Python creating 2D array in object constructor with variable length

Question

I have a problem with creating objects, where 2D array has variable length. I keep getting error

self.state[i] = listt[i]
IndexError: list assignment index out of range

If i try to create object with array with 2 rows and 3 columns, it works. If i try create an object with 3 rows and 3 columns it fails with the error.

class node:
    def __init__(self, m, n, listt=None):
        self.not_in_place = -1
        self.m = m
        self.n = n
        self.state = [[m],[n]]
        if listt is not None:
            for i in range(self.m):
              self.state[i] = listt[i] 

start = [[0,1,2],[3,4,5],[6,7,8]] 
node(3, 3, start) # doesn't work


start_2 = [[0,1,2],[3,4,5]] 
node(2, 3, start_2) # work

What am i doing wrong ?

azro · Accepted Answer · 2020-03-21 08:37:37Z

1

In python you can't initialize an array with a given size, just by giving the size, and also you don't need to

To assign the given list to state, copy it by doing

if listt is not None:
    self.state = list(listt)

The reason it worked in the second case, is that you created a list with 2 sublist, so the 2 boxes are created, and it's okay when you iterate over them, and also why it failed when you try a length of 3

state = [[2], [3]]  # [[2], [3]], and not [[,,], [,,]]

If you want to initialize empty, in case listt is None, you can do

class node:
    def __init__(self, m, n, listt=None):
        self.not_in_place = -1
        self.m = m
        self.n = n
        self.state = [[]] * m
        if listt is not None:
            self.state = list(listt)

    def __str__(self):
        return str(self.state)

if __name__ == '__main__':
    start = [[0, 1, 2], [3, 4, 5], [6, 7, 8]]
    n = node(3, 3, start)
    print(n)  # [[0, 1, 2], [3, 4, 5], [6, 7, 8]]

    n = node(2, 3, None)
    print(n)  # [[], []]

answered Mar 21, 2020 at 8:37

azro

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2 Comments

Tomáš Rončák Over a year ago

great thanks ! Now i have another problem. How can i get position/index [x][y] of a number for instance 0 in the array ? I need it's position so i can do stuff with the array.

azro Over a year ago

@TomášRončák to read [0, 1, 2] do state[0], to read 0 do state[0][0]

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