Adding rows to an empty 2D NumPy array

Question

I want to start with an empty 2D NumPy array, and then add some rows to it. However, so far I have only been able to do this with a 1D array. Here is what I have tried so far:

a = numpy.array([])
a = numpy.append(a, [1, 2])
a = numpy.append(a, [8, 8])
print a

The output I get is:

[1, 2, 8, 8]

Whereas I want the output to be:

[[1, 2], [8, 8]]

How can I do this?

stackoverflow.com/questions/6667201/…

RST
– RST

2014-11-21 13:41:27 +00:00
Commented Nov 21, 2014 at 13:41 — RST
– RST, Commented Nov 21, 2014 at 13:41

Riyaz · Accepted Answer · 2014-11-21 13:42:31Z

12

Try this:

>>> a = numpy.empty((0,2),int)
>>> a = numpy.append(a, [[1, 2]], axis=0)
>>> a = numpy.append(a, [[8, 8]], axis=0)
>>> a
array([[ 1,  2],
       [ 8,  8]])

answered Nov 21, 2014 at 13:42

Riyaz

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Comments

YXD · Accepted Answer · 2014-11-21 13:40:58Z

0

>>> import numpy
>>> numpy.vstack(([1, 2], [8, 8]))
array([[1, 2],
       [8, 8]])

answered Nov 21, 2014 at 13:40

YXD

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2 Comments

Riyaz Over a year ago

numpy.array(([1, 2], [8, 8])) would do the same. The question is how to append rows to an initially empty array and your code doesn't address that.

hpaulj Over a year ago

Both vstack and append end up using concatenate. They just differ in how they tweak the inputs before.

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