Re: Re: Want to use Module
- To: mathgroup at smc.vnet.net
- Subject: [mg9863] Re: Re: Want to use Module
- From: Allan Hayes <hay at haystack.demon.co.uk>
- Date: Sat, 29 Nov 1997 00:10:53 -0500
- References: <347EC033.60F2@haystack.demon.co.uk>
- Sender: owner-wri-mathgroup at wolfram.com
Re: Want to use Module
Wouter Meeussen <vdmcc at vandemoortele.be>
Wouter,
Your function foo below does not clear the variable temp when foo is
returned unevaluated, because Clear itself is not then evaluated.
y[x_]:=x Exp[x]+2
foo[x_/;((temp=N at y[x])<100)]:= Module[{it =temp},Clear[temp]; 2/(3+it)]
{foo[2],foo[20], temp,
Do[{foo[2],foo[20]},{100}]//Timing//First}//OutputForm
9
{0.101122, foo[20], 9.7033 10 , 0.316667 Second}
Here is a way round: the condition is inside Module (see Help Browser) I
have also rearranged N so that the value of foo is exact.
foo2[x_]:= Module[{temp2}, 2/(3+temp2)/;(N[temp2=y[x]]<100)]
{foo2[2],foo2[20], temp2,
Do[{foo2[2],foo2[20]},{100}]//Timing//First}//OutputForm
2
{--------, foo2[20], temp2, 0.233333 Second}
2
5 + 2 E
Allan
--
Allan Hayes
Mathematica Training and Consulting
Leicester, UK
hay at haystack.demon.co.uk
http://www.haystack.demon.co.uk
voice: +44 (0)116 271 4198
fax: +44 (0)116 271 4198
Wouter Meeussen wrote
---------------------------------------------------------------
>
> hi Ted,
>
> are you in for a "pas-de deux" ?
>
> In[1]:=y[x_]:=x Exp[x]+2
>
> In[2000987]:=foo[x_/;((temp=N at y[x])<100)]:= Module[{it
> =temp},Clear[temp]; 2/(3+it)]
>
> but maybe I "bent the rules" by using Clear[temp]?
>
> It took me some time to find out I had to convert y[x] into a number (N[
> ..]) if I wanted the condition "/;( stuff < 100)" to evaluate to
> either False or True.
>
> Maybe check with Xah's notebooks to see wether
>
> foo[x_/;(condition)]:= ...
> works faster than
> foo[x_]/;(condition):= ...
>
> A slight variation:
>
> In[2000993]:=foo[x_/;(N@(temp=y[x])<100)]:= Module[{it
> =temp},Clear[temp]; 2/(3+it)]
>
> now, temp is left symbolic, only the condition is forced to numeric.
> This allows you to evaluate stuff like :
>
> NestList[foo,1,7]
> as being different from
> NestList[foo,1.,7]
>
> have fun,
>
> wouter.
>
> At 12:07 25.11.97 -0500, Ersek_Ted%PAX1A at mr.nawcad.navy.mil wrote:
> >
> >I need to write a function like foo[x] below.
> >
> >In[1]:=
> >y[x_]:=x Exp[x]+2;
> >
> >In[2]:=
> >foo[x_]/;(temp=y[x]; temp<100):= 2/(3+temp)
> >
> >In[3]:=
> >foo[2]
> >
> >Out[3]=
> >2/(5 + 2*E^2)
> >
> >In[4]:=
> >foo[20]
> >
> >Out[4]=
> >foo[20]
> >
> >In[5]:=
> >Clear[s];
> >foo[s]
> >
> >Out[5]=
> >foo[s]
> >
> >Notice foo[x] is only defined when (y[x]<100). (y[20]<100)->False, so
> >foo[20] is returned unevaluated. (y[s]<100) is not defined, so foo[s]
> >is returned unevaluated.
> >
> >I used (temp=y[x]) so I wouldn't have to evaluate y[x] twice. How can I
> >do this with (temp) inside a Module, so it is local to (foo[x])?
> >
> > Thanks
> >
> > Ted Ersek
> > ersek_ted%pax1a at mr.nawcad.navy.mil
> >
> >
> >
>
> NV Vandemoortele Coordination Center Oils & Fats Applied Research
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