# Create an empty list in Python with certain size

## Create an empty list in Python with certain size

You cannot assign to a list like `lst[i] = something`, unless the list already is initialized with at least `i+1` elements. You need to use append to add elements to the end of the list. `lst.append(something)`.

(You could use the assignment notation if you were using a dictionary).

Creating an empty list:

``````>>> l = [None] * 10
>>> l
[None, None, None, None, None, None, None, None, None, None]
``````

Assigning a value to an existing element of the above list:

``````>>> l[1] = 5
>>> l
[None, 5, None, None, None, None, None, None, None, None]
``````

Keep in mind that something like `l[15] = 5` would still fail, as our list has only 10 elements.

range(x) creates a list from [0, 1, 2, … x-1]

``````# 2.X only. Use list(range(10)) in 3.X.
>>> l = range(10)
>>> l
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
``````

Using a function to create a list:

``````>>> def display():
...     s1 = []
...     for i in range(9): # This is just to tell you how to create a list.
...         s1.append(i)
...     return s1
...
>>> print display()
[0, 1, 2, 3, 4, 5, 6, 7, 8]
``````

List comprehension (Using the squares because for range you dont need to do all this, you can just return `range(0,9)` ):

``````>>> def display():
...     return [x**2 for x in range(9)]
...
>>> print display()
[0, 1, 4, 9, 16, 25, 36, 49, 64]
``````

``````lst = [None] * 10
``````

The above will create a list of size 10, where each position is initialized to `None`. After that, you can add elements to it:

``````lst = [None] * 10
for i in range(10):
lst[i] = i
``````

Admittedly, thats not the Pythonic way to do things. Better do this:

``````lst = []
for i in range(10):
lst.append(i)
``````

Or even simpler, in Python 2.x you can do this to initialize a list with values from 0 to 9:

``````lst = range(10)
``````

And in Python 3.x:

``````lst = list(range(10))
``````

#### Create an empty list in Python with certain size

`````` >>> l = [None] * 10
>>> l
[None, None, None, None, None, None, None, None, None, None]
``````

Works well for non-reference types like numbers. Unfortunately if you want to create a list-of-lists you will run into referencing errors. Example in Python 2.7.6:

``````>>> a = [[]]*10
>>> a
[[], [], [], [], [], [], [], [], [], []]
>>> a[0].append(0)
>>> a
[[0], [0], [0], [0], [0], [0], [0], [0], [0], [0]]
>>>
``````

As you can see, each element is pointing to the same list object. To get around this, you can create a method that will initialize each position to a different object reference.

``````def init_list_of_objects(size):
list_of_objects = list()
for i in range(0,size):
list_of_objects.append( list() ) #different object reference each time
return list_of_objects

>>> a = init_list_of_objects(10)
>>> a
[[], [], [], [], [], [], [], [], [], []]
>>> a[0].append(0)
>>> a
[[0], [], [], [], [], [], [], [], [], []]
>>>
``````

There is likely a default, built-in python way of doing this (instead of writing a function), but Im not sure what it is. Would be happy to be corrected!

Edit: Its `[ [] for _ in range(10)]`

Example :

``````>>> [ [random.random() for _ in range(2) ] for _ in range(5)]
>>> [[0.7528051908943816, 0.4325669600055032], [0.510983236521753, 0.7789949902294716], [0.09475179523690558, 0.30216475640534635], [0.3996890132468158, 0.6374322093017013], [0.3374204010027543, 0.4514925173253973]]
``````