# How do I create an empty array/matrix in NumPy?

### Question

I can't figure out how to use an array or matrix in the way that I would normally use a list. I want to create an empty array (or matrix) and then add one column (or row) to it at a time.

At the moment the only way I can find to do this is like:

``````mat = None
for col in columns:
if mat is None:
mat = col
else:
mat = hstack((mat, col))
``````

Whereas if it were a list, I'd do something like this:

``````list = []
for item in data:
list.append(item)
``````

Is there a way to use that kind of notation for NumPy arrays or matrices?

1
264
12/30/2014 6:33:54 PM

You have the wrong mental model for using NumPy efficiently. NumPy arrays are stored in contiguous blocks of memory. If you want to add rows or columns to an existing array, the entire array needs to be copied to a new block of memory, creating gaps for the new elements to be stored. This is very inefficient if done repeatedly to build an array.

In the case of adding rows, your best bet is to create an array that is as big as your data set will eventually be, and then add data to it row-by-row:

``````>>> import numpy
>>> a = numpy.zeros(shape=(5,2))
>>> a
array([[ 0.,  0.],
[ 0.,  0.],
[ 0.,  0.],
[ 0.,  0.],
[ 0.,  0.]])
>>> a[0] = [1,2]
>>> a[1] = [2,3]
>>> a
array([[ 1.,  2.],
[ 2.,  3.],
[ 0.,  0.],
[ 0.,  0.],
[ 0.,  0.]])
``````
373
4/1/2010 12:10:07 PM

A NumPy array is a very different data structure from a list and is designed to be used in different ways. Your use of `hstack` is potentially very inefficient... every time you call it, all the data in the existing array is copied into a new one. (The `append` function will have the same issue.) If you want to build up your matrix one column at a time, you might be best off to keep it in a list until it is finished, and only then convert it into an array.

e.g.

``````
mylist = []
for item in data:
mylist.append(item)
mat = numpy.array(mylist)
``````

`item` can be a list, an array or any iterable, as long as each `item` has the same number of elements.
In this particular case (`data` is some iterable holding the matrix columns) you can simply use

``````
mat = numpy.array(data)
``````

(Also note that using `list` as a variable name is probably not good practice since it masks the built-in type by that name, which can lead to bugs.)

EDIT:

If for some reason you really do want to create an empty array, you can just use ` numpy.array([])`, but this is rarely useful!