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

Option 1: Reshape your initial All array to 3 columns so that the number of columns match h:

All=np.array([]).reshape((0,3))

for i in data:
    h=i*Weights      
    All=np.concatenate((All,h))

All
#array([[  8.,   8.,   8.],
#       [  8.,   8.,   8.],
#       [  8.,   8.,   8.],
#       [ 12.,  12.,  12.],
#       [ 12.,  12.,  12.],
#       [ 12.,  12.,  12.]])

Option 2: Use a if-else statement to handle initial empty array case:

All=np.array([])
for i in data:
    h=i*Weights      
    if len(All) == 0:
        All = h
    else:
        All=np.concatenate((All,h))

All
#array([[ 8,  8,  8],
#       [ 8,  8,  8],
#       [ 8,  8,  8],
#       [12, 12, 12],
#       [12, 12, 12],
#       [12, 12, 12]])

Option 3: Use itertools.product():

import itertools
np.array([i*j for i,j in itertools.product(data, Weights)])

#array([[ 8,  8,  8],
#       [ 8,  8,  8],
#       [ 8,  8,  8],
#       [12, 12, 12],
#       [12, 12, 12],
#       [12, 12, 12]])