Python creates a two-dimensional array instance (a small pitfall on list), and python two-dimensional array

Python creates a two-dimensional array instance (a small pitfall on list), and python two-dimensional array

0. Directory

1. Problems Encountered

2. How to create a two-dimensional array

• 3.1 Direct Creation Method

• 3.2 List generative Method

• 3.3 create with module numpy

1. Problems Encountered

Today, when I was writing Python code, I encountered a big pitfall, which almost delayed my assignment...

The problem is that I need to create a two-dimensional array, as shown below:

m = n = 3test = [[0] * m] * nprint("test =", test)

The output result is as follows:

test = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]

Does it look okay?

At the beginning, I thought so too. I thought that I used the wrong function in other places. The result was as follows:

m = n = 3test = [[0] * m] * nprint("test =", test) test[0][0] = 233print("test =", test)

The output result is as follows:

test = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]test = [[233, 0, 0], [233, 0, 0], [233, 0, 0]]

Are you surprised ?!

This problem really happened to me at noon. I went to the Internet to search for it. The instructions in the official documents are as follows:

Note also that the copies are shallow; nested structures are not copied. This often haunts new Python programmers; consider:

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

What has happened is that [[[] is a one-element list containing an empty list, so all three elements of [[] * 3 are (pointers) this single empty list. modifying any of the elements of lists modifies this single list. you can create a list of different lists this way:

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

In other words, in the matrix = [array] * 3 operation, only three references pointing to the array are created. Therefore, once the array changes, the three lists in the matrix also change.

2. How to create a two-dimensional array

2.1 Direct Creation Method

test = [0, 0, 0], [0, 0, 0], [0, 0, 0]]

Simple and rude, but too troublesome.

2.2 list Generation Method

test = [[0 for i in range(m)] for j in range(n)]

Learn how to use the list generative method to benefit the whole life. No, you can go to the list builder-liao Xuefeng's official website to learn.

2.3 create with module numpy

import numpy as nptest = np.zeros((m, n), dtype=np.int)

For more information about the module numpy. zeros, goHow to Use numpy. zeros (np. zeros) in pythonLook.

In the above example, Python creates a two-dimensional array instance (a small pitfall on list), which is all the content shared by Alibaba Cloud. I hope to give you a reference and support for the help house.