如何索引python列表中的中间字符

Question

Complete the get_mid_letter() function which is passed a list of strings as a parameter. The function returns a string made up of the concatenation of the middle letter of each word from the parameter list. The string returned by the function should be in lowercase characters. If the parameter list is an empty list, the function should return an empty string.

def get_mid_letter(a_list):
    middle_list = []
    for item in a_list:
        middle_index = int(len(item) / 2)
        middle_letter = a_list.index(middle_index)
        middle_list = middle_list + [middle_letter] 
    return middle_list.lower()

def test_get_mid_letter():
    print("1.", get_mid_letter(["Jess", "Cain", "Amity", "Raeann"]))

In my case, it shows an error message like "2 is not in the list". What can I do to run my code successfully? Thanks!

Answer 1

array.index(element) returns an index not the character or element . So you use array[index_mid] to get the character and then append it to the middle_list

Answer 2

As the other answer pointed out, the mistake is here:

middle_letter = a_list.index(middle_index)

The index() method is looking for an element equal to middle_index (in this case, 2) in the list, trying to return the index of that element, but since there's no element equal to 2 in the list you get that error. To find the middle_letter , you should directly access the list item:

middle_letter = item[middle_index]

Be also aware that you're using the lower() method on a list, which is gonna cause an error. To get around this problem, you can use lower() on middle_letter for each loop iteration and simply return middle_list , like this:

for item in a_list:
    middle_index = int(len(item) / 2)
    middle_letter = item[middle_index]
    middle_letter = middle_letter.lower()
    middle_list = middle_list + [middle_letter] 
return middle_list

Answer 3

There are a few problems with your code.

• The function is supposed to return a string but you declare middle_list as a list not a string.
• The index method returns the position in the list of its argument. You want to use [] for access.
• You are not retrieving the character from the item, but the item from the list.

The corrected code could look like below:

def get_mid_letter(a_list):
    middle_list = ""                        # declare the variable as an empty string
    for item in a_list:
        middle_index = int(len(item) / 2)   # get the middle index
        middle_letter = item[middle_index]  # get the char from the string in the list
        middle_list += middle_letter        # concatenate the char and the string
    return middle_list.lower()

Answer 4

If you turn each word into a list ['J', 'e', 's', 's'] in your for loop then you can just add the letter of the middle index to your growing word of middle letters

def middles(listb):
    middle_word = ''
    for i in listb:
        letters = list(i)
        index = int(len(i)/2)
        letter = letters[index]
        middle_word += letter.lower()
    return middle_word

lista = ['Jess', 'Cain', 'Amity', 'Reann']

print(middles(lista))

 (xenial)vash@localhost:~/python/stack_overflow$ python3.7 middle.py siia