Converting bool to float

Robert Kern robert.kern at
Wed Nov 1 23:17:27 CST 2006

Tim Hochberg wrote:
> 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.

Well, they *are* 64-bit floating point numbers. You simply can't get around 
that. That's why we now have all of the scalar types: you can get any precision 
scalars that you want as long as you are explicit about it (and explicit is 
better than implicit). The spacesaver flag was the only solution before the 
various scalar types existed. I'd like to suggest that the discussion already 
occurred some time ago and has concluded in favor of the scalar types. 
Downcasting should be explicit.

However, whether or not float32 arrays operated with Python float scalars give 
float32 or float64 arrays is tangential to my question. Does anyone actually 
think that a Python float operated with a boolean array should give a float32 
result? Must we *up*cast a boolean array to float64 to preserve the precision of 
the scalar?

Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco

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