CODEWITHSUNDEEP
pythonarrays
Dab
Dab

Reputation: 13

Changing a value in an array of arrays seemingly changing corresponding value in next array

I was making a rudimentary Snake game and ran into this problem. I am completely dumbfounded as to why this is happening, here is the code.

self._body is as array of arrays as such:

[[15, 20], [16, 20], [17, 20]]

The goal of this code is to make that into: 

[[14, 20], [15, 20], [16, 20]]

Seems simple right? Here is the problem code, note the position of the print statements:

        if move == 'w' or move == 'W' :
            print(self._body)
            for i in range(len(self._body)-1,0,-1):
                self._body[i] = self._body[i-1]
            print(self._body)
            self._body[0][0] -= 1
            print(self._body)

The output of this code is:

[[15, 20], [16, 20], [17, 20]]
[[15, 20], [15, 20], [16, 20]]
[[14, 20], [14, 20], [16, 20]]

It is all correct until the last statement.

for some reason, 

 self._body[0][0] -= 1

changes both

self._body[0][0]

and

self._body[1][0]

I cannot for the life of me figure out why. Even if I could work around I still really want to know why this happens.

Thank you so much!

Upvotes: 1

Views: 74

Answers (1)

TheSoundDefense
TheSoundDefense

Reputation: 6945

Here's the problem:

self._body[i] = self._body[i-1]

This will not make a copy of the right-hand list. It will assign the exact same list object to both variables. So if you change one, you change the other. You need to make a copy of the list instead, like so:

self._body[i] = list(self._body[i-1])

This trips up a lot of people. Just remember that you have to explicitly copy lists.

Upvotes: 1

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