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)

Answer 3

Code should look like this. board = [] for i in range(0, 5): board.append(["O"]*5) print(board)

Answer 4

I think the problem is simple: when you run your code, your array is empty!

You could issue a

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

to initialize it as a nested, five places array.

You could also use a dict to simulate a indefinitely nested array:

board=defaultdict(lambda:defaultdict(lambda:[]))

Answer 5

This is because each time you run board.append(["O"] *5) you create a new list (because of the square brackets used inside the append statement).

If you would have used board.append("O" *5) instead the you would get a single list with five elements like:

['OOOOO', 'OOOOO', 'OOOOO', 'OOOOO', 'OOOOO']

The *5 multiplies the string. if you just run from an interactive prompt 'O' * 5 you get:

'OOOOO'

However, if you run ['O'] * 5 it multiplies the text within the list and you get a list with five elements.

['O', 'O', 'O', 'O', 'O']

You appended five lists, each having 5 elements.

Answer 6

board = []
board_size=5

for i in range(board_size):

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

In this code board starts as an empty list. ["O"] * 5 will create a list that is equal to ["O", "O", "O", "O", "O"] . It will then be appended to the list board . This will happen 5 times in this example since board_size == 5 .

for i in range(board_size):

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

This wouldn't work unless the item at index i was a list since append() is a method of the list class. If you wanted to insert something at i you would use the list method insert()

Answer 7

First step in codeacademy

board = []
for i in range(5):
    board.append(["O"]*5)

print board

Second step in codeacademy

board = []

def print_board(board):
    for x in range(5):
        board= (["O"] * 5)
        print board
    return board

print_board(board)

third step in codeacademy

board = []

def print_board(board):
    for x in range(5):
        board= (["O"] * 5)
        print " ".join(board)
    return board

print_board(board)

4th Step

from random import randint 

board = []

for x in range(0, 5):
    board.append(["O"] * 5)

def print_board(board):
    for row in board:
        board = " ".join(row)
        return  board

def random_row(board):
    return randint(0, len(board) - 1)

def random_col(board):
    return randint(0, len(board) - 1)

print print_board(board)
print random_row(board)
print random_col(board)

5th step of buttleship on codeacademy

from random import randint

board = []

for x in range(0,5):
    board.append(["O"] * 5)

def print_board(board):
    for row in board:
        print " ".join(row)

def random_row(board):
    return randint(0, len(board) - 1)

def random_col(board):
    return randint(0, len(board[0]) - 1)

ship_row = random_row(board)
ship_col = random_col(board)


guess_row = int(raw_input("Guess Row: "))
guess_col = int(raw_input("Guess Col: "))

Final Step Lets .. Play game BettleShip CodeAcademy

from random import randint

board = [] for x in range(5): board.append(["O"] * 5)

def print_board(board): for row in board: print " ".join(row)

print "Let's play Battleship!" print_board(board)

def random_row(board): return randint(0, len(board) - 1)

def random_col(board): return randint(0, len(board[0]) - 1)

ship_row = random_row(board) ship_col = random_col(board) print ship_row print ship_col

turns=0 win = 0 for turn in range(4): if turn <= 5: guess_row = int(raw_input("Guess Row:")) guess_col = int(raw_input("Guess Col:"))

    if guess_row == ship_row and guess_col == ship_col:
        print "Congratulations! You sunk my battleship!"
        win +=1
        if win == 2:
            break
    else:
        turns +=1
        if (guess_row < 0 or guess_row > 4) or (guess_col < 0 or guess_col > 4):
            print "Oops, that's not even in the ocean."
        elif(board[guess_row][guess_col] == "X"):
            print "You guessed that one already."
        else:
            print "You missed my battleship!"
            board[guess_row][guess_col] = "X"
            print (turn + 1)
        print_board(board)
elif turns == 3:
    print "Game Over"
else:
    print "Max Turn try is over Try Again"