2D数组的Python列表拼接

Question

matrix = [[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 14, 15, 16]]
matrix[3][1:] = matrix[3][0:-1]
print(matrix)

gives the following output:

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 13, 14, 15]]

But:

rows = 3
k = 0
matrix[rows-k][1+k:-k] = matrix[rows-k][k:-1-k]
print(matrix)

gives the following output:

[[1, 2, 3, 4], [5, 6, 7, 8], [9, 10, 11, 12], [13, 13, 14, 15, 14, 15, 16]]

Can someone please explain to me what's going on?

Answer 1

Interesting question.

This happens because

matrix[rows-k][1+k:-k] = matrix[rows-k][k:-1-k]

is equivalent to

matrix[3][1:0] = matrix[3][0:-1]

which is equivalent to

matrix[3][1:1] = matrix[3][0:-1]

So when you do matrix[3][1:0] => matrix[index][start:stop:None]

Your start value is 1, which is more than your stop value of 0. In case of start >= stop Python sets stop = start and returns an empty slice.

You can check the value of matrix[3][1:1] which is an empty slice like matrix[3][1:0]

>>> matrix[3][1:0]
[]
>>> matrix[3][1:1]
[]

Read more: Python doc

If i is greater than or equal to j, the slice is empty

Answer 2

If you insert rows = 3 and k = 0:

matrix[3][1:0] = matrix[3][0:-1]

And it is not the same like your second line:

matrix[3][1:] = matrix[3][0:-1]

Added.

In first example you insert some data from 1. position to the end :

a = [1, 2, 3, 4]
a[1:] = [100, 200, 300]
print(a)
OUTPUT: [1, 100, 200, 300]

In the second case you insert a value between the 0. and 1. position :

a = [1, 2, 3, 4]
a[1:0] = [100, 200, 300]
print(a)
OUTPUT: [1,100, 200, 300, 2, 3, 4]