在Python中打印二维列表的列

Question

Suppose if A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Then A[0][:] prints [1, 2, 3]

But why does A[:][0] print [1, 2, 3] again ?

It should print the column [1, 4, 7] , shouldn't it?

Answer 1

[:] is equivalent to copy.

A[:][0] is the first row of a copy of A. A[0][:] is a copy of the first row of A.

The two are the same.

To get the first column: [a[0] for a in A] Or use numpy and np.array(A)[:,0]

Answer 2

如果未指定开始或结束索引，Python将返回整个数组：

A[:] = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Answer 3

[:] matches the entire list.

So A[:] is the same as A . So A[0][:] is the same as A[0] .

And A[0][:] is the same as A[0] .

Answer 4

Note that [:] just gives you a copy of all the content of the list. So what you are getting is perfectly normal. I think you wanted to use this operator as you would in numpy or Matlab. This does not do the same in regular Python.

---

A[0] is [1, 2, 3]

Therefore A[0][:] is also [1, 2, 3]

---

A[:] is [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

Therefore A[:][0] is [1, 2, 3]

---

If you wanted the first column you should try:

[e[0] for e in A]
# [1, 4, 7]

Answer 5

A[:] returns a copy of the entire list. which is A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] A[:][0] Thus selects [1, 2, 3] . If you want the first column, do a loop:

col = []
for row in A:
    col.append(row[0])

Answer 6

Problem

A is not a 2-D list: it is a list of lists. In consideration of that:

1. A[0] is the first list in A:

    >>> A = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] >>> A[0] [1, 2, 3] 

   Consequently, A[0][:] : is every element of the first list:

    >>> A[0][:] [1, 2, 3] 

2. A[:] is every element of A, in other words it is a copy of A:

    >>> A[:] [[1, 2, 3], [4, 5, 6], [7, 8, 9]] 

   Consequently, A[:][0] is the first element of that copy of A.

    >>> A[:][0] [1, 2, 3] 

Solution

To get what you want, use numpy:

>>> import numpy as np
>>> A = np.array( [[1, 2, 3], [4, 5, 6], [7, 8, 9]] )

A is now a true two-dimensional array. We can get the first row of A :

>>> A[0,:]
array([1, 2, 3])

We can similarly get the first column of A :

>>> A[:,0]
array([1, 4, 7])

`

Answer 7

A is actually a list of list, not a matrix. With A[:][0] You are accessing the first element (the list [1,2,3] ) of the full slice of the list A. The [:] is Python slice notation (explained in the relevant Stack Overflow question ).

To get [1,4,7] you would have to use something like [sublist[0] for sublist in A] , which is a list comprehension , a vital element of the Python language.