Why do two arrays share values?

Question

I have an array array1 = [[0, 1, 2, 3], [4, 5, 6, 7]] and I want to switch array1[0] and array1[1], which would ideally look like this: array1 = [[4, 5, 6, 7], [0, 1, 2, 3]]. So far I have tried this:

array1 = [[0, 1, 2, 3], [4, 5, 6, 7]]
array2 = array1
array1[0] = array2[1]
array1[1] = array2[0]

But that returns [[4, 5, 6, 7], [4, 5, 6, 7]]. Does anyone know why this is happening?

`array2 = array1` just means that two names now refer to the same list. In other words, names aren't memory locations, they're just labels attached to objects. — Steven Rumbalski, Jan 18 '21 at 20:12
Use this to see what happens: http://www.pythontutor.com/visualize.html#mode=edit — mkrieger1, Jan 18 '21 at 20:13
You could just have used `array1[0], array1[1] = array1[1], array1[0]`. — mkrieger1, Jan 18 '21 at 20:14
[Facts and myths about Python names and values](https://nedbatchelder.com/text/names.html) — wwii, Jan 18 '21 at 20:15
You don't have two arrays, you have one list with two names. — Samwise, Jan 18 '21 at 20:16

score 1 · Answer 1 · answered Jan 18 '21 at 20:15

When you say:

array2 = array1

You are making it so array2 references the same object in memory that array1 does. This is called a "binding" assignment. After that the expression:

array1[0] = array2[1]

is no different than saying:

array1[0] = array1[1]

There no longer is any difference between array1 and array2.

score 1 · Accepted Answer · answered Jan 18 '21 at 20:15

1

You can try:

array1 = [array1[1], array1[0]]

Then you don't need array2.

answered Jan 18 '21 at 20:15

Tomer Shetah

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score 0 · Answer 3 · answered Jan 18 '21 at 20:15

0

You wanted to copy an array, but:

array2 = array1

is not copying. You just created a new label array2 and pointed it to array1.

To copy an array, you can have a loop at copy.copy() and copy.deepcopy() here: https://docs.python.org/3/library/copy.html

answered Jan 18 '21 at 20:15

pavelsaman

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Why do two arrays share values?

3 Answers3