Unsuccessful append to an empty NumPy array

Question

I am trying to fill an empty(not np.empty!) array with values using append but I am gettin error:

My code is as follows:

import numpy as np result=np.asarray([np.asarray([]),np.asarray([])]) result[0]=np.append([result[0]],[1,2]) 

And I am getting:

ValueError: could not broadcast input array from shape (2) into shape (0) 

Answer 1

I might understand the question incorrectly, but if you want to declare an array of a certain shape but with nothing inside, the following might be helpful:

Initialise empty array:

>>> a = np.zeros((0,3)) #or np.empty((0,3)) or np.array([]).reshape(0,3) >>> a array([], shape=(0, 3), dtype=float64) 

Now you can use this array to append rows of similar shape to it. Remember that a numpy array is immutable, so a new array is created for each iteration:

>>> for i in range(3): ...     a = np.vstack([a, [i,i,i]]) ... >>> a array([[ 0.,  0.,  0.],        [ 1.,  1.,  1.],        [ 2.,  2.,  2.]]) 

np.vstack and np.hstack is the most common method for combining numpy arrays, but coming from Matlab I prefer np.r_ and np.c_:

Concatenate 1d:

>>> a = np.zeros(0) >>> for i in range(3): ...     a = np.r_[a, [i, i, i]] ... >>> a array([ 0.,  0.,  0.,  1.,  1.,  1.,  2.,  2.,  2.]) 

Concatenate rows:

>>> a = np.zeros((0,3)) >>> for i in range(3): ...     a = np.r_[a, [[i,i,i]]] ... >>> a array([[ 0.,  0.,  0.],        [ 1.,  1.,  1.],        [ 2.,  2.,  2.]]) 

Concatenate columns:

>>> a = np.zeros((3,0)) >>> for i in range(3): ...     a = np.c_[a, [[i],[i],[i]]] ... >>> a array([[ 0.,  1.,  2.],        [ 0.,  1.,  2.],        [ 0.,  1.,  2.]])