Counting length-2 substrings that are common to two strings at the same offset -- Python

Question

I solved the CodingBat task:

Given 2 strings, a and b, return the number of the positions where they contain the same length 2 substring. So "xxcaazz" and "xxbaaz" yields 3, since the "xx", "aa", and "az" substrings appear in the same place in both strings.

stringMatch("xxcaazz", "xxbaaz") → 3  
stringMatch("abc", "abc") → 2  
stringMatch("abc", "axc") → 0

import doctest


def all_two_chars_occurencies(string):
    """
    >>> list(all_two_chars_occurencies('abcd'))
    ['ab', 'bc', 'cd']
    >>> list(all_two_chars_occurencies('xxcaazz'))
    ['xx', 'xc', 'ca', 'aa', 'az', 'zz']
    """
    for index, char in enumerate(string[:-1]):
        yield char + string[index + 1]


def common_two_chars_occurences(a, b):
    """
    Given 2 strings, a and b, return the number of the positions where
    they contain the same length 2 substring.

    >>> common_two_chars_occurences('xxcaazz', 'xxbaaz')
    3
    >>> common_two_chars_occurences('abc', 'abc')
    2
    >>> common_two_chars_occurences('abc', 'axc')
    0
    """
    equal_duets = 0
    for a_duet, b_duet in zip(all_two_chars_occurencies(a),
                              all_two_chars_occurencies(b)):
        if a_duet == b_duet:
            equal_duets += 1
    return equal_duets


if __name__ == "__main__":
    doctest.testmod()

Answer 1

There is no such word as "occurencies". In any case, all_two_chars_occurencies is a very long name. I suggest pairwise_chars. Note that it could also be written in a style similar to the pairwise() recipe in Python's documentation.

In Python, explicit looping is slightly cumbersome. Fortunately, Python offers many ways to just do what you want as a "one-liner", without looping. Here's one approach, which uses a generator expression, and this technique to find its length.

def common_two_chars_occurences(a, b):
    """
    Given 2 strings, a and b, return the number of the positions where
    they contain the same length 2 substring.

    >>> common_two_chars_occurences('xxcaazz', 'xxbaaz')
    3
    >>> common_two_chars_occurences('abc', 'abc')
    2
    >>> common_two_chars_occurences('abc', 'axc')
    0
    """
    return sum(1 for pair
                 in zip(pairwise_chars(a), pairwise_chars(b))
                 if pair[0] == pair[1])

Answer 2

You should refactor further writing an equal_at_same_index function:

def equal_at_same_index(list_1, list_2):
    """
    >>> equal_at_same_index("abcde", "xxcae")
    2
    >>> equal_at_same_index("qwerty", "xqwerty")
    0
    """
    return sum((i == j for i, j in zip(list_1, list_2)))


def common_two_chars_occurences(a, b):
    """
    Given 2 strings, a and b, return the number of the positions where
    they contain the same length 2 substring.

    >>> common_two_chars_occurences("xxcaazz", "xxbaaz")
    3
    >>> common_two_chars_occurences("abc", "abc")
    2
    >>> common_two_chars_occurences("abc", "axc")
    0
    """
    return equal_at_same_index(all_two_chars_occurencies(a),
                               all_two_chars_occurencies(b))