Accessing object data slicing numpy array

Question

So i'm working on optimizing some python code by using numpy arrays instead of for each loops. Is there any way to access fields from a class during slicing of a numpy array?

class foo:

    the_int = 0

    def __init__(self, i):
        self.bar = i

one, two = foo(5), foo(10)
ar = array([one, two])

int_array = ar[0:2].the_int

#I want int_array = [5, 10]

If that isn't possible in that manner, how would I efficiently generate my "int_array" variable without using a for each loop to loop through "ar", gather "the_int" from each entry?

Thanks, Kyle

Community · Accepted Answer · 2017-05-23 12:24:06Z

2

Why are you using a numpy array to store PyObjects? You won't get the performance improvement you think. See here.

Using a list, you can use a list comprehension instead:

class foo:

    the_int = 0

    def __init__(self, i):
        self.bar = i

one, two = foo(5), foo(10)

ar = [one, two]

int_array = [i.bar for i in ar[0:2]]

edited May 23, 2017 at 12:24

CommunityBot

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answered Apr 15, 2016 at 14:07

user3684792

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4 Comments

Kyle Hunter Over a year ago

Ahh, that makes sense. Didn't realize that numpy arrays shouldn't store objects. So there's no way with objects to avoid the for each loop?

user3684792 Over a year ago

even though the syntax is similar, the list comprehension I have used is not actually a for loop

Kyle Hunter Over a year ago

Yeah, just noticed that. It's significantly faster that my original for loop. Thanks a lot for your help!

hpaulj Over a year ago

Something, for loop or otherwise, has to query each object (or the list or array) for its bar attribute. Your object has been coded in Python, so that query is a Python operation.

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