Merge pull request #5 from r0e/master

[rust-101.git] / workspace / src / part05.rs

diff --git a/workspace/src/part05.rs b/workspace/src/part05.rs
index a2da18f2a435b8fe66595de2fecad8aed74ec87b..65f6f7de10fd3142c9a240ebe5d1ef0799d0e18e 100644 (file)
--- a/workspace/src/part05.rs
+++ b/workspace/src/part05.rs
@@ -4,14 +4,14 @@
 // ## Big Numbers
 
 pub struct BigInt {
-    pub data: Vec<u64>,
+    pub data: Vec<u64>, // least significant digit first, no trailing zeros
 }
 
 // Now that we fixed the data representation, we can start implementing methods on it.
 impl BigInt {
     pub fn new(x: u64) -> Self {
         if x == 0 {
-            BigInt { data: vec![] }
+            unimplemented!()
         } else {
             unimplemented!()
         }
@@ -26,12 +26,14 @@ impl BigInt {
     }
 
     // We can convert any vector of digits into a number, by removing trailing zeros. The `mut`
-    // declaration for `v` here is just like the one in `let mut ...`, it says that we will locally
-    // change the vector `v`.
+    // declaration for `v` here is just like the one in `let mut ...`: We completely own `v`, but Rust
+    // still asks us to make our intention of modifying it explicit. This `mut` is *not* part of the
+    // type of `from_vec` - the caller has to give up ownership of `v` anyway, so they don't care anymore
+    // what you do to it.
     // 
     // **Exercise 05.1**: Implement this function.
     // 
-    // *Hint*: You can use `pop()` to remove the last element of a vector.
+    // *Hint*: You can use `pop` to remove the last element of a vector.
     pub fn from_vec(mut v: Vec<u64>) -> Self {
         unimplemented!()
     }
@@ -59,7 +61,7 @@ impl<T: Clone> Clone for SomethingOrNothing<T> {
 }
 
 // **Exercise 05.2**: Write some more functions on `BigInt`. What about a function that returns the number of
-// digits? The number of non-zero digits? The smallest/largest digit?
+// digits? The number of non-zero digits? The smallest/largest digit? Of course, these should all just borrow `self`.
 
 // ## Mutation + aliasing considered harmful (part 2)
 enum Variant {
@@ -72,7 +74,7 @@ fn work_on_variant(mut var: Variant, text: String) {
         Variant::Number(ref mut n) => ptr = n,
         Variant::Text(_) => return,
     }
-    /* var = Variant::Text(text); */
+    /* var = Variant::Text(text); */                                /* BAD! */
     *ptr = 1337;
 }