[SciPy-User] Populating a recarray from 0 size

[Joe Kington]

On Sun, Aug 22, 2010 at 12:44 PM, Benjamin Root <ben.root at ou.edu> wrote:

> On Sat, Aug 21, 2010 at 8:18 PM, Sergi Pons Freixes <spons at utm.csic.es>wrote:
>
>> Hi everybody,
>>
>> I'm interested in populating a recarray a row at a time. I thought in
>> using:
>>
>> -------
>> # Creation of empty recarray
>> names = ["cruise", "SEQ", "param", "r2", "stderr", "slope", "intercept"
>> formats = ["i4", "i4", "S10", "f4", "f4", "f4", "f4"]
>> statsc = scipy.empty(0, dtype={"names":names, "formats":formats})
>>
>> # Adding row and setting values
>> statsc = scipy.resize(statsc, statsc.size + 1)
>> statsc["cruise"][-1] = cr
>> statsc["SEQ"][-1] = ca
>> statsc["param"][-1] = yvar
>> ...
>> -------
>>
>> But scipy.resize complains with:
>> statsc = scipy.resize(statsc, statsc.size + 1)
>>  File "/usr/lib/python2.6/site-packages/numpy/core/fromnumeric.py",
>> line 833, in resize
>>    if not Na: return mu.zeros(new_shape, a.dtype.char)
>> ValueError: Empty data-type
>>
>> So, is scipy not happy because statsc.size is 0 on the first resize?
>> In this case, how could I overcome this limitation?
>>
>> Regards,
>> Sergi
>>
>>
> Sergi,
>
> I had a similar need recently.  I was populating recarrays where I didn't
> know the final size beforehand.  The way I solved the problem was to create
> an iterator function and use .fromiter() to build the array.
>
> A simplified example:
>
> import numpy
>
> class foobar(object) :
>     def __init__(self) :
>        self._myval = 0
>
>     def __iter__(self) :
>         return self
>
>     def next(self) :
>         if self._myval >= 7 :
>             raise StopIteration
>
>         self._myval += 1
>         return self._myval
>
> gen = foobar()
> a = numpy.fromiter(gen, dtype=[('Column', 'i4')])
> print a
>
>
> However... for some reason, I can't seem to get this working.  Maybe
> someone else can spot my error?
>

I think the iterator needs to return a tuple, rather than a "bare" int.  At
any rate, if you change "return self._myval" to "return (self._myval,)",
things work perfectly.


>
> Ben Root
>
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>
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