Index multiple, non-adjacent ranges in numpy

I’d like to select multiple, non-adjacent ranges from a 1d numpy array (or vector).

Suppose:

>>> idx = np.random.randint(100, size=10)
array([82,  9, 11, 94, 31, 87, 43, 77, 49, 50])

This works, of course:

>>> idx[0:3]
array([82,  9, 11])

And this works to fetch via individual indices:

>>> idx[[0,3,4]]
array([82, 94, 31])

But what if I want to select the ranges 0:3, and 7:?

I’ve tried:

>>> idx[[0:3,7:]]
SyntaxError: invalid syntax

Is there a simple way to do this, or do I need to generate them separately and concatenate?

Answers:

Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

You need to concatenate, either before or after indexing. np.r_ makes it easy

In [116]: idx=np.array([82,  9, 11, 94, 31, 87, 43, 77, 49, 50])
In [117]: np.r_[0:3,7:10]
Out[117]: array([0, 1, 2, 7, 8, 9])
In [118]: idx[np.r_[0:3,7:10]]
Out[118]: array([82,  9, 11, 77, 49, 50])

np.r_ expands the slices and concatenates them.

You can mix slices and lists:

In [120]: np.r_[0:3,7:10,[0,3,4]]
Out[120]: array([0, 1, 2, 7, 8, 9, 0, 3, 4])

Concatenating before indexing is probably faster than after, but for 1d array like this, I don’t think the difference is significant.

All methods was sourced from stackoverflow.com or stackexchange.com, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0