recarray with lists: how to reference first element in list

I want to copy contents of a few fields in a record array into a ndarray (both type float64). I know how to do this when the recarray data has a single value in each field:

my_ndarray[:,0]=my_recarray['X']  #(for field 'X')

Now I have a recarray with a list of 5 floats in each field, and I only want to copy the first element of each list. When I use the above with the new recarray (and list), I get this error:

ValueError: could not broadcast input array from shape (92,5) into shape (92)

That makes total sense (in hindsight).

I thought I could get just the first element of each with this:

my_ndarray[:,0]=my_recarray['X'][0]  #(for field 'X')

I get this error:

ValueError: could not broadcast input array from shape (5) into shape (92)

I sorta understand...numpy is only taking the first row (5 elements) and trying to broadcast into a 92 element column.

So....now I'm wondering how to get the first element of each list down the 92 element column, Scratchin my head.... Thanks in advance for advice.

Solution

My guess is that the recarray has a dtype where one of the fields has shape 5:

In [48]: dt = np.dtype([('X',int,5),('Y',float)])
In [49]: arr = np.zeros(3, dtype=dt)
In [50]: arr
Out[50]: 
array([([0, 0, 0, 0, 0], 0.), ([0, 0, 0, 0, 0], 0.),
       ([0, 0, 0, 0, 0], 0.)], dtype=[('X', '<i8', (5,)), ('Y', '<f8')])

Accessing this field by name produces an array that is (3,5) shape (analogous to your (92,5):

In [51]: arr['X']
Out[51]: 
array([[0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0],
       [0, 0, 0, 0, 0]])

This could be described as a list of 5 items for each record, but indexing with field name produces a 2d array, which can be indexing like any 2d numpy array.

Let's set those values to something interesting:

In [52]: arr['X'] = np.arange(15).reshape(3,5)
In [53]: arr
Out[53]: 
array([([ 0,  1,  2,  3,  4], 0.), ([ 5,  6,  7,  8,  9], 0.),
       ([10, 11, 12, 13, 14], 0.)],
      dtype=[('X', '<i8', (5,)), ('Y', '<f8')])

We can fetch the first column of this field with:

In [54]: arr['X'][:,0]
Out[54]: array([ 0,  5, 10])

If you have several fields with a structure like this, you'll probably have to access each one by name. There's a limit to what you can do with multi-field indexing.