Re: [Maxima-discuss] draw2d parametric fails on a function defined by cases

[Robert D. <rob...@gm...>]

On Wed, Oct 9, 2024 at 2:33 PM Eduardo Ochs <edu...@gm...> wrote:

>   P(t) := if t <= 6 then [t,4] else [6,10-t];
>   draw2d(explicit(P(t)[1], t,0,10),
>          explicit(P(t)[2], t,0,10));
>   draw2d(parametric(P(t)[1], P(t)[2], t,0,10));
>
> The first three "draw2d"s work perfectly, but the last one
> yields this error:
>
>   draw2d (parametric): non defined variable

This is due to a bug in the way that parametric verifies that the
first two arguments are functions of the parameter t alone.
`listofvars` is called, but listofvars(foo(x)[1]) returns [foo(x)[1]],
not [x] as one might hope.

I guess this could be interpreted as a bug in listofvars. It is
supposed to return [x[1]] for listofvars(x[1] + 2), for example, but
maybe if the expression looks like listofvars(foo(x)[1] + 2) it should
return [x].

As a workaround, one could say something like

P1(t) := if t <= 6 then t else 6;
P2(t) := if t <= 6 then 4 else 10 - t;
draw2d (parametric (P1(t), P2(t), t, 0, 10));

I know that's less elegant in a way.

Robert