Multiplying a list by a number creates items have relation with the original one?

Asked Aug 04 '18 at 22:11

Active Aug 04 '18 at 22:11

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I tried creating Identity matrix, but there was a problem creating a 2D array (list inside a list so to say) that can be determined by an input/parameter.

I expected from this:-

def eye(limit):
    lst = [[0] * limit] * limit
    for i in range(limit):
        lst[i][i] =1
    return lst


for item in eye(4):
    print(item)

to output this:-

[1, 0, 0, 0]
[0, 1, 0, 0]
[0, 0, 1, 0]
[0, 0, 0, 1]

but instead, go this:-

[1, 1, 1, 1]
[1, 1, 1, 1]
[1, 1, 1, 1]
[1, 1, 1, 1]

I tried to create a fixed 4x4 array, and it surprisingly worked and out-putted the expected answer.

And lead me to think that by multiplying a list won't create another list and concatenate them, but instead printing them by the number they got multiplied of. What do you think and how can i create a variable one.

asked Aug 04 '18 at 22:11

Kepler 186

1

Thats a classic problem. You have actually N references to the same object. Will find a dupe on that, 1 sec – rafaelc Aug 04 '18 at 22:14
NumPy offers inbuilt function `np.identity()` to create an identity matrix. For your case, just use `np.identity(4)`. You can also use `np.eye(4)` – Sheldore Aug 04 '18 at 22:16
[One more dupe](https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly) – DYZ Aug 04 '18 at 22:16
Also `[n*[0] for _ in range(n)]` will do what you need – Zach Gates Aug 04 '18 at 22:17
You can try using as `lst = [[0] * limit for i in range(limit)] ` – niraj Aug 04 '18 at 22:18
@ZachGates How do you get an identity matrix which OP wanted, from your way? – Sheldore Aug 04 '18 at 22:18
@Bazingaa That gives you an NxN matrix filled with zeroes. Same as student's. – Zach Gates Aug 04 '18 at 22:20
Ok, well I got your points. You were aiming at how to create the `lst`. Point taken. I rest my case. – Sheldore Aug 04 '18 at 22:22

Multiplying a list by a number creates items have relation with the original one?

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