[Numpy-discussion] please change mean to use dtype=float

Charles R Harris charlesr.harris at gmail.com
Tue Sep 19 19:37:01 CDT 2006


On 9/19/06, Travis Oliphant <oliphant at ee.byu.edu> wrote:
>
> Sebastian Haase wrote:
>
> >On Tuesday 19 September 2006 15:48, Travis Oliphant wrote:
> >
> >
> >>Sebastian Haase wrote:
> >>
> >>
> ><snip>
> >
> >
> >>>can we please change dtype to default to float64 !?
> >>>
> >>>
> >>The default is float64 now (as long as you are not using
> >>numpy.oldnumeric).
> >>
> >>I suppose more appropriately, we could reduce over float for integer
> >>data-types when calculating the mean as well (since a floating point is
> >>returned anyway).
> >>
> >>
> >>
> >
> >Is now mean() always "reducing over" float64 ?
> >The svn note """Log:
> >Fix mean, std, and var methods so that they reduce over double data-type
> with
> >integer inputs.
> >"""
> >makes it sound that a float32 input is stays float32 ?
> >
> >
> Yes, that is true.  Only integer inputs are changed because you are
> going to get a floating point output anyway.
>
> >For mean calculation this might introduce large errors - I usually would
> >require double-precision for *any*  input type ...
> >
> >
> Of course.  The system is not fool-proof.  I hesitate to arbitrarily
> change this.  The advantage of using single-precision calculation is
> that it is faster.  We do rely on the user who expressly requests these
> things to be aware of the difficulties.


Speed depends on the achitecture. Float is a trifle slower than double on my
Athlon64, but faster on PPC750. I don't know about other machines. I think
there is a good argument for accumlating in double and converting to float
for output if required.

Chuck
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