       Re: Confusing results with N[expr]?

• To: mathgroup at smc.vnet.net
• Subject: [mg62353] Re: Confusing results with N[expr]?
• From: Steven HANCOCK <Steven.Hancock at cern.ch>
• Date: Tue, 22 Nov 2005 04:41:55 -0500 (EST)
• Organization: CERN
• References: <dlp320\$1bs\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

There may be a clue in the following, equally distressing nonsense...

In:=
\$Version

Out=
5.1 for Microsoft Windows (October 25, 2004)

(* Map[f, expr] or f /@ expr applies f to each element on the first
level in expr. *)

In:=
Map[f, a^b]

Out=
f[b]
f[a]

In:=
f /@ a^b

Out=
b
a

(* MapAll[f, expr] or f //@ expr applies f to every subexpression in
expr. *)

In:=
MapAll[f, a^b]

Out=
f[b]
f[f[a]    ]

In:=
f //@ a^b

Out=
b
f[a]

Thomas Bohl wrote:

> 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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