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Finding first and last index of some value in a list in Python

Hi I was wondering if someone could help me with Python. I am trying to create a code that returns the last index of the last occurrence of an item in a list in a recursive way. So in a list [1,2,3,4,5,2] the last it should return 4. It only takes in 2 variables which are the list and the item that it is searching for. If it does not find any matching variable then it returns -1.

So far I have this:

def lstIndex(lst, item):
    if len(lst) == 0:
        return -1
    place = lst[0]
    if place == item:
        print(place)
        #just return the index
        return lst.index(place)
    else:
        return lstIndex(lst[1:],item)

Answer 1

I’m not quite 100% sure I know what you want. Your statement that “… in a list of [1,2,3,4,5,2] the last it should return 4 …” has me a bit confused; I think you want to return the index of the last appearance of your specified item. So, for 4 to be the result in the list specified, item must be 5.

As noted elsewhere, a recursive function would not be the most efficient or Pythonic solution here. I’d prefer a solution like the first one in nneonneo’s answer.

However, if it must be recursive, I believe the code below gets what you want. Instead of stepping through the list from the front (by using [1:]), you need to step backwards by using [:-1] as the index range when passing the list in the recursive call:

def lstIndex(lst, item):
    if len(lst) == 0:
        return -1
    elif lst[-1] == item:
        return len(lst) - 1
    else:
        return lstIndex(lst[0:-1], item)

I tested with the following:

the_list = [1,2,3,4,5,2]
print lstIndex(the_list, 2)
print lstIndex(the_list, 1)
print lstIndex(the_list, 3)
print lstIndex(the_list, 4)
print lstIndex(the_list, 5)
print lstIndex(the_list, 6)
print lstIndex(the_list, 0)

With the following output:

5
0
2
3
4
-1
-1