What is 'IndexError: list index out of range'?

Question

This is my code:

from tkinter import messagebox, simpledialog, Tk

def is_even(number):
    return number % 2 == 0

def get_even_letters(message):
    even_letters = []
    for counter in range(0, len(message)):
        if is_even(counter):
            even_letters.append(message[counter])
        return even_letters

def get_odd_letters(message):
    odd_letters = []
    for counter in range(0, len(message)):
        if not is_even(counter):
            odd_letters.append(message[counter])
    return odd_letters

def swap_letters(message):
    letter_list = []
    if not is_even(len(message)):
        message = message + 'x'
    even_letters = get_even_letters(message)
    odd_letters = get_odd_letters(message)
    for counter in range(0, int(len(message)/2)):
        letter_list.append(odd_letters[counter])
        letter_list.append(even_letters[counter])
    new_message = ''.join(letter_list)
    return new_message


def get_task():
    task = simpledialog.askstring('Task', 'Do you want to encrypt or decrypt?')
    return task

def get_message():
    message = simpledialog.askstring('Message', 'Enter the secret message')
    return message
root = Tk()

while True:
    task = get_task()
    if task == 'encrypt':
        message = get_message()
        encrypted = swap_letters(message)
        messagebox.showinfo('Ciphertext of the secret message is:', encrypted)
    elif task == 'decrypt':
        message = get_message()
        decrypted = swap_letters(message)
        messagebox.showinfo('Plain text of the secret message is: ', decrypted)
    else:
        break
root.mainloop()

Currently using IDLE, Python 3.6.3. Whenever I run the program, the first prompt works, 'encrypt', but when I get to the 'Enter the secret message' prompt, if I type anything with more than 2 letters, it gives me the error:

Traceback (most recent call last):
  File "/Users/richarddao/Documents/secret_message.py", line 46, in <module>
    encrypted = swap_letters(message)
  File "/Users/richarddao/Documents/secret_message.py", line 28, in swap_letters
    letter_list.append(even_letters[counter])
IndexError: list index out of range

What is IndexError: list index out of range? How do I fix this?

Answer 1

Index Error: List index out of range means you are trying to get an item in a list that doesn't exist.

For Example, I have this list:

["a","b","c"]

So a is the 0th item, b is the 1st, and c is the 2nd.

I can get b by doing ["a","b","c"][1] because it is the first item and I am calling for the first item with [1].

An index error is what would happen if I tried to do this:

["a","b","c"][4]

There is no 4th item, so it errors.