回文检查器：列表索引混乱，这是如何工作的？

Question

I was trying to make a program that takes a string as an input and checks whether it is a palindrome. I wrote this and it works but I'm confused as to how it works. Specifically, the list indexing when making the reverse list. Could someone please explain this step wise, I've been trying to follow it. Now I'm wondering how I even wrote this then.

word = input("Enter a word. ")
letter_list = []

for i in word:
    letter_list.append(i)
print(letter_list)
reverse = []
len = len(letter_list)

for i in letter_list:
    reverse.insert(-(len - 1), i)
print(reverse)

if letter_list == reverse:
    print("This word is a palindrome.")
else:
    print("This word is not a palindrome.")

Answer 1

I was trying to make a program that takes a string as an input and checks whether it is a palindrome.

If you want to check if a word is palindrome you can do it in a very simple way:

>>> def is_palindrome(word):
...     return word == word[::-1]
...
>>> word = input("Enter a word: ")
Enter a word: kajak
>>> word
kajak
>>> is_palindrome(word)
True
>>>

word[::-1] returns word string in reverse.

But if you want to make it as for loop in a list of chars, then here is a code:

def is_palindrome(letters):
    l = len(letters)
    for i, letter in enumerate(letters):
        if letters[i] != letters[l-1 - i]:
            return False
    return True

letter_list = list(input("Enter a word: "))
print(is_palindrome(letter_list))

Result:

Enter a word: kajak
True

I recommend you first method. It's much faster

Edit - Explanation

for i in word:
        letter_list.append(i)

letter_list is filled with every letter that user typed, eg for "kajak" is ['k', 'a', 'j', 'a', 'k'] .

for i in letter_list:
    reverse.insert(-(len - 1), i)
print(reverse)

Insert every letter from letter_list to reverse at index -(len - 1) .

if letter_list == reverse:
    print("This word is a palindrome.")
else:
    print("This word is not a palindrome.")

If lists are equal then print that word is palindrome. Else not.

NOTE: * You loop with reverse is wrong. You are constantly inserting a letter to the same position, which makes other words shift. It works, but it's not good. Also try not to name your variables with keywords ( len is built-in function. You should do something as letters_len = len(letter_list) )

Answer 2

The insert command receives an index and an element to be inserted. The elements are shifted to the right in order to store the new element. Since the command

reverse.insert(-(len - 1), i)

in your code is always inserting the element i at the beginning of the new list (- (len - 1) (insertion of negative indexes goes through the end to the beginning), and i is varying from the beginning to the end of the original list letter_list ,

for i in letter_list:

then the new list will contain the original reversed. Then you just compare to see whether they are equal.

letter_list == reverse

By the way, the command reverse.insert(-(len - 1), i) has the same effect as

reverse.insert(0, i)

You could use this site in order to follow the execution step by step.