(Python) Adding elements to the list using loop, appending to this list actual state of processing element after every iteration

Question

Hello i'm fighting with one kata on Codewars. My idea is to create list of coordinates based on list of directions(NORTH,SOUTH...) but here a have a problem. I'm going to use loop while, to check every direction and simultaneously appending created coordinates to list t. But I'm surprised because my loop doesn't append coordinates to t just after creating, but add final result of all iterations of the loop. Could you explain me how it works and how can I add result of particular iterartion of loop to list t?

def dirReduc(arr):
    t=[[0,0]]

    a=[0,0]
    b=0
    while b<len(arr):

         if arr[b]=='NORTH':
            print(b)

            a[0]=a[0]+1
            print(a)
            t.append(a)

        b=b+1
    print(t)



print(dirReduc(["NORTH", "SOUTH", "SOUTH", "EAST", "WEST", "NORTH", "WEST"]))
# I wish to modify this to [[0,0],[1,0],[0,0],[-1,0],[-1,1],[-1,0],[0,0],[0,-1]]
# if 'North' [a+1,b], if 'South' [a-1,b],if 'West' [a,b-1],if 'East' [a,b+1]

Run:
0
[1, 0]
5
[2, 0]
[[0, 0], [2, 0], [2, 0]] #here i suspect:[[0, 0], [1, 0], [2, 0]]
None

cwalvoort · Answer 1 · 2020-03-27T20:41:21.360

0

When you are appending a to the list, you are appending the reference (see here), not the "snapshot in time" that you think you are getting. You need to instead use copy():

t.append(a.copy())

edited Mar 27 '20 at 20:41

answered Mar 27 '20 at 20:12

cwalvoort

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(Python) Adding elements to the list using loop, appending to this list actual state of processing element after every iteration

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