How to recycle elememts when dimensions of two numpy arrays do not matches?

Question

I would like to do something similar to the following R code with numpy, where y is recycled.

R> x=rbind(c(1,2,3), c(4,5,6))
R> y=c(1,2)
R> x/y
     [,1] [,2] [,3]
[1,]    1  2.0    3
[2,]    2  2.5    3

Obviously, the following code does not work with numpy. Does anybody know what is the equivalent python code that works? Thanks.

>>> x=numpy.array([[1,2,3], [4, 5, 6]])
>>> y=numpy.array([1,2])
>>> x/y
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: operands could not be broadcast together with shapes (2,3) (2,)

The title of your question is a bit confusing - what do you mean by "recycling y"? — Amos Egel, Dec 27 '17 at 19:09

score 1 · Accepted Answer · answered Dec 27 '17 at 19:02

1

How about

x=numpy.array([[1,2,3], [4, 5, 6]])
y=numpy.array([1,2])
x/y[:, None]

y[:, None] turns the (2,) array to a (2,1) array, thereby allowing broadcast division with x.

answered Dec 27 '17 at 19:02

Amos Egel

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Where is this feature documented? – user1424739 Dec 27 '17 at 21:09
@user1424739 https://docs.scipy.org/doc/numpy-1.13.0/user/basics.broadcasting.html – Divakar Dec 27 '17 at 22:09
@user1424739 Another source of information is here: https://docs.scipy.org/doc/numpy-1.13.0/reference/arrays.indexing.html Note that `y[:, None]` is equivalent to `y[:, numpy.newaxis]`. – Amos Egel Dec 28 '17 at 11:10

How to recycle elememts when dimensions of two numpy arrays do not matches?

1 Answers1