Question

Given that in recursion you’re using a divide-and-conquer method to break down problems into smaller pieces, how would you be able to read data from beginning of the problem, at the end of problem?

Example: I have a an encrypt() function that replaces characters in a string with characters 3 indices to the right:

A in string "ABCDEF" becomes D for example

So far I did it recursively up to the point where it stops when 3 indices to the right are undefined, but this leaves the last bit of the string the same as the original.

Example: "ABCDEF" becomes "DEFDEF"

Is there a way that I can pass the beginning of the string to the innermost functions during the recursive calls in an efficient way?

This is my code currently:

def shift_cipher_noloop(plain):

    encrypted = ""

    if(plain == ""):
        encrypted = ""

    else:
        if(len(plain) > 3):            
            temp_sub = plain[3]          
            encrypted = encrypted + temp_sub
            encrypted = encrypted + shift_cipher_noloop(plain[1:])

        else:
            temp_sub = plain[0] 
            encrypted = encrypted + temp_sub
            encrypted = encrypted + shift_cipher_noloop(plain[1:])

    return encrypted

x = "ABCDEFGHIJK"

y = shift_cipher_noloop(x)

print(y)

Answer 1

May be this does not exactly solves your problem, but you may have to mould it a little bit to make it fit. As I see somewhere you want Recursion. I have just shown how you can move to the beginning, when you reach the end of the string.

Take the modulus of i + 3 with the length of string to move to the beginning automatically: –

>>> my_str = "ABCDEF"
>>> length = len(my_str)

>>> for i in range(length):
        print my_str[(i + 3) % length],


D E F A B C

So, when i = 3, i + 3 = 6, and 6 % 6 = 0 -> back to the first character

---

If you want to use Recursion, here’s your modified program: –

def shift_cipher_noloop(plain, i):

    if(plain == ""):
        return ""

    else:
        # Iterate with your string, moving first character to last on each iteration
        if len(plain) > 3 and i > 0: 
            return shift_cipher_noloop(plain[1:] + plain[0], i - 1)

        else:
            # else Will be executed when either len <= 3, or `i <= 0`.
            # when (i <= 0) is true, you have created a new string, 
            # with each character shifted by `original i` indices. 
            # So, just return it.

            return plain


x = "ABCDEF"
index_to_shift = 3
y = shift_cipher_noloop(x, len(x) - index_to_shift)

print(y)

OUTPUT : –

DEFABC

I added one more parameter to your method, which I use to take the appropriate index. And also, I’m passing the complete string to the method each time moving the first character to the end.

Also, if the len(plain) <= 3, you can simply return the string.