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

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bastardo
bastardo 2024-11-28
编辑： Stephen23 2024-11-28
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?

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Stephen23
Stephen23 2024-11-28
编辑：Stephen23 2024-11-28
Ran in:
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

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