       Re: Rule to Function, Frontend funnies

• To: mathgroup at smc.vnet.net
• Subject: [mg5335] Re: [mg5307] Rule to Function, Frontend funnies
• From: jpk at apex.mpe.FTA-Berlin.de (Jens-Peer Kuska)
• Date: Wed, 27 Nov 1996 01:47:50 -0500
• Sender: owner-wri-mathgroup at wolfram.com

```> how can I convert a rule to a function?
> if I get a result from
>
> rulesoln = DSolve[{y'[x]==y[x]/(c+1/x),y==1},y[x],x]
>
> in the form :
>
>                                2
>            x/c - Log[1 + c x]/c
> {{y[x] -> E                     }}
>
> how do I get the definition :
>
> y[x_]:= E^(x/c - Log[1 + c x]/c^2)
>
> Anybody out there to whom this happens too?
> After some {soul,FullForm}-searching, I came up with this rule-eating-rule :
>
> Flatten[rulesoln]/.y[x]->y[x_]/.Rule->SetDelayed
>
> {Null}
>
> you don't get to see much except "{Null}", but it results in:
>
> ?y
>
> Global`y
> y[x_] := E^(x/c - Log[1 + c*x]/c^2)
>
> (*************************************************************************)
>
> Is there a fast & easy Cut-and-Paste way to do it, not leaving the FrontEnd?
> Is this the same on a Mac-machine?
> Is this the same in Mma 3.0 for Windows?
>
What do You want? The SetDelayed is evaluated and the result of it is
Null. The best is to make a definition

thisSolutionY[x_]:=y[x] /. Flatten[rulesoln]

How ever if You don't want to evaluate Your delayed rule You can do the
following
In:= Clear[y]

In:= rulesoln=DSolve[{y'[x]==y[x]/(c+1/x),y==1},y[x],x] //InputForm

Out=
{{y[x] -> E^(x/c)/(1 + c*x)^c^(-2)}}

In:= sol=rulesoln[[1,1,1]] //InputForm
Out//InputForm= y[x] ->  E^(x/c)/(1 + c*x)^c^(-2)

In:= Hold[Evaluate[sol] ]/. {y[x] :> HoldPattern[y[x_]],Rule->SetDelayed}
Out= Hold[HoldPattern[y[x_]] := E^(x/c)/(1 + c*x)^c^(-2)]

now You see the definition. A simple

In:=ReleaseHold[%]

will evaluate this. With Mma 2.2 You must use Literal instead of HoldPattern.
Somewehre in Your 2.2 Frontend is the command - Copy Output from above.

Hope that helps
Jens

```

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