Python two-dimensional array operations, python two-dimensional array operations

Python two-dimensional array operations, python two-dimensional array operations

You need to use a two-dimensional array in the program. You can find this usage on the Internet:

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# Create an array with a width of 3 and a height of 4#[[0,0,0], # [0,0,0],# [0,0,0],# [0,0,0]]myList = [[0] * 3] * 4

However, when myList [0] [1] = 1, the entire second column is assigned a value and becomes

[[0, 1, 0],

[0, 1],

[0, 1],

[0, 1]

Why? I don't understand it for a moment. I will refer to The Python Standard Library to find The answer later.

List * n-> nShallow copiesOf list concatenated, n listsShortest copyConnection

Example:

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>>> lists = [[]] * 3>>> lists[[], [], []]>>> lists[0].append(3)>>> lists[[3], [3], [3]]

[[] Is a list containing an empty list element. Therefore, [[] * 3 indicates three references pointing to this empty list element. modify any

An element changes the entire list:

Therefore, you need to create a multi-dimensional array in another way to avoid copying:

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>>> lists = [[] for i in range(3)]>>> lists[0].append(3)>>> lists[1].append(5)>>> lists[2].append(7)>>> lists[[3], [5], [7]]

The previous two-dimensional array creation method is as follows:

1	myList = [([0] * 3) for i in range(4)]