Problem when appending to 2D list in python

Asked Mar 19 '20 at 17:11

Active Mar 19 '20 at 17:11

Viewed 15 times

First I have initialized mylist as follows :

buf = []
mylist = []
for i in range(0, 10):
    mylist.append(buf)

Now mylist is like this :

mylist:

[[], [], [], [], [], [], [], [], [], []]

Now when I want to append a value to the first index like so:

mylist[0].append(3)

And I print mylist again I get this:

[[3], [3], [3], [3], [3], [3], [3], [3], [3], [3]]

Where it should only add the number 3 to the first list inside mylist as so:

[[3], [], [], [], [], [], [], [], [], []]

What is wrong with my code? when I don't use the above for loop to create embedded lists and I do it manually, I don't have this problem. And I need to do by using for loop because I need a huge embedded list, I put 10 there just for the sake of simplicity.

asked Mar 19 '20 at 17:11

John Sall

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You are appending a reference to the *same* list over and over. Don't append `buf` just append a new `[]` with `mylist.append([])`. – Mark Mar 19 '20 at 17:14
Replace this `mylist.append(buf) ` with `mylist.append([])` . Reason is very well explained by Mark – Ch3steR Mar 19 '20 at 17:16
@MarkMeyer nice, thanks – John Sall Mar 19 '20 at 17:18

Problem when appending to 2D list in python

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