[FIXED] Concatenating one dimensional numpyarrays with variable size numpy array in loop

Issue

nNumbers = [1,2,3]
baseVariables = ['a','b','c','d','e']

arr = np.empty(0)
    
    for i in nNumbers:
        x = np.empty(0)
        for v in baseVariables:
            x = np.append(x, y['result'][i][v])
            print(x)
        arr = np.concatenate((arr, x))

I have one Json input stored in y. need to filter some variables out of that json format. the above code works in that it gives me the output in an array, but it is only in a one dimensional array. I want the output in a two dimensional array like:

[['q','qr','qe','qw','etc']['','','','','']['','','','','']]

I have tried various different ways but am not able to figure it out. Any feedback on how to get it to the desired output format would be greatly appreciated.

Solution

A correct basic Python way of making a nested list of strings:

In [57]: nNumbers = [1,2,3]
    ...: baseVariables = ['a','b','c','d','e']

In [58]: alist = []
    ...: for i in nNumbers:
    ...:     blist = []
    ...:     for v in baseVariables:
    ...:         blist.append(v+str(i))
    ...:     alist.append(blist)
    ...:     

In [59]: alist
Out[59]: 
[['a1', 'b1', 'c1', 'd1', 'e1'],
 ['a2', 'b2', 'c2', 'd2', 'e2'],
 ['a3', 'b3', 'c3', 'd3', 'e3']]

That can be turned into an array if necessary - though numpy doesn't provide much added utility for strings:

In [60]: np.array(alist)
Out[60]: 
array([['a1', 'b1', 'c1', 'd1', 'e1'],
       ['a2', 'b2', 'c2', 'd2', 'e2'],
       ['a3', 'b3', 'c3', 'd3', 'e3']], dtype='<U2')

Or in a compact list comprehension form:

In [61]: [[v+str(i) for v in baseVariables] for i in nNumbers]
Out[61]: 
[['a1', 'b1', 'c1', 'd1', 'e1'],
 ['a2', 'b2', 'c2', 'd2', 'e2'],
 ['a3', 'b3', 'c3', 'd3', 'e3']]

You are starting with lists! And making strings! And selecting items from a JSON, with y['result'][i][v]. None of that benefits from using numpy, especially not the repeated use of np.append and np.concatenate.

Answered By - hpaulj