Re: [boost] [Math/nextafter] A question of naming functions...

From: dherring_at_[hidden]
Date: 2008-04-29 15:27:56

On Tue, 29 Apr 2008, John Maddock wrote:

> Folks I'm looking for some good names for some functions I'm on the brink of
> adding to Boost.Math (all in namespace boost::math::):
>
> T nextafter(T val, T dir)
> T next_greater(T val)
> T next_less(T val)

Given the first name is fixed, this is a reasonable family of names; 
though dropping the underscore would increase consistency.  However, there 
is the ambiguity whether greater/less reference zero or infinity.

nextpositive() and nextnegative() might be clearer.  Why does this remind 
me of IEEE-754 rounding modes?

> T edit_distance(T a, T b)
>
> Returns the number of floating point representations between values a and b.
>
> So the questions are: can you think of any better names, or are these OK?
>
> And should edit_distance return a signed or absolute value?

I'm at a loss for a better name (representation_distance seems overly 
verbose but interval_size might be ok), but do have a couple questions 
about this function's behavior.

Why is the return value templated?  Shouldn't it be some fixed diff_t?  I 
tentatively agree with the other comment that a signed distance is 
preferable to unsigned.

Shouldn't this return the number of gaps, rather than the number of 
representations?  Consider the following examples

float x=1.0;
float y=2.0;
int d=edit_distance(x, y);
float z=x;
for(int i=0; i<d; i++)
{
   z=next_greater(z);
}
// Does y==z?

z=x;
for(int i=0; i<d/2; i++)
{
   z=next_greater(z);
}
// Does y==(x+z)/2, given the right rounding mode?

Thanks,
Daniel