Re: Defining N for a new entity

*To*: mathgroup at smc.vnet.net*Subject*: [mg66794] Re: Defining N for a new entity*From*: Peter Pein <petsie at dordos.net>*Date*: Tue, 30 May 2006 05:49:10 -0400 (EDT)*References*: <200605280103.VAA23345@smc.vnet.net> <e5bsuq$afo$1@smc.vnet.net> <e5ei6d$800$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Giuseppe Bilotta schrieb: > On Sun, 28 May 2006 10:10:34 +0000 (UTC), Carl K. Woll wrote: > >> Giuseppe Bilotta wrote: >>> So I was looking for a way to tell Mathematica that applying N to an >>> AffineExpression should only actually apply it to c and xi, something >>> like >>> >>> N[AffineExpression[c_, dev_]] := >>> AffineExpression[N[c], MapAt[N,#,{2}]&/@ dev] >>> >>> but if I actually do this and then call >>> >>> AffineExpression[1, {{1, 1}}] >>> N[%] >>> >>> the Mathematica kernel (5.2.0.0) dies without any message. >>> >>> Does anybody have an idea of what could the reason be? And what could >>> I do as a workaround? >> One idea is to use the attribute NHoldRest (or if you change your data >> structure, NHoldFirst). For example: >> >> SetAttributes[AffineExpression, NHoldRest] >> >> In[42]:= N[AffineExpression[.5, {{1, .2}, {2, .1}}]] >> >> Out[42]= AffineExpression[0.5, {{1, 0.2}, {2, 0.1}}] > > Thank you very much, this does the trick. It has the rather small > side-effect of preventi N from working on the coefficients > (N at AffineExpression[.5,{{1,Pi/6}}] will not numerize Pi/6), but it's > good enough for my application. > > Do you have any ideas on why the Mathematica kernel dies without a > message? > Salute Giuseppe, just guessing... N[AffineExpression[c_,dev_]]:=... might evaluate the left hand side before applying SetDelayed. This is an explanation, why Mathematica doesn't complain about N being protected. But N[AffineExpression[c_,dev_]] evaluates to AffineExpression[c_,dev_], leaving you with a hidden recursion: N[f[x_]]:=f[N[x]] N[f[13]] (* choose your favourite unlucky number here *) reproduces the crash. If you don't want the NHoldRest-solution, you might try: In[1]:= a1=AffineExpression[1,{{1,1},{2,Pi}}]; AffineExpression /: HoldPattern[N][AffineExpression[c_, dev_]] := AffineExpression[N[c], MapAt[N, #, 2]& /@ dev] N[a1] --> AffineExpression[1.,{{1,1.},{2,3.14159}}] Ciao, Peter

**References**:**Defining N for a new entity***From:*Giuseppe Bilotta <bilotta78@hotpop.com>