Why is python automatically sorting an array I didn't tell it to sort [duplicate]

Question

Why is python sorting an array I'm not asking it to sort, and how do I get it to stop?

For example, I have an NumPy array A:

A = np.array[4, 7, 1, 3, 2], [3, 2, 4, 2, 1]

I wish to sort this array, but preserve its original structure so I can index it later. So my idea was to make a copy of the array and sort the copy:

B = A
B.sort()

Why is it then, that when I print A after this sort command, it prints the sorted array? Even though I never called sort on it?

print(a)


[[1 2 3 4 7]
[1 2 2 3 4]]

Is there a way around this?

Correct, B just references the location in memory where A is stored, so they are pointing to the same thing. — whege
– whege, Commented Oct 2, 2020 at 16:20
@jonrsharpe I think that's a thing that's hard for people to wrap their head around with Python: = never makes a copy. If you want a copy, you have to be explicit about it. See Zen of Python: Explicit is better than Implicit. — Mark Ransom
– Mark Ransom, Commented Oct 2, 2020 at 16:22
@Yoddlenod read the following: nedbatchelder.com/text/names.html — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Oct 2, 2020 at 16:32

whege · Accepted Answer · 2020-10-02 16:22:27Z

0

As mentioned above, B just points to the same location in memory as A, so modifying B also modifies A. You need to call the .copy() method, as shown below:

answered Oct 2, 2020 at 16:22

whege

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