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

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 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]


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] * 10for 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))


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]]


matomo