3rd

src/main.rs

@@ -1,3 +1,49 @@
+use std::fmt;
+
+// use the automated debug as below
+//#[derive(fmt::Debug)]
+struct Vector2D {
+    x: isize,
+    y: isize,
+}
+
+// Or write your own
+impl fmt::Debug for Vector2D {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        // The `f` value implements the `Write` trait, which is what the
+        // write! macro is expecting. Note that this formatting ignores the
+        // various flags provided to format strings.
+        write!(f, "My Vector2D debug({}, {})", self.x, self.y)
+    }
+}
+
+// display cannot be automatically generated
+impl fmt::Display for Vector2D {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        // The `f` value implements the `Write` trait, which is what the
+        // write! macro is expecting. Note that this formatting ignores the
+        // various flags provided to format strings.
+        write!(f, "({}, {})", self.x, self.y)
+    }
+}
+
+// Different traits allow different forms of output of a type. The meaning
+// of this format is to print the magnitude of a vector.
+impl fmt::Binary for Vector2D {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let magnitude: f64 = (self.x * self.x + self.y * self.y) as f64;
+        let magnitude = magnitude.sqrt();
+
+        // Respect the formatting flags by using the helper method
+        // `pad_integral` on the Formatter object. See the method
+        // documentation for details, and the function `pad` can be used
+        // to pad strings.
+        let decimals = f.precision().unwrap_or(3);
+        let string = format!("{:.*}", decimals, magnitude);
+        f.pad_integral(true, "", &string)
+    }
+}
+
 fn main() {
     // In general, the `{}` will be automatically replaced with any
     // arguments. These will be stringified.
@@ -46,4 +92,10 @@ fn main() {
     // handling. This will not work.
     //println!("This struct `{}` won't print...", Structure(3));
     // FIXME ^ Comment out this line.
+
+    let myvector = Vector2D { x: 3, y: 4 };
+
+    println!("{}", myvector); // => "(3, 4)"
+    println!("{:?}", myvector); // => "Vector2D {x: 3, y:4}"
+    println!("{:10.3b}", myvector); // => "     5.000"
 }