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Your friend wants to try to make a word ladder! This is a list of words where each word has a one-letter difference from the word before it.

Here’s an example:


Write a program to help your friend. It should do the following:

  • Ask your friend for an initial word.

  • Repeatedly ask them for an index and a letter.

  • You should replace the letter at the index they provided with the letter they enter.

  • You should then print out the new word.

  • Stop asking for input when the user enters -1 for the index.

My answer to this is:

import os
get_index = 0
word = input("Enter a word: ")
for i in range(len(word)):
    while True:
            get_index = int(input("Enter an index (-1 to quit): "))
            if get_index == -1:
            elif get_index < -1 or get_index > 2:
                raise ValueError
        except ValueError:
            print ("Invalid index!")
            while True:
                    letter = input("Enter a letter: ")
                    for c in letter:
                        if c.isupper():
                            raise ValueError
                except ValueError:
                    print ("Character must be a lowercase letter!")
                    if len(letter) > 1:
                        print ("Must be exactly one character!")
                        word = word[:get_index] + letter + word[get_index + 1:]
                        print (word)

NOTE #1: Here is a link to the program run (output): OUTPUT

NOTE #2: I would also like to use an alternative for os._exit() as it takes time for the program to completely end.

So, I would like to know whether I could make this code shorter and more efficient.

Any help would be highly appreciated.


1 Answer 1


While your code seems to work, the next step should be separating the different responsibilities of your code. As far as I can see, there are three:

  • You have to ask the user for an index. It needs to be an integer in the range [-1, len(s)).
  • You have to ask them for a replacement character. It must be exactly one lower case character.
  • You need to actually run the game.

The first two can be combined into a general function asking the user for input with type and validation:

def ask_user(message, type_=str, valid=lambda: True, invalid="Invalid"):
    while True:
            user_input = type_(input(message))
        except ValueError:
        if not valid(user_input):
        return user_input

And your main program can then use this function. I would also change the internal data of the word to be a list, so you can easily replace characters. When printing the word, just use "".join(word).

def play_word_ladder():
    word = list(input("Enter a word: "))

    def valid_index(i):
        return i in range(-1, len(word))

    def valid_character(c):
        return len(c) == 1 and c.islower()

    while True:
        index = ask_user("Enter an index (-1 to quit): ",
                         int, valid_index, "Invalid index!")
        if index == -1:
        char = ask_user("Enter a letter: ", str, valid_character,
                        "Character must be one lowercase letter!")
        word[index] = char

Note that your outer for loop is completely unnecessary.

The game can be started under a if __name__ == "__main__": guard to allow importing from this script. Note that you don't need to manually exit the script, it will do so automatically at the end.

if __name__ == "__main__":
  • 1
    \$\begingroup\$ Upvoted! I tested it and it worked perfectly. Definitely more efficiently than my code above :) \$\endgroup\$
    – Justin
    Commented May 12, 2019 at 12:50

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