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]]
This post will discuss how to create an empty list with a given size in Python.
To assign any value to a list using the assignment operator at position i, a[i] = x, the list’s size should be at least i+1. Otherwise, it will raise an IndexError, as shown below:
if__name__=='__main__': a =[] a[0]= 1 print[a] # raise IndexError: list assignment index out of range |
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The solution is to create an empty list of None when list items are not known in advance. This can be easily done, as shown below:
if__name__=='__main__': a =[None]*5 print[a] # prints [None, None, None, None, None] |
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The above code will create a list of size 5, where each position is initialized by None. None is frequently used in Python to represent the absence of a value.
Another alternative for creating empty lists with
a given size is to use list comprehensions:
if__name__=='__main__': a =[Noneforxinrange[5]] print[a] # prints [[], [], [], [], []] |
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Which solution to use?
The first solution works well for non-reference types like numbers. But you might run into referencing errors in some cases. For example, [[]] * 5 will result in the list containing the same list object repeated 5
times.
if__name__=='__main__': a =[[]]*5 a[0].append[1] print[a] # prints [[1], [1], [1], [1], [1]] |
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The solution to this problem is using list comprehensions like this:
if__name__=='__main__': a =[[]forxin range[5]] a[0].append[1] print[a] # prints [[1], [], [], [], []] |
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That’s all about creating an empty list with the given size in Python.
Also See:
Initialize a list with the same values in Python