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Re: Re: Putting an If in my function

• To: mathgroup at smc.vnet.net
• Subject: [mg101108] Re: [mg101073] Re: Putting an If in my function
• From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
• Date: Wed, 24 Jun 2009 06:32:52 -0400 (EDT)
• References: <h1neun$8rk$1@smc.vnet.net> <200906231106.HAA08214@smc.vnet.net> <20090623085416.51104n3qjb2plf28@web.mail.umich.edu>

Hi Porscha,

Set (=) evaluates the right hand side immediately. At that time the truth of
the condition g==w cannot be determined as these are still unknown
parameters. You must use SetDelayed (:=) so that the condition will be
evaluated at the time the function is called with known parameters.

Cheers -- Sjoerd

BTW the '==='in the last line of my reply should have been =!=

> -----Original Message-----
> From: Porscha Louise McRobbie [mailto:pmcrobbi at umich.edu]
> Sent: 23 June 2009 14:54
> To: Sjoerd C. de Vries
> Cc: mathgroup at smc.vnet.net
> Subject: Re: [mg101073] Re: Putting an If in my function
> 
> Hi,
> 
> I'm also interested in adding a special case to a function. Using
> what's written below works fine:
> 
> myFun[a_, g_, f_, x_, d_, w_] := d + a f x /; g == w
> 
> But why doesn't the same work when the myFun is defined using "="
> instead of":="?  i.e.,
> 
> myFun[a_, g_, f_, x_, d_, w_] = (d + a f x) /; g == w
> ln[5]:=myFun[a, w, f, x, d, w]
> Out[5]=d + a f x /; w == w
> 
> Thanks,
> Porscha
> 
> 
> Quoting "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>:
> 
> > Dear lobotomy,
> >
> > "Nomen est omen", the old Romans used to say. And they were often
> > right.
> >
> > I'm not sure what you are trying to do here, with your function in an
> > unneccessary, overly complex, pure function notation. Why not write:
> >
> > myFun[a_, g_, f_, x_, d_, w_] := d (1 + (E^g - E^w)/f)^(f x) + (a (-1
> > + (1 + (E^g - E^w)/f)^(f x)) f)/( E^g - E^w)
> >
> > which is much more readable (at least in Mathematica)?
> >
> > In[31]:= Limit[myFun[a, g, f, x, d, w], g -> w]
> >
> > Out[31]= d + a f x
> >
> > You use the undefined 'n' in d + a n. I guess this must be f x...
> >
> > You can add a definition for myFun for this special case like this
> (no
> > If[ ] necessary):
> >
> > myFun[a_, g_, f_, x_, d_, w_] := d + a f x /; g == w
> >
> > In[36]:= myFun[a, w, f, x, d, w]
> >
> > Out[36]= d + a f x
> >
> > If you want to use an If in the definition this would be something
> > like:
> >
> > myFun[a_, g_, f_, x_, d_, w_] := If[g===w, d (1 + (E^g - E^w)/f)^(f=
> >  x)
> > + (a (-1 + (1 + (E^g - E^w)/f)^(f x)) f)/( E^g - E^w),d + a f x]
> >
> > Cheers -- Sjoerd
> >
> > On Jun 22, 10:21 am, Lobotomy <labb... at gmail.com> wrote:
> >> Hi, this is my function
> >>
> >> #5 (1 + ((E^#2 - 1) - (E^#6 -
> >>             1))/#3)^(#3*#4) + (#1*((1 + (((E^#2 - 1) - (E^#6 =
> > -
> >>                  1))/#3))^(#3*#4) -
> >>         1))/(((E^#2 - 1) - (E^#6 - 1))/#3) &[a, g, f, x, d, w]
> >>
> >> in the case when w==g the denominator equals zero.
> >>
> >> in this case i would like to rewrite the formula above to the much
> >> simpler
> >> d+a*n. How is this done? I've tried
> >>
> >> If[w == g, % = d + a*n], but this is not working. Another thing is
> >> where to put the "If"
> >
> >
> >
> >
> >

• References:
  • Re: Putting an If in my function
    • From: "Sjoerd C. de Vries" <sjoerd.c.devries@gmail.com>