X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/66c777f23e032d445e29b1b40faa8e1799b4930c..ab7f9b241429bd675b437d2437799de75d2f409b:/workspace/src/part09.rs

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-// Rust-101, Part 09: Iterators (WIP)
-// ==================================
-
-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`. <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> {
-    num: &'a BigInt,
-    idx: usize, // the index of the last number that was returned
-}
-
-// Now we are equipped to implement `Iterator` for `Iter`.
-impl<'a> Iterator for Iter<'a> {
-    // We choose the type of things that we iterate over to be the type of digits, i.e., `u64`.
-    type Item = u64;
-
-    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.
-            unimplemented!()
-        } else {
-            // 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!()
-    }
-}
-
-// We are finally ready to iterate! Remember to edit `main.rs` to run this function.
-pub fn main() {
-    let b = BigInt::new(1 << 63) + BigInt::new(1 << 16) + BigInt::new(1 << 63);
-    for digit in b.iter() {
-        println!("{}", digit);
-    }
-}
-