best way ( short hand) adding column and row to numpy array

Question

What is the best way to concatenate column or row in numpy?

I know of numpy.append and numpy.vstack.

for example, i have 3x5 array and 3x5 array

a = numpy.zeros((3,5))
b = numpy.ones((3,5))

If I want to concatenate a and b to make 3 x 10 array, i would do

a = numpy.hstack((a,b))

If I want to concatenate a and b to make 6 x 5 array, i would do

a = numpy.vstack((a,b))

is there a more efficient(more array-ish) syntax like in R?

You need to give us a better example, such as a couple of input arrays, and the desired output. — hpaulj
– hpaulj, Commented Jan 31, 2015 at 20:15

user2379410 · Accepted Answer · 2015-02-01 19:58:49Z

3

You can do shorthand like so:

import numpy as np

a = numpy.zeros((3,5))
b = numpy.ones((3,5))

# hstack equivalent
c = np.c_[a, b]

# vstack equivalent
d = np.r_[a, b]

answered Feb 1, 2015 at 19:58

user2379410

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Comments

hpaulj · Accepted Answer · 2015-01-31 21:58:10Z

1

hstack and vstack both end up calling concatenate, which is a compiled function. So it's as efficient as can be.

In [76]: np.concatenate([a,b],1).shape
Out[76]: (3, 10)
In [77]: np.concatenate([a,b],0).shape
Out[77]: (6, 5)
In [79]: np.array([a,b]).shape
Out[79]: (2, 3, 5)

And np.array is the basic constructor, which normally adds a dimension. The input to all of these is a list, a list of arrays, lists, or numbers.

What is special about R syntax that makes more efficient or 'arrayish'?

edited Jan 31, 2015 at 21:58

answered Jan 31, 2015 at 21:50

hpaulj

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1 Comment

user2773013 Over a year ago

I remember it wrong. R uses rbind and cbind. THank you!!

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