在Python中查找最后一个位置

Question

I am learning Python and I am trying to resolve this problem without len() but I am still stuck. Here is a quiz explanation.

Define a procedure, find_last, that takes as input two strings, a search string and a target string, and returns the last position in the search string where the target string appears, or -1 if there are no occurrences.

Example: find_last('aaaa', 'a') returns 3

Make sure your procedure has a return statement.

Here is my code:

def find_last(search, target):
    target = search.find(target)
    while target:
        if target == -1:
            return target
        else:
            return target +1
    return target

Thank you.

Answer 1

You have several errors to fix:

1. target is the target string, not the position where you find it. You're overwriting that value.
2. You may need to call search.find multiple times
3. You only break out of the loop once search.find returns -1. Your function should then return the previous value returned by search.find .

Fixing each of those issues gives you

def find_last(search, target):
    # Find the first occurrence of target
    pos = search.find(target)

    # If you found something, keep looking for it until you don't
    # find it again
    while pos >= 0:
        # You found target once; now look for the next occurrence
        next_pos = search.find(target, pos + 1)
        if next_pos == -1:
            # no more targets, so stop looking
            break
        pos = next_pos 
    return pos

You could also use return pos instead of break .

---

As pointed out in the comments, though, search.rfind(target) already does what you want.