How to use masked array as index in python?

Question

For example, when i want to use numpy masked array as index, i got some trouble:

import numpy as np
a=np.array([0,1,2,3,4,5])
b=np.array([[0,1],[3,5]])
>>> a[b]
   array([[0, 1],
          [3, 5]]

well, THE PROBLEM is number 3 in b is not what i want. The result i want to geti is like

a[b]=array([[0, 1],
              [5]])

so i used the numpy.ma:

b=np.ma.masked_equal(b,3)
>>> b
masked_array(data =
 [[0 1]
  [-- 5]],
         mask =
 [[False False]
 [ True False]],
   fill_value = 3)
>>> a[b]
array([[0, 1],
   [3, 5]])

As we can see, the masked value still works as index, is there a way to fix it ？

What does 'ignore it' mean, in this context? In general masked arrays only work with masked array functions and methods. They aren't a drop in substitute for regular arrays. But, with more information we can suggest ways to use it, or something equivalent. — hpaulj
– hpaulj, Commented May 12, 2017 at 18:16

user3204459 · Accepted Answer · 2017-11-06 15:46:53Z

1

>>> np.ma.masked_array(a[b.data], b.mask)
masked_array(data =
 [[0 1]
 [-- 5]],
             mask =
 [[False False]
 [ True False]],
       fill_value = 999999)

answered Nov 6, 2017 at 15:46

user3204459

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1 Comment

Toby Speight Over a year ago

Thank you for this code snippet, which might provide some limited, immediate help. A proper explanation would greatly improve its long-term value by showing why this is a good solution to the problem, and would make it more useful to future readers with other, similar questions. Please edit your answer to add some explanation, including the assumptions you've made.

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