better handling of setting func_str to an empty string

src/egui_app.rs

@@ -279,7 +279,7 @@ impl MathApp {
                             );
                         }
                     } else {
-                        function.func_str = String::new();
+                        function.empty_func_str();
                     }
                 }
 
@@ -349,7 +349,7 @@ impl epi::App for MathApp {
                             None, // Doesn't matter, updated later
                             Some(self.settings.sum),
                         );
-                        function.func_str = String::new();
+                        function.empty_func_str();
                         function
                     });
                     self.func_strs.push(String::new());

src/function.rs

@@ -23,7 +23,7 @@ impl fmt::Display for RiemannSum {
 
 pub struct Function {
     function: Box<dyn Fn(f64) -> f64>,
-    pub(crate) func_str: String,
+    func_str: String,
     min_x: f64,
     max_x: f64,
     pixel_width: usize,
@@ -248,4 +248,6 @@ impl Function {
         let area: f64 = data2.iter().map(|(_, y)| y * step).sum(); // sum of all rectangles' areas
         (data2, area)
     }
+
+    pub fn empty_func_str(&mut self) { self.func_str = String::new(); }
 }