Accessing properties of objects in a numpy array

Question

I've got a numpy array of custom objects. How can I get a new array containing the values of specific attributes of those objects?

Example:

import numpy as np

class Pos():
   def __init__(self, x, y):
      self.x = x
      self.y = y

arr = np.array( [ Pos(0,1), Pos(2,3), Pos(4,5) ] )

# Magic line
xy_arr = .... # arr[ [arr.x,arr.y] ]

print xy_arr
# array([[0,1],
         [2,3],
         [4,5]])

I should add that my motives for such an operation is to calculate the centre of mass of the objects in the array.

Is there a reason you're using arrays for this? This isn't really how numpy arrays are meant to be used, and using them like this is typically more cumbersome and slower than using python lists. — Bi Rico
– Bi Rico, Commented Apr 7, 2012 at 0:52
I was using an array for its indexing power. I have float array A of the same shape as arr and have to select elements out of arr based on a threshold in A — ajwood
– ajwood, Commented Apr 7, 2012 at 0:57

pwuertz · Accepted Answer · 2012-04-07 18:08:55Z

2

Usually, when I have multiple quantities that belong together and I want to benefit from numpys indexing power I use record arrays. Beware, if you do a lot of append/remove operations, numpy might be rather ineffective in terms of speed.

If I understood your comment correctly, this is an example where two values are selected by a third:

import numpy as np

# create a table for your data
dt = np.dtype([('A', np.double), ('x', np.double), ('y', np.double)])
table = np.array([(1,1,1), (2,2,2), (3,3,3)], dtype=dt)

# define a selection mask
selection = table['A'] > 1.5
columns = ['x', 'y']

print table[selection][columns]

A nice side effect is that saving this table using h5py is very simple and convenient as your data is already labeled.

edited Apr 7, 2012 at 18:08

answered Apr 7, 2012 at 17:59

pwuertz

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