Re: [SOSlib-devel] wrong AST_FUNCTION_ABS implementation

[Rainer M. <ra...@tb...>]

On Tue, 13 Jun 2006, Stefan Mueller wrote:

> hi rainer!
>
> you are right. currently the implementaion is wrong.
> it should be as it's written in the comment.
> f(x)=abs(a(x)) => f' = sig(a)*a'

sig(a) is not an ASTNode type, so I guess we need to construct a piecewise 
function:

if a < 0: -1
if a > 0: +1

What if a = 0?


The next problem then is, that our piecewise implementation is also not 
generally valid.

   case AST_FUNCTION_PIECEWISE:
     if ( evaluateAST(child(n,0),data) )
       result = evaluateAST(child(n,1),data);
     else
       result = evaluateAST(child(n,2),data);

Which is only valid for piecewise/otherwise combinations, but not for more 
then one `piece'.

See e.g
http://www.zvon.org/xxl/MathML/Output/el_piecewise.html
and click example repository for two examples.

Rainer



>
> don't worry about the discontinuity at x=0! :)
>
> cheers, s
>
>
> Am Freitag 09 Juni 2006 17:46 schrieb Rainer Machne:
>> Hi Stefan
>>
>> I am currently going through the code, implementing the style convention
>> and getting familiar with SOSlib again :-)
>>
>> in differentiateAST we have the lines
>>
>>      case AST_FUNCTION_ABS:             /** ABS: WRONG */
>>        /* f(x)=abs(a(x)) => f' = sig(a)*a'
>>  	  WRONG: CAN RESULT IN A DISCONTINUOUS FUNCTION! */
>>        ASTNode_setType(fprime, ASTNode_getType(f)); /* WRONG !!! */
>>        ASTNode_addChild(fprime, differentiateAST(ASTNode_getChild(f,0),x));
>>        break;
>>
>> So it's actually not doing what it claims in the comment, and also there
>> are these uppercase WRONG warnings.
>>
>> What should we do about that. Should we implement it as written in the
>> comment?
>>
>> Rainer
>>
>>
>>
>>
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>