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

[eat]

Hi,


On Tue, Jan 21, 2014 at 8:34 AM, Dr. Leo <fhaxbox66 at 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
>
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