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MathGroup Archive 2005

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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[1]:=
$Version

Out[1]=
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[2]:=
Map[f, a^b]

Out[2]=
     f[b]
f[a]

In[3]:=
f /@ a^b

Out[3]=
  b
a

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

In[4]:=
MapAll[f, a^b]

Out[4]=
       f[b]
f[f[a]    ]

In[5]:=
f //@ a^b

Out[5]=
     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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