python - How to select inverse of indexes of a numpy array?

Question

I have a large set of data in which I need to compare the distances of a set of samples from this array with all the other elements of the array. Below is a very simple example of my data set.

import numpy as np
import scipy.spatial.distance as sd

data = np.array(
    [[ 0.93825827,  0.26701143],
     [ 0.99121108,  0.35582816],
     [ 0.90154837,  0.86254049],
     [ 0.83149103,  0.42222948],
     [ 0.27309625,  0.38925281],
     [ 0.06510739,  0.58445673],
     [ 0.61469637,  0.05420098],
     [ 0.92685408,  0.62715114],
     [ 0.22587817,  0.56819403],
     [ 0.28400409,  0.21112043]]
)


sample_indexes = [1,2,3]

# I'd rather not make this
other_indexes = list(set(range(len(data))) - set(sample_indexes))

sample_data = data[sample_indexes]
other_data = data[other_indexes]

# compare them
dists = sd.cdist(sample_data, other_data)

Is there a way to index a numpy array for indexes that are NOT the sample indexes? In my above example I make a list called other_indexes. I'd rather not have to do this for various reasons (large data set, threading, a very VERY low amount of memory on the system this is running on etc. etc. etc.). Is there a way to do something like..

other_data = data[ indexes not in sample_indexes]

I read that numpy masks can do this but I tried...

other_data = data[~sample_indexes]

And this gives me an error. Do I have to create a mask?

Answer 1

mask = np.ones(len(data), np.bool)
mask[sample_indexes] = 0
other_data = data[mask]

not the most elegant for what perhaps should be a single-line statement, but its fairly efficient, and the memory overhead is minimal too.

If memory is your prime concern, np.delete would avoid the creation of the mask, and fancy-indexing creates a copy anyway.

On second thought; np.delete does not modify the existing array, so its pretty much exactly the single line statement you are looking for.