[Tut] How to Convert a List to a NumPy Array?

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How to Convert a List to a NumPy Array?

To convert a Python list to a NumPy array, use either of the following two methods:

1. The
   Code:

   np.array()
   function that takes an iterable and returns a NumPy array creating a new data structure in memory.
   
   
2. The
   Code:

   np.asarray()
   function that takes an iterable as argument and converts it to the array. The difference to
   Code:

   np.array()
   is that
   Code:

   np.asarray()
   doesn’t create a new copy in memory if you pass a NumPy array. All changes made on the original array are reflected on the NumPy array.
   

https://trinket.io/embed/python/2c445ee795

Exercise: Create array

Code:

b

from array

Code:

a

using both methods. Then change a value in array

Code:

a

. What happens at array

Code:

b

?

NumPy vs Python Lists



The Python built-in list data type is powerful. However, the NumPy array has many advantages over Python lists. What are they?

Advantages NumPy |
Advantages Python Lists

Multi-dimensional Slicing |
Library-Independent

Broadcasting Functionality |
Intuitive

Processing Speed |
Less Complicated

Memory Footprint |
Heterogeneous List Data Allowed

Many Convenience Methods |
Arbitrary Data Shape (Non-Square Matrix)

To read more about the advantages of a NumPy array over a Python list,read my detailed blog tutorial.

How to Convert a 1D Python List to a NumPy Array?

Problem: Given a one-dimensional Python list. How to convert it to a NumPy array?

Example: You have the following 1D Python list of integers.

Code:

lst = [0, 1, 100, 42, 13, 7]

You want to convert it into a NumPy array.

Code:

array([ 0, 1, 100, 42, 13, 7])

Method 1: np.array(…)

The simplest way to convert a Python list to a NumPy array is to use the

Code:

np.array()

function that takes an iterable and returns a NumPy array.

Code:

import numpy as np lst = [0, 1, 100, 42, 13, 7] print(np.array(lst))

The output is:

Code:

# [ 0 1 100 42 13 7]

This creates a new data structure in memory. Changes on the original list are not visible to the variable that holds the NumPy array:

Code:

lst = [0, 1, 100, 42, 13, 7] a = np.array(lst) lst.append(999) print(a) # [ 0 1 100 42 13 7]

The element

Code:

999

which is now part of list

Code:

lst

is not part of array

Code:

a

.

Method 2: np.asarray(…)

An alternative is to use the

Code:

np.asarray()

function that takes one argument—the iterable—and converts it to the NumPy array. The difference to

Code:

np.array()

is that it doesn’t create a new copy in memory IF you pass a NumPy array. All changes made on the original array are reflected on the NumPy array! So be careful.

Code:

lst = [0, 1, 100, 42, 13, 7] a = np.array(lst) b = np.asarray(a) a[0] = 99 print(b) # [ 99 1 100 42 13 7]

The array

Code:

b

is created using the

Code:

np.asarray()

function, so if you change a value of array

Code:

a

, the change will be reflected on the variable

Code:

b

(because they point to the same object in memory).

[Video] How to Convert a List of Lists to a NumPy Array?



Convert List of Lists to 2D Array

Problem: Given a list of lists in Python. How to convert it to a 2D NumPy array?

Example: Convert the following list of lists

Code:

[[1, 2, 3], [4, 5, 6]]

into a NumPy array

Code:

[[1 2 3] [4 5 6]]

Solution: Use the

Code:

np.array(list)

function to convert a list of lists into a two-dimensional NumPy array. Here’s the code:

Code:

# Import the NumPy library import numpy as np # Create the list of lists lst = [[1, 2, 3], [4, 5, 6]] # Convert it to a NumPy array a = np.array(lst) # Print the resulting array print(a) ''' [[1 2 3] [4 5 6]] '''

Try It Yourself: Here’s the same code in our interactive code interpreter:

Code:

<iframe height="700px" width="100%" src="https://repl.it/@finxter/numpylistoflists?lite=true" scrolling="no" frameborder="no" allowtransparency="true" allowfullscreen="true" sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"></iframe>

Hint: The NumPy method

Code:

np.array()

takes an iterable as input and converts it into a NumPy array.

Convert a List of Lists With Different Number of Elements

Problem: Given a list of lists. The inner lists have a varying number of elements. How to convert them to a NumPy array?

Example: Say, you’ve got the following list of lists:

Code:

[[1, 2, 3], [4, 5], [6, 7, 8]]

What are the different approaches to convert this list of lists into a NumPy array?

Solution: There are three different strategies you can use. (source)

(1) Use the standard

Code:

np.array()

function.

Code:

# Import the NumPy library import numpy as np # Create the list of lists lst = [[1, 2, 3], [4, 5], [6, 7, 8]] # Convert it to a NumPy array a = np.array(lst) # Print the resulting array print(a) ''' [list([1, 2, 3]) list([4, 5]) list([6, 7, 8])] '''

This creates a NumPy array with three elements—each element is a list type. You can check the type of the output by using the built-in

Code:

type()

function:

Code:

>>> type(a) <class 'numpy.ndarray'>

(2) Make an array of arrays.

Code:

# Import the NumPy library import numpy as np # Create the list of lists lst = [[1, 2, 3], [4, 5], [6, 7, 8]] # Convert it to a NumPy array a = np.array([np.array(x) for x in lst]) # Print the resulting array print(a) ''' [array([1, 2, 3]) array([4, 5]) array([6, 7, 8])] '''

This is more logical than the previous version because it creates a NumPy array of 1D NumPy arrays (rather than 1D Python lists).

(3) Make the lists equal in length.

Code:

# Import the NumPy library import numpy as np # Create the list of lists lst = [[1, 2, 3], [4, 5], [6, 7, 8, 9]] # Calculate length of maximal list n = len(max(lst, key=len)) # Make the lists equal in length lst_2 = [x + [None]*(n-len(x)) for x in lst] print(lst_2) # [[1, 2, 3, None], [4, 5, None, None], [6, 7, 8, 9]] # Convert it to a NumPy array a = np.array(lst_2) # Print the resulting array print(a) ''' [[1 2 3 None] [4 5 None None] [6 7 8 9]] '''

You use list comprehension to “pad”

Code:

None

values to each inner list with smaller than maximal length.

Related Articles

• How to Convert a List of Lists to a NumPy array?
  
  
• What are Advantages of NumPy arrays over Python lists?
  

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