如何在不更改 python 中的原始值的情况下将二维数组中的元素分配给新变量？

Question

I want to take a value from a 2D array (so this value has a type of array) and assign it to a new variable. But when I make changes to this new variable, its original value in that 2D array also changes. I want to know if there is a correct way that I can do this?

Here is my example code.

a = [[1],[2],[3]]
b = a[1]
b.append(2)
a.append(b)
print(a)
print(b)

In the output, a, b will be

a = [[1], [2, 2], [3], [2, 2]]
b = [2, 2]

Where I don't want a[1] to be changed as well. If someone can help me figure this out? Thanks.

Answer 1

This is probably one of the most confusing concept for beginner, when you are doing subscription slicing, b = a[1] , the reference to first array a has been passed to b , which you whatever action you do to b , will be done to a[1] . unless you make a complete copy of the original array, any action to subscripted will lead to change in original , therefore, whenever you want to work with array without changing its initial value, copy it manually.

a = [[1],[2],[3]]
# in your case it is nested so it will be a bit intricate 
ac = list(map(lambda x:list(map(lambda y:y, x)), a))
b = ac[1]
b.append(2)
a.append(b)
print(a)
print(b)
# [[1], [2], [3], [2, 2]]
# [2, 2]

Answer 2

Because a[1] is an array, passing it to b will pass it by reference and not by value . You can use

b = a[1].copy()

to make a new instance of a[1] without effecting the original a[1] .