ArrayList code last_index issue - Section 9.2

[arielbello]

Chapter 9, Section 2: Python Lists Revisited

The last_index in this code is tracking one position past the last item in the list. It starts pointing to index 0 in an empty list. Then, let's say we add 1 to a fresh instance of ArrayList, it will be the only (and also the last) item on index 0, but last_index will be 1 Up to this point, it's not a problem, but could cause problems when trying to pop an item from the list

class ArrayList:
    def __init__(self):
        self.size_exponent = 0
        self.max_size = 0
        self.last_index = 0
        self.my_array = []

    def append(self, val):
        if self.last_index > self.max_size - 1:  |\label{line:lst_arr_size}|
            self.__resize()
        self.my_array[self.last_index] = val
        self.last_index += 1

But then, there's a problem on this listing:

def insert(self, idx, val):
    if self.last_index > self.max_size - 1:
        self.__resize()
    for i in range(self.last_index, idx - 1, -1):  |\label{line:lst_arrlistins_range}|
        self.my_array[i + 1] = self.my_array[i]
    self.last_index += 1
    self.my_array[idx] = val

Let's say we have a list [1,0] with max_size == 2. According to the first snippet, after appending one item last_index == 1. If we insert anything, the first iteration on the loop will try to access the array on index 2, which should cause an IndexError

[arielbello]

My suggestions to adjust the issue is as follows:

class ArrayList:
    def __init__(self):
        self.size_exponent = 0
        self.max_size = 0
        #last_index initialized with -1
        #could also be useful to check for an empty list
        self.last_index = -1
        self.my_array = []

    def append(self, val):
        if self.last_index > self.max_size - 1:  |\label{line:lst_arr_size}|
            self.__resize()
        #updating last_index prior to appending the item
        self.last_index += 1
        self.my_array[self.last_index] = val

    def __resize(self):
        new_size = 2 ** self.size_exponent
        print("new_size = ", new_size)
        new_array = [0] * new_size
        for i in range(self.max_size):  |\label{line:lst_arr_cop1}|
            new_array[i] = self.my_array[i]

        self.max_size = new_size
        self.my_array = new_array
        self.size_exponent += 1

def insert(self, idx, val):
    #Note the change from > to >=
    #this is to assure we will not go out of bounds when accessing
    #last_index+1 to shift the list
    if self.last_index >= self.max_size - 1:
        self.__resize()
    for i in range(self.last_index, idx - 1, -1):  |\label{line:lst_arrlistins_range}|
        self.my_array[i + 1] = self.my_array[i]
    self.last_index += 1
    self.my_array[idx] = val