从元组列表中的项目构建2D numpy数组

Question

Given a python list of tuples such as:

test = [(1, 'string1', 47.9, -112.8, 6400.0),
        (2, 'string2', 29.7, -90.8, 11.0),
        (3, 'string3', 30.8, -99.1, 1644.0),
        (4, 'string4', 45.8, -110.9, 7500.0),
        (5, 'string5', 43.9, -69.8, 25.0)]

What is the most efficient way to build a 2D numpy array using the 3rd and 4th items from each tuple?

Desired output is:

array([[47.9, 29.7, 30.8, 45.8, 43.9],
       [-112.8, -90.8, -99.1, -110.9, -69.8]]) 

Answer 1

You can prepare the data outside numpy using a list comprehension which selects the 3rd and 4th item. Then you only need to transpose the resulting array:

np.array([x[2:4] for x in test]).T

Answer 2

zip the list, slice it using itertools.islice :

from itertools import islice

np.array(list(islice(zip(*test), 2, 4)))
# array([[  47.9,   29.7,   30.8,   45.8,   43.9],
#        [-112.8,  -90.8,  -99.1, -110.9,  -69.8]])

Answer 3

You could transform the list of tuples directly into an array then use slicing and transposing to get the desired output:

import numpy as np

test = [(1, 'string1', 47.9, -112.8, 6400.0),
        (2, 'string2', 29.7, -90.8, 11.0),
        (3, 'string3', 30.8, -99.1, 1644.0),
        (4, 'string4', 45.8, -110.9, 7500.0),
        (5, 'string5', 43.9, -69.8, 25.0)]

arr = np.array(test, dtype=object)
result = arr[:, 2:4].T.astype(np.float32)
print(result)

Output

[[  47.9   29.7   30.8   45.8   43.9]
 [-112.8  -90.8  -99.1 -110.9  -69.8]]

Note that after doing arr = np.array(test) everything is done at numpy level.

Answer 4

the first list is:

the_first = [item[2] for item in test]

and second is:

 second = [item[3] for item in test]

and the result is:

 result = np.array([the_first, second])

Answer 5

You can try this:

import numpy as np

test = [(1, 'string1', 47.9, -112.8, 6400.0), (2, 'string2', 29.7, -90.8, 11.0), (3, 'string3', 30.8, -99.1, 1644.0), (4, 'string4', 45.8, -110.9, 7500.0), (5, 'string5', 43.9, -69.8, 25.0)]

result = np.array([(item[3], item[4]) for item in test]).T
print(result)

# array([[-112.8,  -90.8,  -99.1, -110.9,  -69.8],
#       [6400. ,   11. , 1644. , 7500. ,   25. ]])