Re: Unexpected Behavior: SetDelayed versus Set

*To*: mathgroup at smc.vnet.net*Subject*: [mg120324] Re: Unexpected Behavior: SetDelayed versus Set*From*: Leonid Shifrin <lshifr at gmail.com>*Date*: Tue, 19 Jul 2011 06:55:10 -0400 (EDT)*References*: <201107181013.GAA26867@smc.vnet.net>

Brian, The short answer is that, when you define a function using SetDelayed, as in f[args]:=rhs, Then having your r.h.s reference global symbols (other than formal parameters) is generally an invitation for trouble (unless you know precisely what you are doing, such as when you create closures with encapsulated mutable state, for example). It is a pity that we don't have FAQ, since this has been discussed many times before. Here are a few links I am aware of, discussing and explaining this problem: http://groups.google.com/group/comp.soft-sys.math.mathematica/browse_thread/thread/59193a89ecb1afba http://forums.wolfram.com/mathgroup/archive/2010/May/msg00102.html http://groups.google.com/group/comp.soft-sys.math.mathematica/browse_thread/thread/63ab8c382dcdfed5 (see David Park's post in the above thread) http://stackoverflow.com/questions/6046626/a-problem-in-mathematica-8-with-function-declaration/ http://stackoverflow.com/questions/6236458/plot-using-with-versus-plot-using-block-mathematica/ I am sure that there are many more discussions on this topic, but even those links above will give you a rather good and comprehensive picture. Regards, Leonid On Mon, Jul 18, 2011 at 2:13 PM, blamm64 <blamm64 at charter.net> wrote: > Group, > > Please consider the following: > > gc = 3217*(10^-2)*12; > orificeCD=65*(10^-2); > kTorificeL=1/(orificeCD^2); > reyTorifice=12; > cLohm=(117*(10^-4))*1714*(385*(10^-2)); > nuStd=19*155*(10^-5); rhoStd=3014*(10^-5); > > aLinQS[area_]:=(kTorificeL*reyTorifice*nuStd*rhoStd)/ > (2*area*2*Sqrt[area/Pi]*gc); > > bQuadQS[area_]:=(kTorificeL*rhoStd)/(2*gc*area^2); > > Clear[taSD,taS,area,l,q]; > > taSD[l_,q_] := area/.Solve[((l*q)/cLohm)^2==q*(aLinQS[area] > +bQuadQS[area]*q),area]; > > taS[l_,q_] = area/.Solve[((l*q)/cLohm)^2==q*(aLinQS[area] > +bQuadQS[area]*q),area]; > > {Dimensions[taSD[l,q]],Dimensions[taS[l,q]]} > {{4},{4}} > > lm=485;qm=1925*(10^-3); > > N[taSD[lm,qm]]//InputForm > > {0.0015350502096417261} > > N[taS[lm,qm]]//InputForm > > {-0.0015301809337073798 - 4.86928826122106*^-6*I, > -0.0015301809337073798 + 4.86928826122106*^-6*I, > 0.0015253116577730662, 0.0015350502096416934} > > Now, this is really unexpected to me, especially in light of the of > the Dimensions result where Symbols are substituted in the rhs of both > definitions. > > For some unknown to me reason the SetDelayed definition returns the > last member of the solution set when the rhs of the SetDelayed > definition has numerial values for <l> and <q>. > > Obviously, again, I am missing something; but this time whatever it is > I am missing has me gasping for air. > > -Brian L. > >

**References**:**Unexpected Behavior: SetDelayed versus Set***From:*blamm64 <blamm64@charter.net>