How to append multiple matrices in python

Question

I have read the following related discussions What's the simplest way to extend a numpy array in 2 dimensions?

However, if I want to expend multiple matrices, for example

A = np.matrix([[1,2],[3,4]])
B = np.matrix([[3,4],[5,6]])
C = np.matrix([[7,8],[5,6]])
F = np.append(A,[[B]],0)

However, python says

ValueError: all the input arrays must have same number of dimensions, but the array at index 0 has 2 dimension(s) and the array at index 1 has 4 dimension(s)

I want to put B "under" the matrix A, and put C "under" the matrix B.
So, F should be a 6X2 matrix.

How to do this? Thanks!

Since all your arrays have the same shape, you can simply np.concatenate([A,B,C], axis=0). I objected to the use of np.append in your linked answer, and will do so again. — hpaulj
– hpaulj, Commented Dec 22, 2020 at 18:51

SCash · Accepted Answer · 2020-12-22 19:33:36Z

2

Try using numpy.concatenate (https://numpy.org/doc/stable/reference/generated/numpy.concatenate.html):

A = np.matrix([[1,2],[3,4]])
B = np.matrix([[3,4],[5,6]])
C = np.matrix([[7,8],[5,6]])
# F = np.append(A,[[B]],0)
F = np.concatenate((A, B, C), axis=1)

Change the axis parameter to 0 to combine the matrices 'vertically':

print(np.concatenate((A, B, C), axis=1))

[[1 2 3 4 7 8]
[3 4 5 6 5 6]]

print(np.concatenate((A, B, C), axis=0))

[[1 2]
[3 4]
[3 4]
[5 6]
[7 8]
[5 6]]

answered Dec 22, 2020 at 19:33

SCash

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Comments

Sahil_Angra · Accepted Answer · 2020-12-23 05:32:02Z

2

I believe np.concatenate should do the trick

    A = np.matrix([[1,2],[3,4]])
    B = np.matrix([[3,4],[5,6]])
    C = np.matrix([[7,8],[5,6]])
    ABC = np.concatenate([A,B,C],axis = 0) # axis 0 stacks it one above the other
    
    print("Shape : ",ABC.shape)
    print(ABC)

Output :

    Shape : (6, 2)
    matrix(
    [[1, 2],
    [3, 4],
    [3, 4],
    [5, 6],
    [7, 8],
    [5, 6]])

edited Dec 23, 2020 at 5:32

answered Dec 22, 2020 at 19:33

Sahil_Angra

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1 Comment

hpaulj Over a year ago

But why do two concatenate? It's more efficient the do one with [A,B,C].

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