Numerical vs Symbolic -- What to do when plots are blank (and other strange errors)
The plotting functions require numerical inputs. This sounds obvious but most people have trouble debugging plots that come out empty. When something does not plot, the first thing to do is to evaluate the expression and look for undefined variables or typos.
Consider the following:
A = 12;
b = 3;
expr = a* b sin[theta];
Plot[expr, {theta, 0, 10}]
which yields a less than informative plot:
The best approach is to take the plot argument and evaluate it at a point somewhere in the plot range and see that it is numerical. It is often a good idea to evaluate it at the two extremes and the middle to make sure it is numerical everywhere.
expr /. theta -> 0
expr /. theta -> 5
expr /. theta -> 10
to get
(* 3 a sin[0]
3 a sin[5]
3 a sin[10] *)
It becomes clear that the a is undefined and that sin does not evaluate (because of the lower case 's')
