Re: [Numpy-discussion] Creating an ndarray from an iterable over sequences

21 Jan 2014


      Hi,
On Tue, Jan 21, 2014 at 8:34 AM, Dr. Leo fhaxbox66@googlemail.com wrote:
...
Hi,
I would like to write something like:
In [25]: iterable=((i, i**2) for i in range(10))
In [26]: a=np.fromiter(iterable, int32)
ValueError                                Traceback (most recent call
last)
<ipython-input-26-5bcc2e94dbca> in <module>()
----> 1 a=np.fromiter(iterable, int32)
ValueError: setting an array element with a sequence.
Is there an efficient way to do this?
Perhaps you could just utilize structured arrays (
http://docs.scipy.org/doc/numpy/user/basics.rec.html), like:
iterable= ((i, i**2) for i in range(10))
a= np.fromiter(iterable, [('a', int32), ('b', int32)], 10)
a.view(int32).reshape(-1, 2)
Out[]:
array([[ 0,  0],
       [ 1,  1],
       [ 2,  4],
       [ 3,  9],
       [ 4, 16],
       [ 5, 25],
       [ 6, 36],
       [ 7, 49],
       [ 8, 64],
       [ 9, 81]])
My 2 cents,
-eat
...
Creating two 1-dimensional arrays first is costly as one has to
iterate twice over the data. So the only way I see is creating an
empty [10,2] array and filling it row by row. This is memory-efficient
but slow. List comprehension is vice versa.
If there is no solution, wouldn't it be possible to rewrite fromiter
so as to accept sequences?
Leo

NumPy-Discussion mailing list
NumPy-Discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion