Re: NDSolve and numeric-only computation

*To*: mathgroup at smc.vnet.net*Subject*: [mg21875] Re: NDSolve and numeric-only computation*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Wed, 2 Feb 2000 22:54:31 -0500 (EST)*Organization*: Universitaet Leipzig*References*: <872j7n$cie@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi Carl, first of all. I think that your compilation by hand is done my NDSolve[]. The problem is that you can't give a compiled function a Pattern[] for the argument. But you may define RHS = Compile[{{x, _Real}}, x^2] fun[x_?NumericQ] := RHS[x] and NDSolve[] run without a prolem NDSolve[{x'[t] == fun[x[t]], x[0] == 0}, x[t], {t, 0, 1}] Hope that helps Jens Carl Bumiller wrote: > > I have a question about NDSolve and numeric-only computation. > > First I have defined RHS=Compile[{{x,_Real}},........] where ........ > is some complicated expression, then I ran > NDSolve[{x'[t]==RHS[x[t]],x[0]==1},{x},{t,0,1}] > and got the error message > > CompiledFunction::cfr: > Cannot use compiled code; Argument x[t] at position 1 > should be a machine-size real number. > > This suggests that NDSolve is trying to solve symbolically rather than > numerically. How do I get NDSolve to evaluate RHS numerically rather > than symbolically?? When I try Compiled->True or Compiled->False > with NDSolve, I get the same error message. Since RHS is complicated, > its symbolic evaluation is time comsuming although the equation is > eventually solved. I am running Mathematica 3.0 for Digital Unix. > > Carl Bumiller > bumi314 at aol.com