如何使用递归在多个列表中查找最长的字符串？ （蟒蛇）

Question

I'm new here and fairly new to python and I have a question. I had a similar question during my midterm a while back and it has bugged me that I cannot seem to figure it out.

The overall idea was that I had to find the longest string in a nested list. So I came up with my own example to try and figure it out but for some reason I just can't. So I was hoping someone could tell me what I did wrong and how I can go about the problem without using the function max but instead with a for loop. This is my own example with my code:

typing_test = ['The', ['quick', 'brown'], ['fox', ['jumped'], 'over'], 'the', 'lazy', 'dog']

def longest_string (nested_list: 'nested list of strings') -> int:
'''return the longest string within the nested list'''

    maximum_length = 0
    for word in nested_list:
        try:
            if type(word) == str:
                maximum_length >= len(word)
                maximum_length = len(word)
            else:
                (longest_string((word)))
        except:
            print('Error')

    return maximum_length 

My code returns 3 but the highest should be 6 because of the length of jumped I'm not sure if it's going through each list and checking each strings length. In short I don't think it is replacing/updating the longest string. So if someone can tell me what I'm doing wrong or how to fix my example I would greatly appreciate it. And thank you very much in advance.

Answer 1

There's a few problems in your code:

• You need to store the value of the recursive call ( maximum_length = (longest_string((word))) )
• You need to replace the current max value if the length of the word is greater (not less as in your code). There's also no if in this part of your code.

This is the modified code of your function (with minimal modifications to make it work):

def longest_string (nested_list):

    maximum_length = 0
    for word in nested_list:
        try:
            if type(word) == str:
                if maximum_length < len(word):
                    maximum_length = len(word)
            else:
                maximum_length = longest_string((word))
        except:
            print('Error')

    return maximum_length

Answer 2

You are going to use the recursion to figure out the longest string in a nested list.

The recursion should be able to have an end condition. For your question, I would like to put "If the parameter is a str".

Then, suppose the parameter will always be str or list, and we do not consider other boundary conditions, the code will be like:

def longest_string(p):
    if type(p) == str:
        return len(p)  # Return the length of the str.

    return max([longest_string(x) for x in p])

Answer 3

You can use a List Comprehension:

def longest(lst):
    return max([0]+[i for i in lst if isinstance(i,int)]+
               [len(s) for s in lst if isinstance(s,str)]+
               [longest(L) for L in lst if isinstance(L,list)])

The [0] is needed for an empty list including at the top level.