Re: Patch for erfinv.m

[Alois Schloegl]

John W. Eaton wrote:
> On 21-Jan-2008, Alois Schloegl wrote:
>
> | >> Consider the following patch for erfinv.
> | >>
> | >> It (1) replaces z_old and z_new by a single variable z, and
> | >> (2) makes the initial checks simpler.
> | >>
> | >> The code becomes leaner, and a bit faster.
>
> I applied the patch but used
>
>   ## x < 1 or x > 1 ==> NaN
>   y(abs (x) >= 1) = NaN;
>
> instead of
>
> | +  y(~(abs(x) < 1)) = NaN;  %% x<1, x>1, x=NaN
>
> Any reason not to make this change?
>   

In the latter case, x=NaN is not properly handled.

The comment line should be

- ## x < 1 or x > 1 ==> NaN
+ ## x < 1 or x > 1 or isnan(x) ==> NaN

Either use the version of the patch, or add an extra check using isnan.
The solution in the patch is simpler, and perhaps faster, too.

Checking it again, I see that an even better solution is

- y(~(abs(x) < 1)) = NaN;  
+ y(~(abs(x) <= 1)) = NaN;  

because no NaN should be assigned in case of abs(x)==1. 

Alois

> Thanks,
>
> jwe
>