// ========================
// ## Big Numbers
-// In the course of the next few parts, we are going to build a data-structure for
-// computations with *bug* numbers. We would like to not have an upper bound
-// to how large these numbers can get, with the memory of the machine being the
-// only limit.
+// In the course of the next few parts, we are going to build a data-structure for computations with
+// *big* numbers. We would like to not have an upper bound to how large these numbers can get, with
+// the memory of the machine being the only limit.
//
// We start by deciding how to represent such big numbers. One possibility here is
-// to use a vector of "small" numbers, which we will then consider the "digits"
-// of the big number. This is like "1337" being a vector of 4 small numbers (1, 3, 3, 7),
-// except that we will use `u64` as type of our base numbers. Now we just have to decide
-// the order in which we store numbers. I decided that we will store the least significant
+// to use a vector "digits" of the number. This is like "1337" being a vector of four digits (1, 3, 3, 7),
+// except that we will use `u64` as type of our digits, meaning we have 2^64 individual digits. Now we just
+// have to decide the order in which we store numbers. I decided that we will store the least significant
// digit first. This means that "1337" would actually become (7, 3, 3, 1).<br/>
// Finally, we declare that there must not be any trailing zeros (corresponding to
// useless leading zeros in our usual way of writing numbers). This is to ensure that
// the same number can only be stored in one way.
// To write this down in Rust, we use a `struct`, which is a lot like structs in C:
-// Just a collection of a bunch of named fields. Every field can be private to the current module
-// (which is the default), or public (which would be indicated by a `pub` in front of the name).
-// For the sake of the tutorial, we make `dat` public - otherwise, the next parts of this
-// course could not work on `BigInt`s. Of course, in a real program, one would make the field
-// private to ensure that the invariant (no trailing zeros) is maintained.
+// Just a bunch of named fields. Every field can be private to the current module (which is the default),
+// or public (which is indicated by a `pub` in front of the name). For the sake of the tutorial, we make
+// `data` public - otherwise, the next parts of this course could not work on `BigInt`s. Of course, in a
+// real program, one would make the field private to ensure that the invariant (no trailing zeros) is maintained.
pub struct BigInt {
pub data: Vec<u64>,
}
// 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`. In this case, we need to make that annotation to be able to call `pop`
- // on `v`.
+ // change the vector `v`.
+ //
+ // **Exercise 05.1**: Implement this function.
+ //
+ // *Hint*: You can use `pop()` to remove the last element of a vector.
pub fn from_vec(mut v: Vec<u64>) -> Self {
- while v.len() > 0 && v[v.len()-1] == 0 {
- v.pop();
- }
- BigInt { data: v }
+ unimplemented!()
}
}
// ## Cloning
// If you have a close look at the type of `BigInt::from_vec`, you will notice that it
-// consumes the vector `v`. The caller hence loses access. There is however something
+// consumes the vector `v`. The caller hence loses access to its vector. There is however something
// we can do if we don't want that to happen: We can explicitly `clone` the vector,
// which means that a full (or *deep*) copy will be performed. Technically,
// `clone` takes a borrowed vector, and returns a fully owned one.
// Making a type clonable is such a common exercise that Rust can even help you doing it:
// If you add `#[derive(Clone)]` right in front of the definition of `BigInt`, Rust will
// generate an implementation of `Clone` that simply clones all the fields. Try it!
+// These `#[...]` annotations at types (and functions, modules, crates) are called *attributes*.
+// We will see some more examples of attributes later.
// We can also make the type `SomethingOrNothing<T>` implement `Clone`. However, that
// can only work if `T` is `Clone`! So we have to add this bound to `T` when we introduce
// Again, Rust will generate this implementation automatically if you add
// `#[derive(Clone)]` right before the definition of `SomethingOrNothing`.
+// **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?
+
// ## Mutation + aliasing considered harmful (part 2)
// Now that we know how to borrow a part of an `enum` (like `v` above), there's another example for why we
// have to rule out mutation in the presence of aliasing. First, we define an `enum` that can hold either
Text(String),
}
// Now consider the following piece of code. Like above, `n` will be a borrow of a part of `var`,
-// and since we wrote `ref mut`, they will be mutable borrows. In other words, right after the match, `ptr`
+// and since we wrote `ref mut`, the borrow will be mutable. In other words, right after the match, `ptr`
// points to the number that's stored in `var`, where `var` is a `Number`. Remember that `_` means
// "we don't care".
fn work_on_variant(mut var: Variant, text: String) {
// that address, and Rust would eat your laundry - or whatever.)
//
// I hope this example clarifies why Rust has to rule out mutation in the presence of aliasing *in general*,
-// not just for the specific
+// not just for the specific case of a buffer being reallocated, and old pointers becoming hence invalid.
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