# 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
print(lst)
# [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'
print(lst)
# ['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
print(lst)
# [[], [], [], [], []]

lst[2].append(42)

print(lst)
# [[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)]
print(lst)
# [[], [], [], [], []]

lst[2].append(42)
print(lst)
# [[], [], [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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