Create and initialize 5x5 grid for Battleships

Question

So I just completed a section of CodeAcademy Battleship problem, and have submitted a correct answer but am having trouble understanding why it is correct.

The idea is to build a 5x5 grid as a board, filled with "O's". The correct code I used was:

board = []
board_size=5

for i in range(board_size):

    board.append(["O"] *5)

However I'm confused as to why this didn't create 25 "O's" in one single row as I never specified to iterate to a separate row. I tried

for i in range(board_size):

    board[i].append(["O"] *5)

but this gave me the error: IndexError: list index out of range. Can anyone explain why the first one is correct and not the second one?

Answer 1

["O"]*5

This creates a list of size 5, filled with "O": ["O", "O", "O", "O", "O"]

board.append(["O"] *5)

This appends (adds to the end of the list) the above list to board[]. Doing this 5 times in a loop creates a list filled with 5 of the above lists.

[["O", "O", "O", "O", "O"],
["O", "O", "O", "O", "O"],
["O", "O", "O", "O", "O"],
["O", "O", "O", "O", "O"],
["O", "O", "O", "O", "O"]]

Your code did not work, because lists are not initialized with a size in python, it just starts as an empty container []. To make yours work, you could have done:

board = [[],[],[],[],[]]

and in your loop:

board[i] = ["O"]*5

Answer 2

Python lists start out empty:

board = [] # empty list

Unlike some programming languages, you do not specify or initialise the bounds of the array, so if you try to access by index:

board[0]

you will get an IndexError; there is not yet any index 0.

In the first version, on each trip through the for loop, you append (add to the end of the list) a new list containing five "O"s. If you want the board list to contain 25 "O"s, you should use extend instead:

for i in range(board_size):
    board.extend(["O"] *5)