X-Git-Url: https://git.ralfj.de/rust-101.git/blobdiff_plain/bae9e47884fdc3fc1a81fb4844572a832fcfb2ce..562558d25054c5be82f11acad0fbe53699de5b1c:/src/part13.rs diff --git a/src/part13.rs b/src/part13.rs index 0121079..bd1fca7 100644 --- a/src/part13.rs +++ b/src/part13.rs @@ -1,21 +1,21 @@ // Rust-101, Part 13: Slices, Arrays, External Dependencies -// ================= +// ======================================================== //@ To complete rgrep, there are two pieces we still need to implement: Sorting, and taking the job options //@ as argument to the program, rather than hard-coding them. Let's start with sorting. // ## Slices //@ Again, we first have to think about the type we want to give to our sorting function. We may be inclined to -//@ pass it a `Vec`. Now, sorting does not actually consume the argument, so we could make that a `&mut Vec`. +//@ pass it a `Vec`. Of course, sorting does not actually consume the argument, so we should make that a `&mut Vec`. //@ But there's a problem with that: If we want to implement some divide-and-conquer sorting algorithm (say, //@ Quicksort), then we will have to *split* our argument at some point, and operate recursively on the two parts. //@ But we can't split a `Vec`! We could now extend the function signature to also take some indices, marking the //@ part of the vector we are supposed to sort, but that's all rather clumsy. Rust offers a nicer solution. -//@ + //@ `[T]` is the type of an (unsized) *array*, with elements of type `T`. All this means is that there's a contiguous -//@ region of memory, where a bunch of `T` are stored. How many` We can't tell! This is an unsized type. Just like for -//@ trait objects, this means we can only operate on pointers to that type, and these pointers will containing the missing -//@ information - namely, the length. Such a pointer is called a *slice*. As we will see, a slice can be split! +//@ region of memory, where a bunch of `T` are stored. How many? We can't tell! This is an unsized type. Just like for +//@ trait objects, this means we can only operate on pointers to that type, and these pointers will carry the missing +//@ information - namely, the length. Such a pointer is called a *slice*. As we will see, a slice can be split. //@ Our function can thus take a borrowed slice, and promise to sort all elements in there. pub fn sort(data: &mut [T]) { if data.len() < 2 { return; } @@ -24,9 +24,11 @@ pub fn sort(data: &mut [T]) { // making sure that everything on the left is no larger than the pivot, and everything on the right is no smaller. let mut lpos = 1; let mut rpos = data.len(); - /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot; [rpos,len) is >= pivot; lpos < rpos */ + /* Invariant: pivot is data[0]; everything with index (0,lpos) is <= pivot; + [rpos,len) is >= pivot; lpos < rpos */ loop { - // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. + // **Exercise 13.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a + // test function for `sort`. unimplemented!() } @@ -38,14 +40,14 @@ pub fn sort(data: &mut [T]) { //@ one in the middle, and set the lengths appropriately such that they don't overlap. This is what `split_at_mut` does. //@ Since the two slices don't overlap, there is no aliasing and we can have them both mutably borrowed. let (part1, part2) = data.split_at_mut(lpos); - //@ The index operation can not only be used to address certain elements, it can also be used for "slicing": Giving a range + //@ The index operation can not only be used to address certain elements, it can also be used for *slicing*: Giving a range //@ of indices, and obtaining an appropriate part of the slice we started with. Here, we remove the last element from //@ `part1`, which is the pivot. This makes sure both recursive calls work on strictly smaller slices. sort(&mut part1[..lpos-1]); /*@*/ sort(part2); /*@*/ } -// **Exercise 13.2*: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part +// **Exercise 13.2**: Since `String` implements `PartialEq`, you can now change the function `output_lines` in the previous part // to call the sort function above. If you did exercise 12.1, you will have slightly more work. Make sure you sort by the matched line // only, not by filename or line number! @@ -61,8 +63,8 @@ fn sort_nums(data: &mut Vec) { //@ numbers, all one right next to the other in memory. Arrays are sized, and hence can be used like any other type. But we can also //@ borrow them as slices, e.g., to sort them. fn sort_array() { - let mut data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1]; - sort(&mut data); + let mut array_of_data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1]; + sort(&mut array_of_data); } // ## External Dependencies @@ -86,8 +88,8 @@ fn sort_array() { //@ path. All of this is explained in the [Cargo Guide](http://doc.crates.io/guide.html). // I