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

Travis Oliphant oliphant at ee.byu.edu
Tue Sep 19 19:17:47 CDT 2006


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.

>(don't know how to say this for complex types !? Are here real and imag 
>treated separately / independently ?)
>  
>

There is a complex add performed at a low-level as two separate adds.  
The addition is performed in the precision requested.

-Travis





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