How to copy list elements in python-CodePudding

regarding the code below:

A = [[1, 2], [3, 4]]
A[0] = A[1]
B = A[:][0]
B[0] = 5
print(A)
print(B)

I'm wondering why printing B gives [5, 4].

I thought that B = A[:][0] is the same as A[0][0], A[1][0], which would then be [3, 3]. Then, B[0] = 5, so it would print [5, 3].

Could someone kindly clear up my confusion, thanks.

CodePudding user response：

after this line:

A[0] = A[1]

A = [[3,4],[3,4]]

A[:] will return A itself, so when you access 0, you will get [3,4]

B = A[:][0]

and after putting 5, B = [5,4]

CodePudding user response：

Here is your confusion:

I thought that B = A[:][0] is the same as A[0][0], A[1][0]

It's not. The : symbol in Python isn't used for that sort of shorthand like in some other languages. A[:] simply returns a copy of A, so A[:][0] is equivalent to A[0], which in this context is [3, 4].

(The more general use of the : is for an operation called slicing, which is when you copy only part of a list, from a given starting index to a given end index. If we omit the indices, they default to the start and end of the entire list, so we copy the whole thing.)

CodePudding user response：

Using the copy() method is relatively straightforward.

a = [[1, 2], [3, 4]]
b = a.copy()

print(a)
print(b)

result

[[1, 2], [3, 4]]
[[1, 2], [3, 4]]

If you want to make copies of the nested lists, then you would extend to this:

a = [[1, 2], [3, 4]]
b = [a[0].copy(), a[1].copy()]