how to append a numpy matrix into an empty numpy array

Question

I want to append a numpy array(matrix) into an array through a loop

data=[[2 2 2] [3 3 3]]
Weights=[[4 4 4] [4 4 4] [4 4 4]]
All=np.array([])  
for i in data:
           


        

Answer 1

A preferred way of constructing an array with a loop is to collect values in a list, and perform the concatenate once, at the end:

In [1025]: data
Out[1025]: 
array([[2, 2, 2],
       [3, 3, 3]])
In [1026]: Weights
Out[1026]: 
array([[4, 4, 4],
       [4, 4, 4],
       [4, 4, 4]])

Append to a list is much faster than repeated concatenate; plus it avoids the 'empty` array shape issue:

In [1027]: alist=[]
In [1028]: for row in data:
      ...:     alist.append(row*Weights)
In [1029]: alist
Out[1029]: 
[array([[8, 8, 8],
        [8, 8, 8],
        [8, 8, 8]]), array([[12, 12, 12],
        [12, 12, 12],
        [12, 12, 12]])]

In [1031]: np.concatenate(alist,axis=0)
Out[1031]: 
array([[ 8,  8,  8],
       [ 8,  8,  8],
       [ 8,  8,  8],
       [12, 12, 12],
       [12, 12, 12],
       [12, 12, 12]])

You can also join the arrays on a new dimension with np.array or np.stack:

In [1032]: np.array(alist)
Out[1032]: 
array([[[ 8,  8,  8],
        [ 8,  8,  8],
        [ 8,  8,  8]],

       [[12, 12, 12],
        [12, 12, 12],
        [12, 12, 12]]])
In [1033]: _.shape
Out[1033]: (2, 3, 3)

I can construct this 3d version with a simple broadcasted multiplication - no loops

In [1034]: data[:,None,:]*Weights[None,:,:]
Out[1034]: 
array([[[ 8,  8,  8],
        [ 8,  8,  8],
        [ 8,  8,  8]],

       [[12, 12, 12],
        [12, 12, 12],
        [12, 12, 12]]])

Add a .reshape(-1,3) to that to get the (6,3) version.

np.repeat(data,3,axis=0)*np.tile(Weights,[2,1]) also produces the desired 6x3 array.