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Create an empty list in python with certain size


Question

I want to create an empty list (or whatever is the best way) that can hold 10 elements.

After that I want to assign values in that list, for example this is supposed to display 0 to 9:

s1 = list();
for i in range(0,9):
   s1[i] = i

print  s1

But when I run this code, it generates an error or in another case it just displays [] (empty).

Can someone explain why?

1
581
4/22/2015 10:11:50 AM

Accepted Answer

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 don't 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]
2018/07/23
891
7/23/2018 9:44:37 AM

Try this instead:

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, that's 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))
2020/05/17

varunl's currently accepted answer

 >>> 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 I'm not sure what it is. Would be happy to be corrected!

Edit: It's [ [] 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]]
2017/09/14

You can .append(element) to the list, e.g.: s1.append(i). What you are currently trying to do is access an element (s1[i]) that does not exist.

2012/05/23

There are two "quick" methods:

x = length_of_your_list
a = [None]*x
# or
a = [None for _ in xrange(x)]

It appears that [None]*x is faster:

>>> from timeit import timeit
>>> timeit("[None]*100",number=10000)
0.023542165756225586
>>> timeit("[None for _ in xrange(100)]",number=10000)
0.07616496086120605

But if you are ok with a range (e.g. [0,1,2,3,...,x-1]), then range(x) might be fastest:

>>> timeit("range(100)",number=10000)
0.012513160705566406
2016/02/15

I'm surprised nobody suggest this simple approach to creating a list of empty lists. This is an old thread, but just adding this for completeness. This will create a list of 10 empty lists

x = [[] for i in range(10)]
2017/12/11