I needed to be able to generate a sequence of letters from a specific index value. Basically, I wanted to loop through a sequence of values and retrieve the corresponding string value. For example, 0 would be A; 4 would be E; 36 would be AK; etc.

    0 = A
    1 = B
    2 = C
    3 = D
    4 = E
    5 = F
    21 = V
    22 = W
    23 = X
    24 = Y
    25 = Z
    26 = AA
    27 = AB
    28 = AC
    36 = AK
    37 = AL
    38 = AM
    39 = AN

I have created a small python script as a proof of concept that does the calculation using two different methods.

Method 1

Method 1, uses a recursive function to build up the correct sequence from the index value. Essentially the code is treating the index integer as a value that has the index into a large 2D array encoded within it. We use the modulus method to extract the index for the particular row. We then subtract the difference from the index to see if there are any more letters stored in the encoded index. Method 1 returns the value of the sequence in the correct order.

    def get_name(index):
        """
        Translate the index to a set of letters in sequential order.
        """

        #termination criteria
        if index < 0:
            return ''

        #Take the modulus of the current index and translate it into one of the
        #characters stored in the letters string.
        selected_index = index % lettersCount

        return letters[selected_index] + get_name(math.floor((index - lettersCount)/lettersCount))

Method 2

Method 2 works in a similar manor, yet simpler, to Method 1. However it calculates the sequence in reverse order. So in order for the result to be useful, the sequence must be reversed.

    def get_name2(index, values = []):
        """
        Translate the index to a set of letters in sequential order, but reversed.
        """

        if not values:
            values = []

        #termination criteria
        if index < 0:
            return values

        #Take the modulus of the current index and translate it into one of the
        #characters stored in the letters string.
        selected_index = index % lettersCount
        values.append(letters[selected_index])

        return get_name2(math.floor((index - lettersCount)/lettersCount), values)

Demonstration Script

Here is a script to demonstrate the methods:

    #!/usr/bin/env python3
    #-*- coding:utf-8 -*-
    """
        index = 0, value = A
        index = 1, value = B
        index = 2, value = C
        index = 3, value = D
        index = 4, value = E
        index = 5, value = F
        index = 6, value = G
        index = 7, value = H
        index = 8, value = I
        index = 9, value = J
        index = 10, value = K
        index = 11, value = L
        index = 12, value = M
        index = 13, value = N
        index = 14, value = O
        index = 15, value = P
        index = 16, value = Q
        index = 17, value = R
        index = 18, value = S
        index = 19, value = T
        index = 20, value = U
        index = 21, value = V
        index = 22, value = W
        index = 23, value = X
        index = 24, value = Y
        index = 25, value = Z
        index = 26, value = AA
        index = 27, value = AB
        index = 28, value = AC
        index = 29, value = AD
        index = 30, value = AE
        index = 31, value = AF
        index = 32, value = AG
        index = 33, value = AH
        index = 34, value = AI
        index = 35, value = AJ
        index = 36, value = AK
        index = 37, value = AL
        index = 38, value = AM
        index = 39, value = AN
    """

    import math
    import sys

    letters = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'
    # letters = 'ABCDEF'
    lettersCount = len(letters)

    def get_name(index):
        """
        Translate the index to a set of letters in sequential order.
        """

        #termination criteria
        if index < 0:
            return ''

        #Take the modulus of the current index and translate it into one of the
        #characters stored in the letters string.
        selected_index = index % lettersCount

        return letters[selected_index] + get_name(math.floor((index - lettersCount)/lettersCount))

    def test1():
        print(letters)
        print('index = 8:  ', get_name(8)[::-1])
        print('index = 15: ', get_name(15)[::-1])
        print('index = 17: ', get_name(17)[::-1])
        print('index = 18: ', get_name(18)[::-1])
        print('index = 75: ', get_name(75)[::-1])
        print('index = 1000: ',get_name(1000)[::-1])
        print()

        for i in range(100):
            print('index = {}, value = {}'.format(i, get_name(i)[::-1]))

    def get_name2(index, values = []):
        """
        Translate the index to a set of letters in sequential order, but reversed.
        """

        if not values:
            values = []

        #termination criteria
        if index < 0:
            return values

        #Take the modulus of the current index and translate it into one of the
        #characters stored in the letters string.
        selected_index = index % lettersCount
        values.append(letters[selected_index])

        return get_name2(math.floor((index - lettersCount)/lettersCount), values)


    def test2():
        print(letters)

        for i in range(1000):
            print('index = {}, value = {}'.format(i,
                                                  ''.join(reversed(get_name2(i)))))


    def main():
        """
        This runs the rest of the functions in this module
        """

        print('test1')
        test1()

        print('test2')
        test2()

        return 0 # success


    if __name__ == '__main__':
        status = main()
        sys.exit(status)