问题描述

所以我刚刚完成了 CodeAcademy Battleship 问题的一部分，并提交了正确的答案，但我无法理解为什么它是正确的.

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.

这个想法是建立一个 5x5 的网格作为板，用O"填充.我使用的正确代码是:

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)

但是我很困惑为什么这没有在一行中创建 25 个O"，因为我从未指定迭代到单独的行.我试过了

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)

但这给了我错误:IndexError: list index out of range.谁能解释一下为什么第一个是正确的而不是第二个?

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?

推荐答案

["O"]*5

这将创建一个大小为 5 的列表，用O"填充:["O", "O", "O", "O", "O"]

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

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

这会将上述列表附加(添加到列表末尾)到 board[].循环执行 5 次会创建一个包含上述 5 个列表的列表.

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"]]

您的代码不起作用，因为列表没有在 python 中使用大小进行初始化，它只是从一个空容器 [] 开始.为了使你的工作，你可以这样做:

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

在你的循环中:

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

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