programming python

Array of list to array python

Introduction

With NumPy, [np.array] objects can be converted to a list with the tolist[] function. The tolist[] function doesn’t accept any arguments. If the array is one-dimensional, a list with the array elements is returned. For a multi-dimensional array, a nested list is returned.

Converting one-dimensional NumPy Array to List

Let’s construct a one-dimensional array of [1, 2, 3]:

import numpy as np

# 1d array to list
arr_1 = np.array[[1, 2, 3]]

print[f'NumPy Array:\n{arr_1}']

This code will output:

NumPy Array:
[1 2 3]

Now, let’s use tolist[]:

import numpy as np

# 1d array to list
arr_1 = np.array[[1, 2, 3]]

print[f'NumPy Array:\n{arr_1}']

list_1 = arr_1.tolist[]

print[f'List: {list_1}']

This new code will output:

List: [1, 2, 3]

The array has been converted from numpy scalars to Python scalars.

Converting multi-dimensional NumPy Array to List

Let’s construct a multi-dimensional array of [ [1, 2, 3], [4, 5, 6] ]:

import numpy as np

# 2d array to list
arr_2 = np.array[[[1, 2, 3], [4, 5, 6]]]

print[f'NumPy Array:\n{arr_2}']

This code will output:

NumPy Array:
[[1 2 3]
 [4 5 6]]

Now, let’s use tolist[]:

import numpy as np

# 2d array to list
arr_2 = np.array[[[1, 2, 3], [4, 5, 6]]]

print[f'NumPy Array:\n{arr_2}']

list_2 = arr_2.tolist[]

print[f'List: {list_2}']

This new code will output:

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

The array has been converted from numpy scalars to Python scalars.

Conclusion

In this article, you learned how to use tolist[] to convert np.array objects to lists. It is applicable to one-dimensional and multi-dimensional arrays.

References

API Documentation

I'm trying to turn a list of 2d numpy arrays into a 2d numpy array. For example,

dat_list = []
for i in range[10]:
    dat_list.append[np.zeros[[5, 10]]]

What I would like to get out of this list is an array that is [50, 10]. However, when I try the following, I get a [10,5,10] array.

output = np.array[dat_list]

Thoughts?

How to Convert List of Lists to NumPy Array?

Short answer: Convert a list of lists—let’s call it l—to a NumPy array by using the standard np.array[l] function. This works even if the inner lists have a different number of elements.

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. Here’s 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: Here’s 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, you’ve 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.

# 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 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.

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How do I convert a list to an array in Python?

To convert a list to array in Python, use the np. array[] method. The np. array[] is a numpy library function that takes a list as an argument and returns an array containing all the list elements.

Is list [] and [] the same in Python?

Technically speaking, one is a function that returns an object casted to a list, and the other is the literal list object itself. Kinda like int[0] vs 0 . In practical terms there's no difference. I'd expect [] to be faster, because it does not involve a global lookup followed by a function call.

Can you convert a list to an array?

By using the toArray[] method, we can easily convert a list into an array. The toArray[] method returns an array having all the elements in the list. The returned array contains elements in the same sequence as the list.

Is [] a list or array in Python?

What Is an Array in Python? An array is also a data structure that stores a collection of items. Like lists, arrays are ordered, mutable, enclosed in square brackets, and able to store non-unique items. But when it comes to the array's ability to store different data types, the answer is not as straightforward.