List values change after saving them

Question

I am currently writing an eight queens problem solving algorithm.

The alogorithm is not the problem tho. Its the storing of the solutions.

I am trying to append the solutions (solutions are stored in a list of lists) to a list, but after saving the solution and continuing through the algorithm, the Values of the saved list keep changing.

I suppose the saved list and the one I am changing are somehow still "connected", but I dont know how.

Does anyone know a solution or a different approach to my saving strategy?

This is the part of the code I am having Problems with.

# The Code only gets executed if there is a valid solution.
if found_solution():
# Saving solution
    solutions.append(board)
# deleting last Queen
    temp = last_queen.pop()
    board[temp[0]][temp[1]] = "-"
# going back one Row
    Reihe -= 1
# return
    return

The board does look like this

board = [["-","-","-","-"],
         ["-","-","-","-"],
         ["-","-","-","-"],
         ["-","-","-","-"]]

the solution list like this

solutions = []

If anyone wants to take a look at the whole code (There are a few german variables tho):

board = [["-","-","-","-"],
         ["-","-","-","-"],
         ["-","-","-","-"],
         ["-","-","-","-"]]
    
Reihe = 0
solutions = []
last_queen = []


def found_solution():
    count = 0
    for i in range(4):
        for j in range(4):
            if board[i][j] == "D":
                count += 1
    if count == 4:
        return True
    return False


def board_clear(temp_Reihe, temp_i):
    #Check Column and Row
    for k in range(4):
        if board[temp_Reihe][k] == "D":
            return False
        if board[k][temp_i] == "D":
            return False
    #Check Diagonals
    #temp_i == x and temp_Reihe == y
    st1_x = temp_i - min(temp_i, temp_Reihe)
    st1_y = temp_Reihe - min(temp_i, temp_Reihe)
    st2_x = temp_i - min(temp_i, 3-temp_Reihe)
    st2_y = temp_Reihe + min(temp_i, 3-temp_Reihe)
    while st1_x < 4 and st1_y < 4:
        if board[st1_y][st1_x] == "D":
            return False
        st1_x += 1
        st1_y += 1
    while st2_x < 4 and st2_y > -1:
        if board[st2_y][st2_x] == "D":
            return False
        st2_x += 1
        st2_y -= 1

    return True



def main():
    global Reihe, board, solutions, last_queen

    if found_solution():

        #Saving solution
        solutions.append(board)
        #deleting last Queen
        temp = last_queen.pop()
        board[temp[0]][temp[1]] = "-"
        #going back one Row
        Reihe -= 1
        return


    for i in range(4):
        #placing Queen if valid spot
        if board_clear(Reihe, i):
            board[Reihe][i] = "D"
            last_queen.append([Reihe,i])
            Reihe += 1
            main()
    #delete last Queen
    temp = last_queen.pop()
    board[temp[0]][temp[1]] = "-"
    #going back one Row
    Reihe -= 1
    return
    
main()

Does this answer your question? [List of lists changes reflected across sublists unexpectedly](https://stackoverflow.com/questions/240178/list-of-lists-changes-reflected-across-sublists-unexpectedly) — G. Anderson, Jan 27 '22 at 20:26
You have only one variable `board` which you append _a reference to_ over and over. Perhaps you mean to append `board.copy()`? — G. Anderson, Jan 27 '22 at 20:27

score -2 · Answer 1 · answered Jan 27 '22 at 20:27

-2

People have suggested using copy or deepcopy

list2 = list1.deepcopy()

answered Jan 27 '22 at 20:27

Bora Varol

192
1
11

List values change after saving them

1 Answers1