Re: Problems with Set, SetDelayed and replacement rules...

*To*: mathgroup at smc.vnet.net*Subject*: [mg72294] Re: Problems with Set, SetDelayed and replacement rules...*From*: dh <dh at metrohm.ch>*Date*: Tue, 19 Dec 2006 06:35:02 -0500 (EST)*References*: <em3a6c$7fq$1@smc.vnet.net>

Hi Johannes, there is nothing wrong with your treatment of parameters, but you have to pay attentio to function arguments. Consider: f[p_]:=Print[p]. If you set p to some value: p=1, what would you expect to be printed by: f[2]. Well, after you got this, we can fix f3. We have to explicitely bring "p" into the definition, exactly what f1=.. and f2:=.. do. You could e.g. use Evaluate: f3[x_, p_] := Evaluate[f[x] /. params] Daniel Johannes wrote: > Hi, > > I am working a lot with physics equations where I have on the one hand variables, and on the other hand paramters, which depend on other parameters which are only given by a polynome. > In the following (very simplified example) f is a function which depends on variable x and parameters g and h which depend both on parameter p: > > f[x_]:= g/h*x; > g[p_]= 1+p; > h[p_]=1+p^2; > params={g->g[p],h->h[p]}; > > For numerical application, I would like to define a second function f with p as second variable. This I have done with different methods, and there I noticed a difference that I can't explain > > f1[x_, p_] = f[x] /. params; > f2[x_, p_] := f[x] /. {g->g[p],h->h[p]}; > f3[x_, p_] := f[x] /. params; > > If I am now calculating f1[1,1], f2[1,1] and f3[1,1], f1 and f2 deliver as expected a numerical value, but for f3[1,1] p isn't replaced by its value 1. > > Can anybody explain me the differences, especially the differences between f2 and f3? I suspected it to be exactly identical. > > And does anybody know a better method to treat parameters? My problem is that when doing analytical transformations (Derivatives, Integrations...), an immediate replacement of the parameters g and h would make the result very hard to read, so I was searching for an easy method to replace them as late as possible. > > Thanks in advance for any help, > Johannes >