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arraysnumpy
bhomass
bhomass

Reputation: 3572

why an extra comma in the shape of a single index numpy array

A numpy array a a = numpy.arange(12)

has shape a.shape = (12,)

Why do we need the comma? is shape (12) reserved for something else?

Upvotes: 36

Views: 10451

Answers (2)

wim
wim

Reputation: 362766

A numpy array's shape property always returns a tuple.

The number of dimensions and items in an array is defined by its shape, which is a tuple of N positive integers that specify the sizes of each dimension.

(12,) is just a one-element tuple, so this indicates that you have a one-dimensional array (because the tuple has length 1) with a size of 12.

Documented here.

Upvotes: 12

user6655984
user6655984

Reputation:

The reason we don't use (12) for a one-element tuple (like [12] for one-element list) is that round parentheses also appear in formulas. E.g., in x = 2*(5+7) the part (5+7) is just a number, not a tuple. But what if we actually meant it to be a one-element tuple? The trailing comma is a way to indicate that. Compare:

>>> 2*(5+7)
24
>>> 2*(5+7,)
(12, 12)

With lists, the trailing comma is not needed although some style guides recommend it for consistency. 

>>> 2*[5+7]
[12, 12]
>>> 2*[5+7,]
[12, 12]

Upvotes: 51

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