Is there a reason these "identical" arrays are being treated differently?

Question

I was having problems with "equal" arrays being treated differently in python 3.X.

I noticed this when I was working on creating an NxN array dynamically. The array I generated didn't work in my function, but when I typed out a 2D array filled with numbers to see exactly where my code was breaking my function worked with the manually created array.

Here, the arrays are created and seem to be exactly the same.

    x = [[0,0,0],[0,0,0],[0,0,0]]
    n = 3
    a = [[0]*n]*n
    print(x)
    print(a)

will return...

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

Now if I send these through a function I created, even though they appear the same, I get different results.

def checkerboard(a):
    '''takes nxn array and returns a checkerboard pattern'''
    for i in range(len(a)):
        for j in range(len(a[i])):
            if((i+j) % 2):
                a[i][j] = "X"
            else:
                a[i][j] = "0"
    for i in range(len(a)):
        print(a[i])

    return a

    checkerboard(x)
    print("-----------")
    checkerboard(a)

gives the result...

['0', 'X', '0']
['X', '0', 'X']
['0', 'X', '0']
-----------
['0', 'X', '0']
['0', 'X', '0']
['0', 'X', '0']

both of these NxN matrices should be the same and should give the same result.

These are lists, not arrays. Big difference in Python (and Java) — Mad Physicist, Oct 24 '19 at 16:39
The 2nd list (`a`) is actually 3 of the exact same list. See the dupe^ — SyntaxVoid, Oct 24 '19 at 16:39
@Carcigenicate. To give credit where it's due, OP figured that part out, and set up a good experiment to verify that. — Mad Physicist, Oct 24 '19 at 16:42
So 'a' is just 3 copies of one list where if you edit one it affects the rest? Ah this is because pythons immutability right? — sntrenter, Oct 24 '19 at 17:03
It is because some objects in python are mutable. Changing a mutable object (often called changing it in-place) affects anything that references that object. — SyntaxVoid, Oct 24 '19 at 17:14

Is there a reason these "identical" arrays are being treated differently?

0 Answers0