So here's the deal: I have variable x which is a numpy.ndarray. The size of this structure is 1000. If I do x[0], then I get a numpy.void, of 4 numbers. If I do x[1], then I get another numpy.void, also of 4 numbers, etc.
What I simply want to do: I want to slice this data structure, so that I extract a numpy matrix, of size 1000x3.
How do I do that? Thanks
Sounds like you have a structured array, something like this simple example:
In [158]: x = np.ones((5,), dtype='i,i,f,f')
In [159]: x
Out[159]:
array([(1, 1, 1., 1.), (1, 1, 1., 1.), (1, 1, 1., 1.),
(1, 1, 1., 1.), (1, 1, 1., 1.)],
dtype=[('f0', '<i4'), ('f1', '<i4'), ('f2', '<f4'), ('f3', '<f4')])
In [160]: x[0]
Out[160]: (1, 1, 1., 1.)
In [161]: type(x[0])
Out[161]: numpy.void
x[0] is a record, displayed as a tuple. You access fields by name (not by 'column' index):
In [162]: x['f0']
Out[162]: array([1, 1, 1, 1, 1], dtype=int32)
In [163]: x['f2'] = np.arange(5)
In [165]: x['f1'] = [10,12,8,0,3]
In [166]: x
Out[166]:
array([(1, 10, 0., 1.), (1, 12, 1., 1.), (1, 8, 2., 1.),
(1, 0, 3., 1.), (1, 3, 4., 1.)],
dtype=[('f0', '<i4'), ('f1', '<i4'), ('f2', '<f4'), ('f3', '<f4')])
In [168]: x[['f2','f3']] # 2 fields at once
Out[168]:
array([( 0., 1.), ( 1., 1.), ( 2., 1.), ( 3., 1.), ( 4., 1.)],
dtype=[('f2', '<f4'), ('f3', '<f4')])
This is handy when 'columns' should contain different things, for example strings in one, integers in another. But it can be awkward to convert such an array to a 2d array of the same numeric type.
view and astype work in limited cases, but tolist is the most robust conversion medium that I'm aware of.
In [179]: x.tolist()
Out[179]:
[(1, 10, 0.0, 1.0),
(1, 12, 1.0, 1.0),
(1, 8, 2.0, 1.0),
(1, 0, 3.0, 1.0),
(1, 3, 4.0, 1.0)]
In [180]: np.array(x.tolist())
Out[180]:
array([[ 1., 10., 0., 1.],
[ 1., 12., 1., 1.],
[ 1., 8., 2., 1.],
[ 1., 0., 3., 1.],
[ 1., 3., 4., 1.]])
Note that the tolist for the structured array is a list of tuples, whereas tolist for a 2d array is a list of lists. Going this direction that difference doesn't matter. Going the other way the difference matters.
How did you generate this array? From a csv with genfromtxt? As output from some other numeric package?
