Re: Confusing results with N[expr]?

*To*: mathgroup at smc.vnet.net*Subject*: [mg62364] Re: Confusing results with N[expr]?*From*: Peter Pein <petsie at dordos.net>*Date*: Tue, 22 Nov 2005 04:42:05 -0500 (EST)*References*: <dlp320$1bs$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Thomas Bohl schrieb: > I have the impression that since some time (since Vers. 5.0?) N[expr] does > not give anymore the results I expect. > > The following code should illustrate this: > > N[a] = 2.; > > k = z*(a*Pi); > N[k] >> 6.28319 z (* "a" and "Pi" are replaced by their numerical value as >> expected *) > > k = z^(a*Pi) > N[k] >> z^(3.14159 a) (* "Pi" is replaced by its numerical value but not "a", >> why not? *) > > > There are more examples of this behaviour: If the expression is Log[z*a*Pi], > the numerical values of "a" and "Pi" are evaluated, if the expression is > Exp[z*a*Pi] the numerical value of "a" is not evaluated. Why not? > > The motivation behind my question is that I was used to write expressions > symbolically and assign numerical values with N[expr] = num. value. This way > you could keep those expressions symbolically and just apply //N when you > needed numerical values. Now it seems that I have lost this possibility. > > Could you please comment my observation and maybe suggest a way out? > > Thank you very much for any idea. > > Kind regards, > Thomas. > > Hi, I think, I've got a workaround, which does not need changes in every occurance of N[f[a]]: In[1]:= a /: N[a, pa_] := N[2, pa] In[2]:= N [{x^a, x^(Pi*a),E^(x + a*Pi)},3] Out[2]= {x^2.00, x^6.28, 2.72^(6.28 + x)} In[3]:= N[x^a, 32] Out[3]= x^2.`32. In[4]:= (#1[Last[%]] & ) /@ {Accuracy, Precision} Out[4]= {31.69897000433602, 32.} In[5]:= Solve[N[x^a == 1], x] Out[5]= {{x -> -1.}, {x -> 1.}} Most likely some of you finds issues, which have been overlooked by me :-\ Peter