Append a NumPy array to a NumPy array


Question

I have a numpy_array. Something like [ a b c ].

And then I want to append it into another NumPy array (just like we create a list of lists). How do we create an array of NumPy arrays containing NumPy arrays?

I tried to do the following without any luck

>>> M = np.array([])
>>> M
array([], dtype=float64)
>>> M.append(a,axis=0)
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
AttributeError: 'numpy.ndarray' object has no attribute 'append'
>>> a
array([1, 2, 3])
1
120
3/22/2016 6:57:54 PM

Accepted Answer

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]])
153
8/2/2016 5:27:02 PM

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.


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