Your plot syntax is wrong. You cannot define a variable (f) inside your plot command. Use the documentation to identify a valid syntax.
The variable XR_1 is not defined in your code, either.
I see you have defined pre as a variable, but are passing it in as a character array to the plot command in what should be the linespec input.
Since you are only plotting a single point, you will want to include a marker style in your linespec. Perhaps try replacing your plot command with this.
plot(c, f(c),'ro')
