Python validation function generates IndexError: list assignment index out of range

Question

I'm building out Battleships game in Python. I have a list and I'm trying to build a validation tool in Python to catch user inputs that are outside the 10x10 range of my list.

Here is the code:

from random import randint

player = "User"
board = []
board_size = 10

ships = {"Aircraft Carrier":5,
            "Battleship":4,
            "Submarine":3,
            "Destroyer":3,
            "Patrol Boat":2}

def print_board(player, board): # to print joined board
    print("Here is " + player + "'s board")
    for row in board:
        print(" ".join(row))

def switch_user(player): # to switch users
    if player == "User":
        player = "Computer"
    elif player == "Computer":
        player = "User"
    else:
        print("Error with user switching")

for x in range(0, board_size): # to create a board
    board.append(["O"] * board_size)

print_board(player,board)

def random_row(board): # generate random row
    return randint(0, len(board) - 1)

def random_col(board): # generate random column
    return randint(0, len(board[0]) - 1)

def user_places_ships(board, ships): # user choses to place its ships by providing starting co-ordinate and direction.
    for ship in ships:
        valid = False
        while(not valid):
            user_input_coordinates = input("Please enter row & column number for your " + str(ship) + ", which is " + str(ships[ship]) + "-cells long (row, column).")
            ship_row, ship_col = user_input_coordinates.split(",")
            ship_row = int(ship_row)
            ship_col = int(ship_col)
            user_input_dir = input("Please enter direction for your " + str(ship) + ", which is " + str(ships[ship]) + "-cells long (h for horizontal or v for vertical).")
            valid = validate_coordinates(board, ships[ship], ship_row, ship_col, user_input_dir)
            if not valid:
                print("The ship coordinates either outside of" , board_size, "X" , board_size, "range, overlap with or too close to another ship.")
        place_ship(board, ships[ship], ship_row, ship_col, user_input_dir)
    print("You have finished placing all your ships.")

def validate_coordinates(board, ship_len, row, col, dir): # validates if the co-ordinates entered by a player are within the board and don't overlap with other ships
    if dir == "h" or dir == "H":
        for x in range(ship_len):
            if row-1 > board_size or col-1+x > board_size:
                return False
            elif row-1 < 0 or col-1+x < 0:
                return False
            elif board[row-1][col-1+x] == "S":
                return False
    elif dir == "v" or dir == "V":
        for x in range(ship_len):
            if row-1+x > board_size or col-1 > board_size:
                return False
            elif row-1+x < 0 or col-1 < 0:
                return False
            elif board[row-1+x][col-1] == "S":
                return False
    return True

def place_ship(board, ship_len, row, col, dir): # to actually place ships and mark them as "S"
    if dir == "h" or dir == "H":
        for x in range(ship_len):
            board[row-1][col-1+x] = "S"
    elif dir == "v" or dir == "V":
        for x in range(ship_len):
            board[row-1+x][col-1] = "S"
    else:
        print("Error with direction.")
    print_board(player,board)

user_places_ships(board,ships)

If a user enters "10,10" for ship coordinates and "h" for horizontal direction, then Python generates the following error message:

Traceback (most recent call last):   File "C:/Users/Elchin's PC/Downloads/battleships game.py", line 85, in <module>
    user_places_ships(board,ships)   File "C:/Users/Elchin's PC/Downloads/battleships game.py", line 49, in user_places_ships
    valid = validate_coordinates(board, ships[ship], ship_row, ship_col, user_input_dir)   File "C:/Users/Elchin's PC/Downloads/battleships game.py", line 62, in validate_coordinates
    elif board[row-1][col-1+x] == "S": IndexError: list index out of range

I know that the error is in this line:

elif board[row-1][col-1+x] == "S":
    return False

But I don't know how to fix it. Could you please help me figure out the solution?

Answer 1

If a list has length n, you can access indices from 0 to n-1 (both inclusive).

Your if statements however check:

if row-1+x > board_size or col-1 > board_size:  # greater than n
    return False
elif row-1+x < 0 or col-1 < 0:                  # less than 0
    return False
elif board[row-1+x][col-1] == "S":
    return False

So as a result, if we reach the last elif part, we have guarantees that the indices are 0 < i <= n. But these should be 0 < i < n.

So you should change the first if statement to:

if row-1+x >= board_size or col-1 >= board_size:  # greater than or equal n
    return False
elif row-1+x < 0 or col-1 < 0:                    # less than 0
    return False
elif board[row-1+x][col-1] == "S":
    return False

You can make the code more elegant by writing:

if not (0 < row-1+x < board_size and 0 < col-1 < board_size) or \
        board[row-1+x][col-1] == "S":
    return False