Command findobj('T​ype','Func​tionLine') produces error if used after fplot invoked with three arguments, fplot(X,Y,LIMITS) - 0×0 empty GraphicsPlaceholder array

I am preparing a short test in MATLAB Grader, but I am struggling with one particular feature. Namely, I can't seem to assess the validity of a symbolic function plot, using the commands assert, fplot(X,Y,LIMITS) and findobj.

Here is an example code derived from MathWorks samples. If I run

clear variables
syms x y real
figure;
f(x) = sin(x)/x;
fplot(x,f,[-4*pi,4*pi])
legend show
lineobj = findobj('Type','FunctionLine')

I get error message lineobj =

0×0 empty GraphicsPlaceholder array.

If one changes the last line with the following (losing X argument)

fplot(f,[-4*pi,4*pi])

then it works.

Is there a way to introduce flexibility so that the student can use both variants of the fplot?

Answer 1

[Stephen23]

A curve defined by a function of X and a function of Y is a parametric curve, so you would need to search for the corresponding object type 'ParameterizedFunctionLine' :

syms x y real
f(x) = sin(x)/x;
fplot(x,f,[-4*pi,4*pi])

lineobj = findobj('Type','ParameterizedFunctionLine')

lineobj = 
  ParameterizedFunctionLine with properties:

    XFunction: x
    YFunction: [1x1 symfun]
       TRange: [-12.5664 12.5664]
        Color: [0 0.4470 0.7410]
    LineStyle: '-'
    LineWidth: 0.5000

  Use GET to show all properties

You could thus search for both line objects and check which one is empty/scalar/whatever size.

Also note that FPLOT does return the line objects as outputs.

Read more about the line objects: https://www.mathworks.com/help/matlab/ref/fplot.html#bu6xntj-8