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MathGroup Archive 2000

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Re: InterpolatingFunction in NDSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21647] Re: InterpolatingFunction in NDSolve
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Fri, 21 Jan 2000 03:59:56 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <86196r$la7@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

the code
Clear[sss];
Clear[solution];
Clear[sol2];

solution = 
  NDSolve[{Derivative[1][y][x] == -10*y[x], y[0] == 1}, y, {x, 0, 0.5}]


sss = (y[x] /. solution[[1]])[[0]];
(*interpolating function seems to be ok : can Plot it*)

Plot[sss[t], {t, 0, 0.5}];


Derivs[y_, x_?NumericQ] := If[x < 0.25, sss[x], -sss[x]]

sol2 = NDSolve[{Derivative[1][y][x] == Derivs[y, x], y[0] == 0}, 
    y, {x, 0, 0.5}]
Plot[Evaluate[y[x] /. sol2], {x, 0, 0.5}];

works fine. What is your problem ?  Look at my Derivs[] function
and never test a unrestricted pattern against a number.

Hope that helps
  Jens


eborzova at my-deja.com wrote:
> 
> Hi everybody.
> My question could be simple, but I just can't
> figure it out. I need to use InterpolatingFunction
> inside yet another function (Derivs) that is rhs
> for NDSolve (please refer to the code below). The
> problem is that if my rhs "Derivs" has some sort
> of conditional logic (e.g. "If"), then it stops
> working (see a comment with Return[sss[x]]).
> 
> Any help/hints/feedback are greatly appreciated!
> 
>           Thanks.
>                   Elena.
> Clear[sss];
> Clear[solution];
> Clear[sol2];
> solution = NDSolve[{Derivative[1][y][x]==-10*y
> [x], y[0] == 1},
>                     y,{x, 0, 0.5}]
> 
> sss = (y[x] /. solution[[1]])[[0]];
> 
> (* interpolating function seems to be ok: can
> Plot it *)
> Plot[sss[t], {t, 0, 0.5}];
> 
> Derivs[y_, x_] := Block[{},
> 
>      (* uncomment below to make it work *)
> 
>      (* Return[sss[x]]; *)
> 
>      If[x < 0.25, Return[sss[x]], Return[-sss
> [x]]];
> 
>      Print["Ooops!!!!"];
> 
>      Abort[];
> ];
> 
> sol2 = NDSolve[{Derivative[1][y][x] == Derivs[y,
> x], y[0] == 0},
>                 y,{x, 0, 0.5}]
> 
> Plot[Evaluate[y[x] /. sol2], {x, 0, 0.5}];
> 
> Sent via Deja.com http://www.deja.com/
> Before you buy.


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