Note 1: None of the answers given to this question work in my case.
Note 2: The solution must work in NumPy 1.14.
Assume I have the following structured array:
arr = np.array([(105.0, 34.0, 145.0, 217.0)], dtype=[('a', 'f4'), ('b', 'f4'), ('c', 'f4'), ('d', 'f4')]).
Now I'm slicing into the structured data type like so:
arr2 = arr[['a', 'b']]
And now I'm trying to convert that slice into a regular array:
out = arr2[0].view((np.float32, 2))
which results in
ValueError: Changing the dtype of a 0d array is only supported if the itemsize is unchanged
What I would like to get is just a regular array like so:
[105.0, 34.0]
Note that this example is simplified in order to be minimal. In my real use case I'm obviously not dealing with an array that holds one element.
I know that this solution works:
out = np.asarray(list(arr2[0]))
but I thought there must be a more efficient solution than copying data that is already in a NumPy array into a list and then back into an array. I assume there is a way to stay in NumPy an maybe not actually copy any data at all, I just don't know how.
The 1d array does convert with view:
In [270]: arr = np.array([(105.0, 34.0, 145.0, 217.0)], dtype=[('a', 'f4'), ('b','f4'), ('c', 'f4'), ('d', 'f4')])
In [271]: arr
Out[271]:
array([(105., 34., 145., 217.)],
dtype=[('a', '<f4'), ('b', '<f4'), ('c', '<f4'), ('d', '<f4')])
In [272]: arr.view('<f4')
Out[272]: array([105., 34., 145., 217.], dtype=float32)
It's when we try to convert a single element, that we get this error:
In [273]: arr[0].view('<f4')
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
<ipython-input-273-70fbab8f61ba> in <module>()
----> 1 arr[0].view('<f4')
ValueError: Changing the dtype of a 0d array is only supported if the itemsize is unchanged
Earlier view often required a tweak in the dimensions. I suspect that with recent changes to handling of structured arrays (most evident when indexing several fields at once), this error is a result, either intentionally or not.
In the whole array case it changed the 1d, 4 field array into a 1d, 4 element array, (1,) to (4,). But changing the element, goes from () to (4,).
In the past I have recommended tolist as the surest way around problem with view (and astype):
In [274]: arr[0].tolist()
Out[274]: (105.0, 34.0, 145.0, 217.0)
In [279]: list(arr[0].tolist())
Out[279]: [105.0, 34.0, 145.0, 217.0]
In [280]: np.array(arr[0].tolist())
Out[280]: array([105., 34., 145., 217.])
item is also a good way of pulling an element out of its numpy structure:
In [281]: arr[0].item()
Out[281]: (105.0, 34.0, 145.0, 217.0)
The result from tolost and item is a tuple.
You worry about speed. But you are just converting one element. It's one thing to worry about the speed when using tolist on a 1000 item array, quite another when working with 1 element.
In [283]: timeit arr[0]
131 ns ± 1.31 ns per loop (mean ± std. dev. of 7 runs, 10000000 loops each)
In [284]: timeit arr[0].tolist()
1.25 µs ± 11.9 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
In [285]: timeit arr[0].item()
1.27 µs ± 2.39 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
In [286]: timeit arr.tolist()
493 ns ± 17.2 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
In [287]: timeit arr.view('f4')
1.74 µs ± 18.7 ns per loop (mean ± std. dev. of 7 runs, 1000000 loops each)
You could index the element in a way that doesn't reduce the dimension to 0 (not that it helps much with speed):
In [288]: arr[[0]].view('f4')
Out[288]: array([105., 34., 145., 217.], dtype=float32)
In [289]: timeit arr[[0]].view('f4')
6.54 µs ± 15.9 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
In [290]: timeit arr[0:1].view('f4')
2.63 µs ± 105 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
In [298]: timeit arr[0][None].view('f4')
4.28 µs ± 160 ns per loop (mean ± std. dev. of 7 runs, 100000 loops each)
view still requires a change in shape; consider a big array:
In [299]: arrs = np.repeat(arr, 10000)
In [301]: arrs.view('f4')
Out[301]: array([105., 34., 145., ..., 34., 145., 217.], dtype=float32)
In [303]: arrs.shape
Out[303]: (10000,)
In [304]: arrs.view('f4').shape
Out[304]: (40000,)
The view is still 1d, where as we'd probably want a (10000,4) shaped 2d array.
A better view change:
In [306]: arrs.view(('f4',4))
Out[306]:
array([[105., 34., 145., 217.],
[105., 34., 145., 217.],
[105., 34., 145., 217.],
...,
[105., 34., 145., 217.],
[105., 34., 145., 217.],
[105., 34., 145., 217.]], dtype=float32)
In [307]: _.shape
Out[307]: (10000, 4)
This works with the 1 element array, whether 1d or 0d:
In [308]: arr.view(('f4',4))
Out[308]: array([[105., 34., 145., 217.]], dtype=float32)
In [309]: _.shape
Out[309]: (1, 4)
In [310]: arr[0].view(('f4',4))
Out[310]: array([105., 34., 145., 217.], dtype=float32)
In [311]: _.shape
Out[311]: (4,)
This was suggested in one of the answers in your link: https://stackoverflow.com/a/10171321/901925
Contrary to your comment there, it works for me:
In [312]: arr[0].view((np.float32, len(arr.dtype.names)))
Out[312]: array([105., 34., 145., 217.], dtype=float32)
In [313]: np.__version__
Out[313]: '1.14.0'
With the edit:
In [84]: arr = np.array([(105.0, 34.0, 145.0, 217.0)], dtype=[('a', 'f4'), ('b','f4'), ('c', 'f4'), ('d', 'f4')])
In [85]: arr2 = arr[['a', 'b']]
In [86]: arr2
Out[86]:
array([(105., 34.)],
dtype={'names':['a','b'], 'formats':['<f4','<f4'], 'offsets':[0,4], 'itemsize':16})
In [87]: arr2.view(('f4',2))
...
ValueError: Changing the dtype to a subarray type is only supported if the total itemsize is unchanged
Note that the arr2 dtype includes an offsets value. In a recent numpy version, multiple field selection has changed. It is now a true view, preserving the original data - all of it, not just the selected fields. The itemsize is unchanged:
In [93]: arr.itemsize
Out[93]: 16
In [94]: arr2.itemsize
Out[94]: 16
arr.view(('f4',4) and arr2.view(('f4',4)) produce the same thing.
So you can't view (change dtype) a partial set of the fields. You have to first take the view of the whole array, and then select rows/columns, or work with tolist.
I'm using 1.14.0. Release notes for 1.14.1 says:
The change in 1.14.0 that multi-field indexing of structured arrays returns a view instead of a copy has been reverted but remains on track for NumPy 1.15. Affected users should read the 1.14.1 Numpy User Guide section "basics/structured arrays/accessing multiple fields" for advice on how to manage this transition.
https://docs.scipy.org/doc/numpy-1.14.2/user/basics.rec.html#accessing-multiple-fields
This is still under development. That doc mentions a repack_fields function, but that doesn't exist yet.