Question

I have to write a program that finds all of the index values for specific element in list or a string. I have to use recursion, and my function can only take two arguments.

My problem is that my program finds only the first index and then stop. How can I change it to meet my requirements?

My code:

def find_all(L, v):
    return 0 if L[0] == v else 1 + find_all(L[1:], v)

Input:

1. find_all( [1,2,3,4,2,4,5,2,1], 2)
2. find_all("hello wonderful world", "w")

Desired output:

1. [1,4,7]
2. [6,16]

Answer 1

You can use Pythons ability to walk backwards through a list and grab the last element. Then put lists together with the + operator. By going through the list backwards you’re able to find the indice when a value is found, rather than losing it when you move from the start of the list to the end.

def find_all(L, v):
    if not L:
            return []

    result = []
    if L[-1] == v:
            result = [len(L)-1]

    return find_all(L[:-1], v) + result