-// Rust-101, Part 09: Iterators (WIP)
-// ==================================
+// Rust-101, Part 09: Iterators
+// ============================
use part05::BigInt;
-// In the following, we will look into the iterator mechanism of Rust and make our `BigInt` compatible
-// with the `for` loops. Of course, this is all about implementing particular traits again. In particular,
-// an iterator is something that implements the `Iterator` trait. As you can see in [the documentation](http://doc.rust-lang.org/beta/std/iter/trait.Iterator.html),
-// this trait mandates a single function `next` returning an `Option<Self::Item>`, where `Item` is an
-// associated type chosen by the implementation. (There are many more methods provided for `Iterator`,
-// but they all have default implementations, so we don't have to worry about them right now).
-//
-// For the case of `BigInt`, we want our iterator to iterate over the digits in normal, notational order: The most-significant
-// digit comes first. So, we have to write down some type, and implement `Iterator` for it such that `next` returns the digits
-// one-by-one. Clearly, the iterator must somehow be able to access the number it iterates over, and it must store its current
-// location. However, it cannot *own* the `BigInt`, because then the number would be gone after iteration! That'd certainly be bad.
-// The only alternative is for the iterator to *borrow* the number.
-// In writing this down, we again have to be explicit about the lifetime of the borrow: We can't just have an
-// `Iter`, we must have an `Iter<'a>` that borrowed the number for lifetime `'a`. This is our first example of
-// a datatype that's polymorphic in a lifetime, as opposed to a type. <br/>
-// `usize` here is the type of unsigned, pointer-sized numbers. It is typically the type of "lengths of things",
-// in particular, it is the type of the length of a `Vec` and hence the right type to store an offset into the vector of digits.
-struct Iter<'a> {
+pub struct Iter<'a> {
num: &'a BigInt,
idx: usize, // the index of the last number that was returned
}
fn next(&mut self) -> Option<u64> {
// First, check whether there's any more digits to return.
if self.idx == 0 {
- // We already returned all the digits.
+ // We already returned all the digits, nothing to do.
unimplemented!()
} else {
- // Decrement, and return next digit.
+ // Otherwise: Decrement, and return next digit.
unimplemented!()
}
}
// All we need now is a function that creates such an iterator for a given `BigInt`.
impl BigInt {
- // Notice that when we write the type of `iter`, we don't actually have to give the lifetime parameter of `Iter`. Just as it is
- // the case with functions returning borrowed data, you can elide the lifetime. The rules for adding the lifetimes are exactly the
- // same. (See the last section of [part 06](part06.html).)
fn iter(&self) -> Iter {
unimplemented!()
}
// Of course, we don't have to use `for` to apply the iterator. We can also explicitly call `next`.
fn print_digits_v1(b: &BigInt) {
let mut iter = b.iter();
- // `loop` is the keyword for a loop without a condition: It runs endlessly, or until you break out of
- // it with `break` or `return`.
loop {
// Each time we go through the loop, we analyze the next element presented by the iterator - until it stops.
- match iter.next() {
- None => break,
- Some(digit) => println!("{}", digit)
- }
+ unimplemented!()
}
}
-// Now, it turns out that this combination of doing a loop and a pattern matching is fairly common, and Rust
-// provides some convenient syntactic sugar for it.
fn print_digits_v2(b: &BigInt) {
let mut iter = b.iter();
- // `while let` performs the given pattern matching on every round of the loop, and cancels the loop if the pattern
- // doesn't match. There's also `if let`, which works similar, but of course without the loopy part.
while let Some(digit) = iter.next() {
println!("{}", digit)
}
}
-// ## Iterator invalidation and lifetimes
-// You may have been surprised that we had to explicitly annotate a lifetime when we wrote `Iter`. Of
-// course, with lifetimes being present at every borrow in Rust, this is only consistent. But do we at
-// least gain something from this extra annotation burden? (Thankfully, this burden only occurs when we
-// define *types*, and not when we define functions - which is typically much more common.)
+// **Exercise 09.1**: Write a testcase for the iterator, making sure it yields the corrects numbers.
//
-// It turns out that the answer to this question is yes! This particular aspect of the concept of
-// lifetimes helps Rust to eliminate the issue of *iterator invalidation*. Consider the following
-// piece of code.
+// **Exercise 09.2**: Write a function `iter_ldf` that iterators over the digits with the least-significant
+// digits coming first. Write a testcase for it.
+
+// ## Iterator invalidation and lifetimes
+
fn iter_invalidation_demo() {
let mut b = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
for digit in b.iter() {
/*b = b + BigInt::new(1);*/ /* BAD! */
}
}
-// If you enable the bad line, Rust will reject the code. Why? The problem is that we are modifying the
-// number while iterating over it. In other languages, this can have all sorts of effects from inconsistent
-// data or throwing an exception (Java) to bad pointers being dereferenced (C++). Rust, however, is able to
-// detect this situation. When you call `iter`, you have to borrow `b` for some lifetime `'a`, and you obtain
-// `Iter<'a>`. This is an iterator that's only valid for lifetime `'a`. Gladly, we have this annotation available
-// to make such a statement. Now, since we are using the iterator throughout the loop, `'a` has to span the loop.
-// This `b` is borrowed for the duration of the loop, and we cannot mutate it. This is yet another example for
-// how the combination of mutation and aliasing leads to undesired effects (not necessarily crashes, like in Java),
-// which Rust successfully prevents.
-//
-// Technically speaking, there's one more subtlety that I did not explain yet. We never explicitly tied the lifetime `'a` of the
-// iterator to the loop so how does this happen? The answer lies in the full type of `next()`:
-// `fn<'a, 'b>(&'b mut Iter<'a>) -> Option<u64>`. Since `next()` takes a *borrowed* iterator, there are two lifetimes involved:
-// The lifetime of the borrow of the iterator, and the lifetime of the iterator itself. In such a case of nested lifetimes,
-// Rust implicitly adds the additional constraint that the inner lifetime *outlives* the outer one: The borrow of an iterator
-// cannot be valid for longer than the iterator itself is valid. This means that the lifetime `'a` of the iterator needs
-// to outlive every call to `next()`, and hence the loop. Lucky enough, this all happens without our intervention.
+// ## Iterator conversion trait
+
+impl<'a> IntoIterator for &'a BigInt {
+ type Item = u64;
+ type IntoIter = Iter<'a>;
+ fn into_iter(self) -> Iter<'a> {
+ self.iter()
+ }
+}
+// With this in place, you can now replace `b.iter()` in `main` by `&b`. Go ahead and try it! <br/>
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