Why does my list only keep the last line after looping?

Question

def main():
    row = 5
    col = 3
    count = 0
    board = list([[""]*col]*row)
    for i in range(0,row):
        for n in range(0,col):
            board[i][n] = str(count)
            print board[i][n] , count, i, n
            count+=1

    print board
main()

When I run this code, I expect my list to be :

[[0,1,2],[3,4,5],[6,7,8],[9,10,11],[12,13,14]]

and when I print the individual indices inside the loop, it assigns properly but after exiting the loop, I fine the output ends up being

[[12,13,14],[12,13,14],[12,13,14],[12,13,14],[12,13,14]]

`board = list([[""]*col]*row)` is a well-known aliasing bug. Rather than multiple distinct rows, you just have multiple references to a single row. — John Coleman, Jan 31 '18 at 02:29
@JohnColeman How could I change it in order to fix the error? — Dee, Jan 31 '18 at 02:33
Read the other question -- this is a duplicate and will likely be closed as such within another minute or two — John Coleman, Jan 31 '18 at 02:35
Do `[[""]*col for i in range(row)]`. The duplicate question John mentions explains why. — hostingutilities.com, Jan 31 '18 at 02:35

score 0 · Answer 1 · answered Jan 31 '18 at 02:34

0

Try replacing this line

board = list([[""]*col]*row)

with

board = [[0 for x in range(col)] for y in range(row)]

This change makes the code work for me.

answered Jan 31 '18 at 02:34

pgngp

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Why does my list only keep the last line after looping?

1 Answers1