cargo new calculator
cd calculator

2. * `Cargo.toml`.

```toml
[dependencies]
gtk = "0.9.0"
regl = "0.2.1"
glib = "0.9.0"

3. **

   use gtk::prelude::*;
   use gtk::{Button, Entry, Grid, Label, Window, WindowType};
   use std::ops::Add;

   struct Calculator {
   entry: Entry,
   operator: Option,
   first_operand: Option,
   }

   impl Calculator {
   fn new() -> Self {
   let entry = Entry::new();
   entry.set_editable(false);

       Self {
           entry,
           operator: None,
           first_operand: None,
       }
   }
   
   fn add_button(&self, grid: &Grid, label: &str, row: usize, col: usize) {
       let button = Button::with_label(label);
       let entry = self.entry.clone();
       
       button.connect_clicked(move |_| {
           entry.set_text(&label);
       });
   
       grid.attach(&button, col as i32, row as i32, 1, 1);
   }
   
   fn add_operations(&mut self, grid: &Grid) {
       let calc_op = |operator, entry: &Entry, first_operand: &mut Option<f64>| {
           let current_text = entry.get_text().unwrap();
           if let Ok(value) = current_text.parse::<f64>() {
               *first_operand = Some(value);
               entry.set_text(&operator);
           }
       };
   
       let operator_buttons = vec![("+", "add"), ("-", "sub"), ("*", "mul"), ("/", "div")];
   
       for (index, (label, op)) in operator_buttons.iter().enumerate() {
           let entry = self.entry.clone();
           let first_operand = &mut self.first_operand;
           
           let button = Button::with_label(label);
           button.connect_clicked(move |_| {
               calc_op(op, &entry, first_operand);
           });
   
           grid.attach(&button, index as i32, 5, 1, 1);
       }
   }
   

   }

   fn main() {
   gtk::init().expect("Failed to initialize GTK.");

   let window = Window::new(WindowType::Toplevel);
   window.set_title("Calculator");
   window.set_default_size(200, 200);
   
   let grid = Grid::new();
   let calculator = Calculator::new();
   
   grid.attach(&calculator.entry, 0, 0, 4, 1);
   
   let buttons = [
       "7", "8", "9",
       "4", "5", "6",
       "1", "2", "3",
       "0", "+", "=",
   ];
   
   for (index, label) in buttons.iter().enumerate() {
       calculator.add_button(&grid, label, (index / 3) + 1, index % 3);
   }
   
   window.add(&grid);
   window.connect_delete_event(|_, _| {
       gtk::main_quit();
       Inhibit(false)
   });
   
   window.show_all();
   gtk::main();
   

   }

4. **

   cargo run

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