Short answer: Convert a list of listslets call it lto a NumPy array by using the standard np.array[l] function. This works even if the inner lists have a different number of elements.
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
[[1, 2, 3], [4, 5, 6]]into a NumPy array
[[1 2 3] [4 5 6]]Solution: Use the np.array[list] function to convert a list of lists into a two-dimensional NumPy array. Heres the 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: Heres the same code in our interactive code interpreter:
Hint: The NumPy method 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, youve got the following list of lists:
[[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 np.array[] function.
This creates a NumPy array with three elementseach element is a list type. You can check the type of the output by using the built-in type[] function:
>>> type[a][2] Make an array of arrays.
# 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.
# 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 None values to each inner list with smaller than maximal length.
Where to Go From Here?
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