Understanding - Advanced, exercise 9

Text

The variable my_fn contains the string of a family name in lowercase (e.g. "doi"), and the variable my_matcontains the string of ten 0-9 digits (e.g. "0000123456"). Study the execution of the following functions when they are called as follows: c(my_fn, my_mat).

def c(fn, mat):
    r = 0
    kind = 1
    for char in mat:
        r = r + (int(char) * kind)
        kind = kind * -1

    if r < 0:
        r = r * -1

    last = 0
    r = (r + 2) % len(fn)
    chars = list(fn)
    for idx in range(r):
        last = (last + idx) % r
        tmp = chars[idx]
        chars[idx] = chars[last]
        chars[last] = tmp

    return "".join(chars)

Hints

The recursive function c processes all the digits in mat to get a value. According to this new value, the characters in the input family name are permuted and returned at the end of the execution of the algorithm.

Additional material

The runnable Python file is available online. You can run it executing the command python exercise_9.py in a shell, and then following the instructions on screen to specify the intended input.