CODEWITHSUNDEEP
pythonnumpysliceellipsis
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Reputation: 882

How slicing and ellipsis works in numpy?

I have been reading a very old documentation of Numpy and found out a weird notation which eludes my understanding. The documentation says a[i:...] is a shortcut for a[i,:,:,:].

The documentation being old is very vague and I would welcome any comments.

Thanks, Prerit

Upvotes: 1

Views: 2069

Answers (2)

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Reputation: 882

Three full stops ... (and not … (U+2026)), refers to the Ellipsis singleton object. It has no built-in special operations but is often used in slicing expressions.

No built-in classes utilise the Ellipsis object however NumPy uses ... as a shortcut when slicing arrays, for example, where x is a 4D array: x[i, ...] is equivalant to x[i, :, :, :].

NumPy - Indexing

Upvotes: 5

user6454145
user6454145

Reputation:

arr[:,:,1] is fancy indexing used by numpy that selects the first element of the last column in arr. Fancy indexing can only be used in numpy arrays and not in python's traditional lists. Also, like its pointed out in the comments, a[,:,:,] is a syntax error.

It is helpful because you can select columns easily

Upvotes: 2

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