       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:= a /: N[a, pa_] := N[2, pa]
In:= N [{x^a, x^(Pi*a),E^(x + a*Pi)},3]
Out= {x^2.00, x^6.28, 2.72^(6.28 + x)}

In:= N[x^a, 32]
Out= x^2.`32.

In:= (#1[Last[%]] & ) /@ {Accuracy, Precision}
Out= {31.69897000433602, 32.}

In:= Solve[N[x^a == 1], x]
Out= {{x -> -1.}, {x -> 1.}}

Most likely some of you finds issues, which have been overlooked by me :-\

Peter

```

• Prev by Date: Re: Export[] to XLS
• Next by Date: Re: Confusing results with N[expr]?
• Previous by thread: Re: Re: Confusing results with N[expr]?
• Next by thread: Re: Confusing results with N[expr]?