为二维python数组中的单个单元格赋值 [英] Assign value to an individual cell in a two dimensional python array

问题描述

假设我在 Python 中有以下空二维数组:

Let's say I have the following empty two dimensional array in Python:

q = [[None]*5]*4

我想给q第一列的第一行赋值5.本能地，我做了以下事情:

I want to assign a value of 5 to the first row in the first column of q. Instinctively, I do the following:

q[0][0] = 5

然而，这会产生:

 [[5, None, None, None, None], 
  [5, None, None, None, None], 
  [5, None, None, None, None], 
  [5, None, None, None, None]]

每个数组的第一个元素被初始化为5，我认为只有第一个数组的第一个元素会得到更新.我有两个问题:

The first element of every array is being initialized to 5, where I thought only the first element of the first array would get the update. I have two questions:

1. 为什么 Python 初始化每个数组的第一个值而不是第一个?
2. 有没有更好的方法来完成我想要做的事情?

推荐答案

这不符合您的期望.

q = [[None]*5]*4

它多次重用 list 对象.正如您在对一个单元格进行更改时所看到的，该单元格位于重复使用的列表对象中.

It reuses list objects multiple times. As you can see when you made a change to one cell, which was in a reused list object.

值为 [None] 的单个列表被使用了五次.

A single list with a value of [None] is used five times.

值为 [[None]*5] 的单个列表使用了四次.

A single list with a value of [[None]*5] is used four times.

q = [ [ None for i in range(5) ] for j in range(4) ]

可能更符合您的要求.

这明确避免了重复使用列表对象.

This explicitly avoids reusing a list object.

80% 的情况下，您真正​​想要的是字典.

80% of the time, a dictionary is what you really wanted.

q = {}
q[0,0]= 5

也能用.您不会从 None 值的预定义网格开始.但一开始就很少需要它们.

Will also work. You don't start with a pre-defined grid of None values. But it's rare to need them in the first place.

在 Python 2.7 及更高版本中，您可以这样做.

In Python 2.7 and higher, you can do this.

q = { (i,j):0 for i in range(5) for j in range(4) }

这将构建一个由 2 元组索引的网格.

That will build a grid indexed by 2-tuples.

{(0, 1): 0, (1, 2): 0, (3, 2): 0, (0, 0): 0, (3, 3): 0, (3, 0): 0, (3, 1): 0, (2, 1): 0, (0, 2): 0, (2, 0): 0, (1, 3): 0, (2, 3): 0, (4, 3): 0, (2, 2): 0, (1, 0): 0, (4, 2): 0, (0, 3): 0, (4, 1): 0, (1, 1): 0, (4, 0): 0}

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