[FIXED] Python 2D array not appending additions

Issue

I'm trying to create an eight-puzzle game solver, and I have a little problem. My states are being stored as grids(represented by 2D- lists). I am trying to keep track of every resulting state by appending to a succ_states list whenever a move is made by the computer. However, my succ_states 2D array only shows the current state and erases previous states in the list.

Here's my code snippet for the downward move:

def actions(self, state):
    actions = []
    succ_states = []
    print()
    rows = [row for row in state]
    for row in rows:
        for i in row:
            # D
            if i == 0 and state.index(row) != 2:
                print(row, "down action, finally!")
                print("Index of i = ", row.index(i))
                new_row = rows[state.index(row) + 1]
                zero = i
                zero_index = row.index(zero)
                row[zero_index] = new_row[zero_index]
                new_row[zero_index] = zero
                actions.append("D")
                succ_states.append(rows)
    return actions, succ_states

Using the initial state [[1, 3, 0], [6, 7, 8], [4, 2, 5]], actions returns [[[8, 7, 3], [5, 4, 6], [2, 1, 0]], [[8, 7, 3], [5, 4, 6], [2, 1, 0]]]. I don't get why it removes the first resulting state and appends the second state twice. I tried changing the position of the append statements and even tried to extend, but nothing is working.

Please help,I am relatively new to this.

Solution

You are appending rows, but this appends a reference to the list, not a copy. This means that when you later modify rows (or one of the items in row, which are lists in themselves), you modify the contents of succ_states as well.

The fix is to append a copy of rows. And, in this case that rows is a list of lists, then you need to use deepcopy.

I believe this will implement the behavior you seek:

import copy
def actions(self,state):
    actions = []
    succ_states = []
    print()
    rows = [row for row in state]
    for row in rows:
        for i in row:
            # D
            if i == 0 and state.index(row) != 2:
                print(row, "down action, finally!")
                print("Index of i = ", row.index(i))
                new_row = rows[state.index(row) + 1]
                zero = i
                zero_index = row.index(zero)
                row[zero_index] = new_row[zero_index]
                new_row[zero_index] = zero
                actions.append("D")
                succ_states.append(copy.deepcopy(rows))
    return actions, succ_states

Answered By - jokogarcia