Converting bool to float

Tim Hochberg tim.hochberg at ieee.org
Wed Nov 1 21:16:59 CST 2006


Travis Oliphant wrote:
> Robert Kern wrote:
>
>   
>> Travis Oliphant wrote:
>>
>>  
>>
>>     
>>> It looks like 1.0-x is doing the right thing.
>>>
>>> The problem is 1.0*x for matrices is going to float64.  For arrays it 
>>> returns float32 just like the 1.0-x
>>>    
>>>
>>>       
>> Why is this the right thing? Python floats are float64.
>>  
>>
>>     
> Yeah, why indeed.  Must be something with the scalar coercion code...

This is one of those things that pops up every few years. I suspect that 
the best thing to do here is to treat 1.0, and all Python floats as 
having a kind (float), but no precision. Or, equivalently treat them as 
the smallest precision floating point value. The rationale behind this 
is that otherwise float32 array will be promoted whenever they are 
multiplied by Python floating point scalars. If Python floats are 
treated as Float64 for purposes of determining output precision then 
anyone using float32 arrays is going to have to wrap all of their 
literals in float32 to prevent inadvertent upcasting to float64. This 
was the origin of the (rather clunky) numarray spacesaver flag.

It's no skin off my nose either way, since I pretty much never use 
float32, but I suspect that treating python floats equivalently to 
float64 scalars would be a mistake. At the very least it deserves a bit 
of discussion.

-tim


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