How to Create a Python List of Size n?

To create a list of n placeholder elements, multiply the list of a single placeholder element with n. For example, use [None] * 5 to create a list [None, None, None, None, None] with five elements None. You can then overwrite some elements with index assignments. In the example, lst[2] = 42 would result in the changed list [None, None, 42, None, None].

Let’s play with an interactive code shell before you’ll dive into the detailed solution!

Exercise: Initialize the list with n=20 placeholder elements -1 and run the code.

Next, you’ll learn about the more formal problem and dive into the step-by-step solution.

Problem: Given an integer n. How to initialize a list with n placeholder elements?

# n=0 --> []
# n=1 --> [None]
# n=5 --> [None, None, None, None, None]

Solution: Use the list concatenation operation *.

n = 5
lst = [None] * n
# [None, None, None, None, None]

You can modify the element n as you like. In subsequent operations, you can overwrite all placeholder None list elements using simple index assignment operations:

lst[0] = 'Alice'
lst[1] = 0
lst[2] = 42
lst[3] = 12
lst[4] = 'hello'
# ['Alice', 0, 42, 12, 'hello']

However, there’s a small problem if you want to create a list with mutable objects (such as a list of lists):

lst = [[]] * n
# [[], [], [], [], []]


# [[42], [42], [42], [42], [42]]

Changing one list element changes all list elements because all list elements refer to the same list object in memory:

The solution is to use list comprehension (see my detailed blog tutorial on list comprehension for a complete guide):

lst = [[] for _ in range(n)]
# [[], [], [], [], []]

# [[], [], [42], [], []]

In the following visualization, you can see how each element now refers to an independent list object in memory:

Exercise: Run the visualization and convince yourself that only one element is modified! Why is this the case?

Where to Go From Here?

Enough theory, let’s get some practice!

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