Re: Confusing results with N[expr]?

*To*: mathgroup at smc.vnet.net*Subject*: [mg62327] Re: [mg62318] Confusing results with N[expr]?*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Sun, 20 Nov 2005 04:50:31 -0500 (EST)*Reply-to*: hanlonr at cox.net*Sender*: owner-wri-mathgroup at wolfram.com

Map N onto the expression or use a replacement rule N[a]=2.; k=z*(a*Pi); {N[k],N/@k,k/.a->2.} {6.28319 z,6.28319 z,6.28319 z} k=z^(a*Pi); {N[k],N/@k,k/.a->2.} {z^(3.141592653589793*a), z^6.283185307179586, z^6.283185307179586} Bob Hanlon > > From: "Thomas Bohl" <Thomas.Bohl at cern.ch> To: mathgroup at smc.vnet.net > Date: 2005/11/19 Sat PM 11:19:01 EST > Subject: [mg62327] [mg62318] Confusing results with N[expr]? > > 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. > > >