On Tue, Jan 21, 2014 at 07:34:19AM +0100, Dr. Leo 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?
>
> 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.
You could use itertools:
>>> from itertools import chain
>>> g = ((i, i**2) for i in range(10))
>>> import numpy
>>> numpy.fromiter(chain.from_iterable(g), numpy.int32).reshape(-1, 2)
array([[ 0, 0],
[ 1, 1],
[ 2, 4],
[ 3, 9],
[ 4, 16],
[ 5, 25],
[ 6, 36],
[ 7, 49],
[ 8, 64],
[ 9, 81]], dtype=int32)
Oscar
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