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Re: What to return on symbolic evaluation of own heads ?



Is this what you want?

F[x_?NumericQ] := MyCharFunc[0, 1, x]

MyCharFunc[left_, right_, var_] := 
  If[var < left || var > right, 0, 1]

{F[x], F[3], F[x] /. x -> 3, F[Pi/3], F[3/Pi]}

{F[x], 0, 0, 0, 1}

Bob Hanlon

In a message dated 5/24/98 2:25:03 AM, a.keese@tu-bs.de wrote:

>I'm trying to build my own Datatype - as an example, I'll try to
>construct a characteristic function. I define a characteristic function
>like this:
>
>F[x_] := MyCharFunc[0,1,x]
>(* this is the char func of the interval [0,1] *)
>
>And I try to define an evaluation-rule like this:
>
>MyCharFunc[left_, right_, var_]:=
>  If[NumberQ[var],
>    If[var < left || var > right,
>      0,
>      1], (* else - symbolic variable *)
>    MyCharFunc[left, right, var]
>  ]
>
>Of course, this doesn't work - while I can write F[0.5] and get a
>correct result, I can't write F[x] as this results in an infinite
>recursion --- but If I want to manipulate my data-object, I somehow
>need to return the original object. E.G. I want to be able to write
>   F[x] /. x-> 3
>and have this evaluated as F[3]...
>
>What should I return in my evaluation-rule when the parameter is
>symbolic ?



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