CODEWITHSUNDEEP
pythonmultidimensional-array
aley
aley

Reputation: 23

Indexing 2 dimensional array in python

I've been trying to change a single item in a 2-dimensional array in python using the syntax x[2][3]=1 but instead of just changing the item in the 2nd row 3rd column, it ends up changing the values of all of the 3rd column. My code is below:

population = [[0]*20]*5

population[2][3] = 1

for row in population:
    print(row)

This outputs

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

but I only want

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

How would I index the item such that it only changes the 2nd row and 3rd column?

I'm using python 3.7.4 on repl.it

Link here: https://repl.it/@ajqe/2d-array-test

Upvotes: 1

Views: 320

Answers (1)

Toby Petty
Toby Petty

Reputation: 4660

Use :

population = [[0]*20 for _ in range(5)]

to generate the lists instead. The method you are using is referencing the same object 5 times, instead of creating 5 separate lists. To check this you can use the is operator:

>>> population = [[0]*20]*5
>>> population[0] is population[1]
True

>>> population = [[0]*20 for _ in range(5)]
>>> population[0] is population[1]
False

Upvotes: 1

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