Removing Array Elements in Python while keeping track of their position

Question

Hi!

I'v got two numpy arrays. The first array contains some zeros (which are distributed randomly over the length of the array), which I would like to remove.

My issue is that I would also like to remove the entries of the second array at the index positions where the first array elements are zero.

I only came up with a very cumbersome for-loop. Does anyone have an "elegant" method for doing this?

Thx!

Answer 1

Is it what you want? I am a NumPy newbie.

In [1]: import numpy as np

In [2]: a = np.array([1,2,0,3,0,4])

In [3]: b = np.array([1,2,3,4,5,6])

In [4]: b[np.where(a)]  
Out[4]: array([1, 2, 4, 6])

In [5]: np.where(a)  
Out[5]: (array([0, 1, 3, 5]),)

In [6]: a[np.where(a)]  
Out[6]: array([1, 2, 3, 4])

Answer 2

You can use boolean indexing. x!=0 gives you a boolean array with True where x!=0 false where x==0. If you index either x or y with this array (ie x_nozeros=x[x!=0]) then you will get only the elements where x!=0. eg:

In [1]: import numpy as np
In [2]: x = np.array([1,2,0,3,0,4])
In [3]: y = np.arange(1,7)
In [4]: indx = x!=0
In [5]: x_nozeros = x[indx]
In [6]: y_nozeros = y[indx]
In [7]: x_nozeros
Out[7]: array([1, 2, 3, 4])
In [8]: y_nozeros
Out[8]: array([1, 2, 4, 6])