如何在python中正确递归？

Question

so, we have two lists of int, and all i am trying to do is to check if first int of list 2 exists in list 1. If so, then negate the first int of list 1. I tried to use recursion, however, i get an error saying "TypeError: 'int' object is unsubscriptable". Help?

def search(l1,l2):
    if l1 == [] and l2 == []:
        return []
    elif l1[0] == l2[0]:
        return [-(l1[0]),search(l1[1:],l2[1:])]
    else:
        return search(l1[1:],l2[0])

print search([1,2,1],[1,3,4])

Answer 1

A recursive function has to terminate. Your function will only terminate if both lists are empty, which can only happen if you get a hit on your first elif all times, if the last elif kicks in then you will have lists of unequal lengths (also if you make sure you pass a list as an argument and not an integer).

So your first elif suggest you want to check if the first element of l1 is equal to the first of l2, the second of l1 equal to the second of l2, the nth of l1 equal to l2, and so on. The second, however suggests that you want check if any elements in l1 is equal to the first element of the original l2. If you are just looking for list membership of l2[0] in l1, you can do it a lot easier.

So it seems that you need to be more explicit about what you want to return here. The function as it stands cannot work as you try to pass an integer (in the last elif) to the function search that expects a list.