error: isalnum: not defined for class
error: called from
fplot at line 100 column 3
Sprongfuncties_in_Octave at line 19 column 1
stepping through the code, the error occurs at the line:
fplot(M(x),[0,6]);
where M(x) is defined as a symbolic function. It appears that at least currently, fplot in Octave cannot handle a symbolic function. (i did try with a simple polynomial function to verify it wasn't something unique to the function you used.) I can't confirm whether or not this is a regression from a previous version of symbolic, but did confirm that Matlab 2019a is able to execute the same fplot command without issue.