Alina Nicolae Alina Nicolae - 3 months ago 17
Python Question

Numpy array of numpy arrays with certain format

I want to declare a numpy array (

arr
) with elements having a certain format, like this:

dt1 = np.dtype([('sec', '<i8'), ('nsec', '<i8')])
dt2 = np.dtype([('name', 'S10'), ('value', '<i4')])

arr = np.array([('x', dtype=dt1), ('y', dtype=dt2)])


The structure is something like this:

arr['x'] = elements with the format dt1 (sec and nsec)
arr['y'] = array of n elements with the format dt2 (name and value)


And the elements within should be accessed like this:

arr['x']['sec'], arr['x']['nsec'], arr['y'][0]['name'] etc.


But I get an
invalid syntax
error.
What is the correct syntax in this situation?

Answer

A doubly compound dtype might work:

In [834]: dt1 = np.dtype([('sec', '<i8'), ('nsec', '<i8')])
     ...: dt2 = np.dtype([('name', 'S10'), ('value', '<i4')])
     ...: 
In [835]: dt = np.dtype([('x', dt1),('y', dt2)])

In [837]: z=np.ones((3,), dtype=dt)
In [838]: z
Out[838]: 
array([((1, 1), (b'1', 1)), ((1, 1), (b'1', 1)), ((1, 1), (b'1', 1))], 
      dtype=[('x', [('sec', '<i8'), ('nsec', '<i8')]), ('y', [('name', 'S10'), ('value', '<i4')])])

In [839]: z['x']['sec']
Out[839]: array([1, 1, 1], dtype=int64)

In [841]: z['y'][0]['name']='y0 name'
In [842]: z
Out[842]: 
array([((1, 1), (b'y0 name', 1)), ((1, 1), (b'1', 1)), ((1, 1), (b'1', 1))], 
      dtype=[('x', [('sec', '<i8'), ('nsec', '<i8')]), ('y', [('name', 'S10'), ('value', '<i4')])])
In [843]: z[0]
Out[843]: ((1, 1), (b'y0 name', 1))

A dictionary would work with similar syntax

In [845]: d={'x':np.ones((3,),dt1), 'y':np.zeros((4,),dt2)}
In [846]: d
Out[846]: 
{'x': array([(1, 1), (1, 1), (1, 1)], 
       dtype=[('sec', '<i8'), ('nsec', '<i8')]),
 'y': array([(b'', 0), (b'', 0), (b'', 0), (b'', 0)], 
       dtype=[('name', 'S10'), ('value', '<i4')])}
In [847]: d['y'][0]['name']='y0 name'