Strange things about numpy indexing using empty ndarray

Question

When I do numpy indexing, sometimes the index can be an empty list, in that way I want numpy to also return a empty array. For example:

a = np.array([1, 2, 3])

b = []

print a[b]

This works perfectly fine! when the result gives me:

result:[]

But when I use ndarray as indexer, strange things happened:

a = np.array([1, 2, 3])

b = []

c = np.array(b)

print a[c]

This gives me an error:

IndexError: arrays used as indices must be of integer (or boolean) type

However, When I do this:

a = np.array([1, 2, 3])

b = []

d = np.arange(0, a.size)[b]

print a[d]

Then it works perfectly again:

result:[]

But when I check the type of c and d, they returns the same! Even the shape and everything:

print type(c), c.shape

print type(d), d.shape

result：<type 'numpy.ndarray'> (0L,)

result：<type 'numpy.ndarray'> (0L,)

So I was wondering if there is anything wrong with it? How come a[c] doesn't work but a[d] works? Can you explain it for me? Thank you!

Paul Panzer · Accepted Answer · 2017-03-07 06:46:57Z

4

There is a simple resolution numpy arrays have in a sense two types. their own type which is numpy.ndarray and the type of their elements which is specified by a numpy.dtype. Arrays used for indexing must have elements of integer or boolean dtype. In your examples you use two array creation methods with different default dtypes. The array factory which defaults to a float dtype if nothing else can be inferred from the template. And arange which uses an integer dtype unless you pass some float parameters. Since the dtype is also a property of the array it is specified and checked even if there are no elements. Empty lists are not checked for dtype because they do not have a dtype attribute.

answered Mar 7, 2017 at 6:46

Paul Panzer

53.3k3 gold badges59 silver badges103 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

piRSquared · Accepted Answer · 2017-03-07 06:37:31Z

3

numpy doesn't know what type the empty array is.

Try:

c = np.array(b, dtype=int)

answered Mar 7, 2017 at 6:37

piRSquared

296k68 gold badges509 silver badges654 bronze badges

2 Comments

Dirk Paul Over a year ago

Thanks this works! But I still think this is a little inconvenient. How come I have to tell numpy the type of an empty array. If this is a list I wouldn't have to worry about it.

hpaulj Over a year ago

It may be inconvenient for you in this context, but not necessarily for others. Most people don't need to make empty arrays as you did. The more common way of making a blank/empty array is np.zeros(n). That too is dtype float.

Collectives™ on Stack Overflow

Strange things about numpy indexing using empty ndarray

2 Answers 2

Comments

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related