Getting the middle character in a odd length strin

问题:

def get_middle_character(odd_string):
    variable = len(odd_string)
    x = str((variable/2))
    middle_character = odd_string.find(x)
    middle_character2 = odd_string[middle_character]
    return middle_character2



def main():
    print('Enter a odd length string: ')
    odd_string = input()
    print('The middle character is', get_middle_character(odd_string))

main()   

I need to figure out how to print the middle character in a given odd length string. But when I run this code, I only get the last character. What is the problem?

回答1:

You need to think more carefully about what your code is actually doing. Let's do this with an example:

def get_middle_character(odd_string):

Let's say that we call get_middle_character('hello'), so odd_string is 'hello':

    variable = len(odd_string)  # variable = 5

Everything is OK so far.

    x = str((variable/2))  # x = '2'

This is the first thing that is obviously odd - why do you want the string '2'? That's the index of the middle character, don't you just want an integer? Also you only need one pair of parentheses there, the other set is redundant.

    middle_character = odd_string.find(x)  # middle_character = -1

Obviously you can't str.find the substring '2' in odd_string, because it was never there. str.find returns -1 if it cannot find the substring; you should use str.index instead, which gives you a nice clear ValueError when it can't find the substring.

Note that even if you were searching for the middle character, rather than the stringified index of the middle character, you would get into trouble as str.find gives the first index at which the substring appears, which may not be the one you're after (consider 'lolly'.find('l')...).

    middle_character2 = odd_string[middle_character]  # middle_character2 = 'o'

As Python allows negative indexing from the end of a sequence, -1 is the index of the last character.

    return middle_character2  # return 'o'

You could actually have simplified to return odd_string[middle_character], and removed the superfluous assignment; you'd have still had the wrong answer, but from neater code (and without middle_character2, which is a terrible name).

---

Hopefully you can now see where you went wrong, and it's trivially obvious what you should do to fix it. Next time use e.g. Python Tutor to debug your code before asking a question here.

回答2:

You need to simply access character based on index of string and string slicing. For example:

>>> s = '1234567'
>>> middle_index = len(s)/2
>>> first_half, middle, second_half = s[:middle_index], s[middle_index], s[middle_index+1:]
>>> first_half, middle, second_half
('123', '4', '567')

Explanation:

• str[:n]: returns string from 0th index to n-1th index
• str[n]: returns value at nth index
• str[n:]: returns value from nth index till end of list

回答3:

Should be like below:

def get_middle_character(odd_string):
    variable = len(odd_string)/2
    middle_character = odd_string[variable +1]
    return middle_character 

回答4:

i know its too late but i post my solution I hope it will be useful ;)

def get_middle_char(string):
    if len(string) % 2 == 0:
        return None
    elif len(string) <= 1:
        return None

    str_len = int(len(string)/2))

    return string[strlen]