在python的列表末尾插入元素？

Question

I have an array

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

I want to add 4th element at the end of each list as

[[1,2,3,9], [4,5,6,36], [7,8,9,81]]

where inserted element is the square of last element.

How to do it?

Answer 1

This seems like a homework question, so I will not give you the code.

I will explain what you're supposed to do.

What you have is a list of lists. Each element of a list can be iterated over like this:

>>> foo = [[1,2,3], [4,5,6], [7,8,9]]
>>> for element in foo:
...     print element

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

The last element of a list can be accessed by doing this:

>>> foo = [1, 2, 3]
>>> print foo[-1]
3

And adding an element to the end of a list can be done like this:

>>> foo = [1, 2, 3]
>>> foo.append(6)
>>> print foo
[1, 2, 3, 6]

Squaring a variable can be done with **

>>> a = 6
>>> print(a ** 2)
36

Rest is left to you to put together. When combining all these you can easily do the task.

Answer 2

You can just go:

array = [[1,2,3,4]...]
for l in array:
    l.append(l[-1]**2)

You will end up just as you said. Basically, it goes through your inner lists and gets the last item of the list and appends that item squared to the end of the list. I hope this helps.

Answer 3

You can use numpy as well.

import numpy as np
a = np.array([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
j=[pow(i[-1], 2) for i in a]
b = np.array(j)
np.column_stack((a,b))