disabled the following module (using a rather bad hack), because it only compiles if `docopt` is linked. However, before enabling it, -// you still have get the external library into the global namespace. This is done with `extern crate docopt;`, and that statement *has* to be -// in `main.rs`. So please go there, and enable this commented-out line. Then remove the attribute of the following module. +// you still have get the external library into the global namespace. This is done with `extern crate docopt`, and that statement *has* to be +// in `main.rs`. So please go there, and enable this commented-out line. Then remove the attribute of the `rgrep` module. #[cfg(feature = "disabled")] pub mod rgrep { // Now that `docopt` is linked and declared in `main.rs`, we can import it with `use`. We also import some other pieces that we will need. @@ -95,7 +97,7 @@ pub mod rgrep { use part12::{run, Options, OutputMode}; use std::process; - // The USAGE string documents how the program is to be called. It's written in a format that `docopt` can parse. + // The `USAGE` string documents how the program is to be called. It's written in a format that `docopt` can parse. static USAGE: &'static str = " Usage: rgrep [-c] [-s] ... @@ -106,7 +108,12 @@ Options: // This function extracts the rgrep options from the command-line arguments. fn get_options() -> Options { - // Parse argv and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/). + // Parse `argv` and exit the program with an error message if it fails. This is taken from the [`docopt` documentation](http://burntsushi.net/rustdoc/docopt/). + //@ The function `and_then` takes a closure from `T` to `Result`, and uses it to transform a `Result` to a + //@ `Result`. This way, we can chain computations that only happen if the previous one succeeded (and the error + //@ type has to stay the same). In case you know about monads, this style of programming will be familiar to you. + //@ There's a similar function for `Option`. `unwrap_or_else` is a bit like `unwrap`, but rather than panicking in + //@ case of an `Err`, it calls the closure. let args = Docopt::new(USAGE).and_then(|d| d.parse()).unwrap_or_else(|e| e.exit()); // Now we can get all the values out. let count = args.get_bool("-c"); @@ -119,15 +126,23 @@ Options: } // We need to make the strings owned to construct the `Options` instance. - //@ If you check all the type carefully, you will notice that `pattern` above if of type `&str`. `str` is the type of a UTF-8 encoded string, that is, a bunch of - //@ bytes in memory (`[u8]`) that are valid according of UTF-8. `str` is unsized. `&str` is a sliced string, and stores the address of the character data, and - //@ their length. String literals like "this one" are of type `&'static str`: They point right to the constant section of the binary, you you cannot claim you - //@ own them. However, the borrow is valid for as long as the program runs, hence it has lifetime `'static`. Calling `to_string` will copy the string data - //@ into an owned buffer on the heap, and thus convert it to `String`. + //@ If you check all the types carefully, you will notice that `pattern` above is of type `&str`. `str` is the type of a UTF-8 + //@ encoded string, that is, a bunch of bytes in memory (`[u8]`) that are valid according of UTF-8. `str` is unsized. `&str` + //@ stores the address of the character data, and their length. String literals like "this one" are + //@ of type `&'static str`: They point right to the constant section of the binary, so + //@ However, the borrow is valid for as long as the program runs, hence it has lifetime `'static`. Calling + //@ `to_string` will copy the string data into an owned buffer on the heap, and thus convert it to `String`. + let mode = if count { + OutputMode::Count + } else if sort { + OutputMode::SortAndPrint + } else { + OutputMode::Print + }; Options { files: files.iter().map(|file| file.to_string()).collect(), pattern: pattern.to_string(), - output_mode: if count { OutputMode::Count } else if sort { OutputMode::SortAndPrint } else { OutputMode::Print }, + output_mode: mode, } } @@ -141,5 +156,6 @@ Options: // **Exercise 13.3**: Wouldn't it be nice if rgrep supported regular expressions? There's already a crate that does all the parsing and matching on regular // expression, it's called [regex](https://crates.io/crates/regex). Add this crate to the dependencies of your workspace, add an option ("-r") to switch // the pattern to regular-expression mode, and change `filter_lines` to honor this option. The documentation of regex is available from its crates.io site. +// (You won't be able to use the `regex!` macro if you are on the stable or beta channel of Rust. But it wouldn't help for our use-case anyway.) //@ [index](main.html) | [previous](part12.html) | [next](main.html)