how to append a numpy array to another-CodePudding

I'm trying to append a numpy array to another array. I have 3 arrays with a shape of (3, ). I want to get a shape of (3, 3), but I get (9, ) in output.

data = []
for i in range(3) :
    data = np.append(data, next[i], axis=0)
print(data.shape, data)

output :

(9,)
['John' 'Andrew' 'Mike' 4 23 45 123.0 31.3 344.0]

the output I want should have shape (3, 3) like this :

(3,3)
['John'  'Andrew'  'Mike' ,  
4 23 45 ,  
123.0 31.3 344.0]

CodePudding user response：

Check one step:

In [6]: np.append([], [1,2,3], axis=0)
Out[6]: array([1., 2., 3.])

np.array([]) is a (0,) shape array; join that with a (3,) shape on axis 0 (the only possible one), and you get a (3,) shape. Join that (3,) with another (3,), again on axis 0, you get (6,). Not the (2,3) that you seem to imagine.

Too many new users seem to think that np.array([]) is a clone of the [] list, and np.append is a clone of the list append. Stick with list append if you don't understand array concatenate and array dimensions.

CodePudding user response：

I am assuming next is a 2D list in your case. Please avoid using this as a variable name since it is a keyword in python.

If next is a 2D list of 3x3, then you can simply use asmatrix in numpy:

data = np.asmatrix(next)
print(data.shape, data)

Alternatively, you can use vstack:

data = np.array(next[0])
for i in range(1,3) :
    data = np.vstack((data,next[i]))
print(data.shape, data)