Concatenate a NumPy array to another NumPy array

Question

In [1]: import numpy as np

In [2]: a = np.array([[1, 2, 3], [4, 5, 6]])

In [3]: b = np.array([[9, 8, 7], [6, 5, 4]])

In [4]: np.concatenate((a, b))
Out[4]: 
array([[1, 2, 3],
       [4, 5, 6],
       [9, 8, 7],
       [6, 5, 4]])

or this:

In [1]: a = np.array([1, 2, 3])

In [2]: b = np.array([4, 5, 6])

In [3]: np.vstack((a, b))
Out[3]: 
array([[1, 2, 3],
       [4, 5, 6]])

Well, the error message says it all: NumPy arrays do not have an append() method. There's a free function numpy.append() however:

numpy.append(M, a)

This will create a new array instead of mutating M in place. Note that using numpy.append() involves copying both arrays. You will get better performing code if you use fixed-sized NumPy arrays.

You may use numpy.append()...

import numpy

B = numpy.array([3])
A = numpy.array([1, 2, 2])
B = numpy.append( B , A )

print B

> [3 1 2 2]

This will not create two separate arrays but will append two arrays into a single dimensional array.

Sven said it all, just be very cautious because of automatic type adjustments when append is called.

In [2]: import numpy as np

In [3]: a = np.array([1,2,3])

In [4]: b = np.array([1.,2.,3.])

In [5]: c = np.array(['a','b','c'])

In [6]: np.append(a,b)
Out[6]: array([ 1.,  2.,  3.,  1.,  2.,  3.])

In [7]: a.dtype
Out[7]: dtype('int64')

In [8]: np.append(a,c)
Out[8]: 
array(['1', '2', '3', 'a', 'b', 'c'], 
      dtype='|S1')

As you see based on the contents the dtype went from int64 to float32, and then to S1

I found this link while looking for something slightly different, how to start appending array objects to an empty numpy array, but tried all the solutions on this page to no avail.

Then I found this question and answer: How to add a new row to an empty numpy array

The gist here:

The way to "start" the array that you want is:

arr = np.empty((0,3), int)

Then you can use concatenate to add rows like so:

arr = np.concatenate( ( arr, [[x, y, z]] ) , axis=0)

See also https://docs.scipy.org/doc/numpy/reference/generated/numpy.concatenate.html