       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)
• 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.
>
>
>

```

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