last, curr)):
    return last in valid and curr in valid

def make_markov(text):
    markov = defaultdict(Counter)
    lowercased = (c.lower() for c in text)
    for p, q in ifilter(valid_pair, pairwise(lowercased)):
        markov[p][q] += 1
    return markov

Answer 1

Untested:

def pairwise(iterable):
    it = iter(iterable)
    last = next(it)+next(it)
    for curr in it:
        yield last, curr
        last = last[1]+curr


def valid_pair((last, curr)):
    return last[0] in valid and last[1] in valid and curr in valid