Initialize a list with given size and values in Python
Posted: 2020-10-19 / Tags: Python, List
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This article describes how to initialize a list with any size [number of elements] and values in Python.
- Create an empty list
- Initialize a list with any size and values
- Notes on initializing a 2D list [list of lists]
- For tuples and arrays
See the following article about the initialization of NumPy array ndarray.
- NumPy: Create an ndarray with all elements initialized with the same value
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Create an empty list
An empty list is created as follows. You can get the number of elements of a list with the built-in function len[].
l_empty = [] print[l_empty] # [] print[len[l_empty]] # 0
source: list_initialize.py
You can add an element by append[] or remove it by remove[].
l_empty.append[100] l_empty.append[200] print[l_empty] # [100, 200] l_empty.remove[100] print[l_empty] # [200]
source: list_initialize.py
See the following articles for details on adding and removing elements from lists,
- Add an item to a list in Python [append, extend, insert]
- Remove an item from a list in Python [clear, pop, remove, del]
Initialize a list with any size and values
As mentioned above, in Python, you can easily add and remove elements from a list, so in most cases, it is not necessary to initialize the list in advance.
If you want to initialize a list of any number of elements where all elements are filled with any values, you can use the * operator as follows.
l = [0] * 10 print[l] # [0, 0, 0, 0, 0, 0, 0, 0, 0, 0] print[len[l]] # 10
source: list_initialize.py
A list is generated that repeats the elements of the original list.
print[[0, 1, 2] * 3] # [0, 1, 2, 0, 1, 2, 0, 1, 2]
source: list_initialize.py
You can generate a list of sequential numbers with range[].
- How to use range[] in Python
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Notes on initializing a 2D list [list of lists]
Be careful when initializing a list of lists.
The following code is no good.
l_2d_ng = [[0] * 4] * 3 print[l_2d_ng] # [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
source: list_initialize.py
If you update one list, all the lists will be changed.
l_2d_ng[0][0] = 5 print[l_2d_ng] # [[5, 0, 0, 0], [5, 0, 0, 0], [5, 0, 0, 0]] l_2d_ng[0].append[100] print[l_2d_ng] # [[5, 0, 0, 0, 100], [5, 0, 0, 0, 100], [5, 0, 0, 0, 100]]
source: list_initialize.py
This is because the inner lists are all the same object.
print[id[l_2d_ng[0]] == id[l_2d_ng[1]] == id[l_2d_ng[2]]] # True
source: list_initialize.py
You can write as follows using list comprehensions.
- List comprehensions in Python
l_2d_ok = [[0] * 4 for i in range[3]] print[l_2d_ok] # [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]]
source: list_initialize.py
Each inner list is treated as a different object.
l_2d_ok[0][0] = 100 print[l_2d_ok] # [[100, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]] print[id[l_2d_ok[0]] == id[l_2d_ok[1]] == id[l_2d_ok[2]]] # False
source: list_initialize.py
Although range[] is used in the above example, any iterable of the desired size is acceptable.
l_2d_ok_2 = [[0] * 4 for i in [1] * 3] print[l_2d_ok_2] # [[0, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]] l_2d_ok_2[0][0] = 100 print[l_2d_ok_2] # [[100, 0, 0, 0], [0, 0, 0, 0], [0, 0, 0, 0]] print[id[l_2d_ok_2[0]] == id[l_2d_ok_2[1]] == id[l_2d_ok_2[2]]] # False
source: list_initialize.py
If you want to generate a multidimensional list, you can nest list comprehensions.
l_3d = [[[0] * 2 for i in range[3]] for j in range[4]] print[l_3d] # [[[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]]] l_3d[0][0][0] = 100 print[l_3d] # [[[100, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]], [[0, 0], [0, 0], [0, 0]]]
source: list_initialize.py
For tuples and arrays
You can initialize tuples as well as lists.
Note that a tuple of one element requires ,.
- One-element tuples require a comma in Python
t = [0,] * 5 print[t] # [0, 0, 0, 0, 0]
source: list_initialize.py
For array type, you can pass the initialized list to the constructor.
- array Efficient arrays of numeric values Python 3.9.0 documentation
import array a = array.array['i', [0] * 5] print[a] # array['i', [0, 0, 0, 0, 0]]
source: list_initialize.py
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