Changing value of a 2D array element changes the complete column

Question

When I print my arr value i get the correct values for my 2D array but when I exit the while loop my values are all wrong.

I am not sure what i am doing wrong.

    #num runs
    n = 4

    x = np.linspace(-1,1,n)
    y = np.linspace(-1,1,n)
    x1,y1 = np.meshgrid(x, y)

    l = np.linspace(0,1000,n)
    x = np.linspace(-1,1,n)
    p1,p2 = np.meshgrid(l,l)

    w020 = 5*(y1**2+x1**2)

    row, cols = (n,n)
    arr = [[0]*cols]*row
  
    i = 0
    p = 0
    while i < n:
        i += 1
        p=0
        while p < n:
            arr[i-1][p] = 2+2*math.cos(2*math.pi*w020[i-1,p])
            p += 1
    print(arr)       

Answer 1

The way you created the 2D array, creates a shallow list

    arr = [[0]*cols]*row

Instead, if you want to update the elements of the list, you should use

   rows, cols = (5, 5) 
   arr = [[0 for i in range(cols)] for j in range(rows)] 

Explanation:

One can create a list using :

   arr = [0]*N 

or

   arr = [0 for i in range(N)] 

In the first case all the indices of the array point to the same integer object

and when you assign a value to a particular index, a new int object is created, for eg arr[4] = 5 creates

Now let us see what happens when we create a list of list, in this case, all the elements of our top list will point to the same list

And if you update the value of any index a new int object will be created. But since all the top-level list indexes are pointing at the same list, all the rows will look the same. And you will get the feeling that updating an element is updating all the elements in that column.

Credits: Thanks to Pranav Devarakonda for the easy explanation here