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Maxima - plot2d: expression evaluates to non-numeric value everywhere in plotting range

I mean to plot a function of one variable, which uses a paramter that is not included as an argument to the function. Now I want to plot2d the function, so I mean to assign a temporary value to the parameter for plotting. I.e.:

  1. Assign parameter value
  2. Plot2d
  3. Forget parameter value

What I did is

f(u):=a*u;
a=1;plot2d(f(u),[u,0.2,1]);

but I get the error

plot2d: expression evaluates to non-numeric value everywhere in plotting range.
plot2d: nothing to plot.

How would I proceed?

PS: I actually mean to work with functions using many parameters, so if excluding them as arguments is workable, I'd rather do it for convenience. Up to now, I have been integrateing, differentiating, factoring, etc., with no problems.

Solution

  • Self-answering: It was probably a silly question (I am a newbie), but just in case it helps others...

    f(u):=a*u;

a:1;
plot2d(f(u),[u,0.2,1]);
kill(a);

    To avoid assigning and clearing:

    params : [a=1];
fnew : subst(params,f(u));
plot2d(fnew,[u,0.2,1]);

    Other options are posted in the comment by Robert Dodier.