fix some typos

solutions/formatted-output/debug-display.md

@@ -42,7 +42,7 @@ fn main() {
     // The problem with `derive` is there is no control over how
     // the results look. What if I want this to just show a `7`?
 
-    /* Make it output: Now 7 will print! */
+    /* Make it print: Now 7 will print! */
     println!("Now {:?} will print!", Deep(Structure(7)));
 }
 ```
@@ -69,10 +69,46 @@ impl fmt::Debug for Point2D {
 }
 
 fn main() {
-
     let point = Point2D { x: 3.3, y: 7.2 };
-    println!("{}", point);
 
-    println!("{:?}", point);
+    assert_eq!(format!("{}",point), "Display: 3.3 + 7.2i");
+    assert_eq!(format!("{:?}",point), "Debug: Complex { real: 3.3, imag: 7.2 }");
+    
+    println!("Success!")
+}
+```
+
+5.
+```rust
+use std::fmt; // Import the `fmt` module.
+
+// Define a structure named `List` containing a `Vec`.
+struct List(Vec<i32>);
+
+impl fmt::Display for List {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        // Extract the value using tuple indexing,
+        // and create a reference to `vec`.
+        let vec = &self.0;
+
+        write!(f, "[")?;
+
+        // Iterate over `v` in `vec` while enumerating the iteration
+        // count in `count`.
+        for (count, v) in vec.iter().enumerate() {
+            // For every element except the first, add a comma.
+            // Use the ? operator to return on errors.
+            if count != 0 { write!(f, ", ")?; }
+            write!(f, "{}: {}",count, v)?;
+        }
+
+        // Close the opened bracket and return a fmt::Result value.
+        write!(f, "]")
+    }
+}
+
+fn main() {
+    let v = List(vec![1, 2, 3]);
+    assert_eq!(format!("{}",v), "[0: 1, 1: 2, 2: 3]");
 }
 ```