"""
 conditional_example.py

 in class Feb 21

"""

a = 'a'
b = 'b'
c = 'c'

text = (a, b, a, a, c, a, b)

pairs = []
for i in range(len(text) - 1):
    pairs.append( (text[i], text[i+1]) )
# print(pairs)
# Or in one line:  pairs = zip(text[:-1], text[1:])

def p_2nd(second, first):
    """ Return p(second=2nd | first=1st) conditional probability """
    # uses global "pairs" variable
    matches_first = [y for (x,y) in pairs if x == first]
    matches_both = [1 for y in matches_first if y == second]
    return len(matches_both) / len(matches_first)

def p_2nd_kenny(second, first):
    """ same (copyright 'the K')"""
    count_matches_first = 0
    count_matches_both = 0
    for (x, y) in pairs:
        if x == first:
            count_matches_first += 1
            if y == second:
                count_matches_both += 1
    return count_matches_first / count_matches_second