Precision and CD's

David Carlisle davidc at
Tue Jul 20 20:09:16 CEST 1999

> Every bigfloat has a precision attribute.  If it is not
> obvious from its face what its precision is, then it should
> be noted explicitly.  

hmm something not explicit in James' proposed construct in the CD.
It could be added though.

> 1. a signed integer F representing the fraction.
> 2. the radix R.  (sometimes understood)
> 3. the precision P.  (generally P = log of F in radix R, rounded up to integer)
> 4. a signed integer E representing the exponent.

so currently we miss 3. (ie your `generally' becomes an implied rule) I
take it that you say this is BAD?

>  (the radix always comes out "10")
well actually it might not, but we'll pass on that for now:-)

> Let me repeat:  if the precision of a bigfloat in some representation
> is not apparent, then THAT IS NOT ACCEPTABLE.

James, what about adding a fourth child to bigfloat constructor?

Actually I think having this clarifies the situation (apart from making
Richard happy). As originally expressed the `floats are exact rationals'
line of argument originally begged the response of `why not just use
rationals' (which we already have).
However this seems perfectly clear:

>  Why not just use the rationals?  The essence of the floats is that
>  the precision  (= length of fraction) is bounded.


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