Accessing NumPy array elements not in a given index list

Question

I have a NumPy array with the shape (100, 170, 256). And I have an array consisting of indexes [0, 10, 20, 40, 70].

I can get the sub-arrays corresponding to the indexes as follows:

sub_array = array[..., index] 

This returns an array with the shape (100, 170, 5) as expected. Now, I am trying to take the complement and get the sub-array NOT corresponding to those indexes. So, I did:

sub_array = array[..., ~index] 

This still returns me an array of shape (100, 170, 5) for some reason. I wonder how to do this complement operation of these indexes in python?

[EDIT]

Also tried:

sub_array = array[..., not(index.any)] 

However, this does not do the thing I want as well (getting array of shape (100, 170, 251).

Answer 1

The way you have your data, the simplest approach is to use np.delete:

sub_array = np.delete(array, index, axis=2) 

Alternatively, the logical operators you were trying to use can be applied with boolean arrays as @DSM suggests:

mask = np.ones(a.shape[2], dtype=bool) mask[index] = False sub_array = array[:,:, mask] 

(I wouldn't call your array array but I followed the names in your question)

Answer 2

have a look at what ~index gives you - I think it is:

array([ -1, -11, -21, -41, -71]) 

So, your call

sub_array = array[..., ~index] 

will return 5 entries, corresponding to indices [ -1, -11, -21, -41, -71] i.e. 255, 245, 235, 215 and 185 in your case

Similarly, not(index.any) gives

False 

hence why your second try doesn't work

This should work:

sub_array = array[..., [i for i in xrange(256) if i not in index]]