From: Nicola 'tekNico' Larosa Date: Sun, 21 Jan 2018 18:29:54 +0000 (+0100) Subject: part14.rs lines shortened X-Git-Url: https://git.ralfj.de/rust-101.git/commitdiff_plain/ff6b1897f21e3753062da8af7e342548b07c15b8 part14.rs lines shortened --- diff --git a/src/part14.rs b/src/part14.rs index 5c00905..059d1b5 100644 --- a/src/part14.rs +++ b/src/part14.rs @@ -1,104 +1,123 @@ // Rust-101, Part 14: 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. +//@ 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`. 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 carry the missing -//@ information - namely, the length (they will be *fat pointers*). Such a reference to an array is called a *slice*. As we will see, a slice can be split. -//@ Our function can thus take a mutable slice, and promise to sort all elements in there. +//@ 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`. 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 carry the missing information - namely, the +//@ length (they will be *fat pointers*). Such a reference to an array is called a *slice*. As we +//@ will see, a slice can be split. Our function can thus take a mutable slice, and promise to sort +//@ all elements in there. pub fn sort(data: &mut [T]) { if data.len() < 2 { return; } - // We decide that the element at 0 is our pivot, and then we move our cursors through the rest of the slice, - // making sure that everything on the left is no larger than the pivot, and everything on the right is no smaller. + // We decide that the element at 0 is our pivot, and then we move our cursors through the rest + // of the slice, 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 */ loop { - // **Exercise 14.1**: Complete this Quicksort loop. You can use `swap` on slices to swap two elements. Write a - // test function for `sort`. + // **Exercise 14.1**: Complete this Quicksort loop. You can use `swap` on slices to swap + // two elements. Write a test function for `sort`. unimplemented!() } // Once our cursors met, we need to put the pivot in the right place. data.swap(0, lpos-1); - // Finally, we split our slice to sort the two halves. The nice part about slices is that splitting them is cheap: - //@ They are just a pointer to a start address, and a length. We can thus get two pointers, one at the beginning and - //@ 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 both of them as unique, mutable slices. + // Finally, we split our slice to sort the two halves. The nice part about slices is that + // splitting them is cheap: + //@ They are just a pointer to a start address, and a length. We can thus get two pointers, one + //@ at the beginning and 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 both of them as unique, mutable slices. 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 - //@ 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. + //@ 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 14.2**: 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 13.1, you will have slightly more work. Make sure you sort by the matched line -// only, not by filename or line number! +// **Exercise 14.2**: 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 13.1, +// you will have slightly more work. Make sure you sort by the matched line only, not by filename +// or line number! // Now, we can sort, e.g., an vector of numbers. fn sort_nums(data: &mut Vec) { - //@ Vectors support slicing, just like slices do. Here, `..` denotes the full range, which means we want to slice the entire vector. - //@ It is then passed to the `sort` function, which doesn't even know that it is working on data inside a vector. + //@ Vectors support slicing, just like slices do. Here, `..` denotes the full range, which + //@ means we want to slice the entire vector. It is then passed to the `sort` function, which + //@ doesn't even know that it is working on data inside a vector. sort(&mut data[..]); } // ## Arrays -//@ An *array* in Rust is given by the type `[T; n]`, where `n` is some *fixed* number. So, `[f64; 10]` is an array of 10 floating-point -//@ 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. +//@ An *array* in Rust is given by the type `[T; n]`, where `n` is some *fixed* number. So, `[f64; +//@ 10]` is an array of 10 floating-point 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 array_of_data: [f64; 5] = [1.0, 3.4, 12.7, -9.12, 0.1]; sort(&mut array_of_data); } // ## External Dependencies -//@ This leaves us with just one more piece to complete rgrep: Taking arguments from the command-line. We could now directly work on -//@ [`std::env::args`](https://doc.rust-lang.org/stable/std/env/fn.args.html) to gain access to those arguments, and this would become -//@ a pretty boring lesson in string manipulation. Instead, I want to use this opportunity to show how easy it is to benefit from -//@ other people's work in your program. +//@ This leaves us with just one more piece to complete rgrep: Taking arguments from the command- +//@ line. We could now directly work on [`std::env::args`](https://doc.rust- +//@ lang.org/stable/std/env/fn.args.html) to gain access to those arguments, and this would become +//@ a pretty boring lesson in string manipulation. Instead, I want to use this opportunity to show +//@ how easy it is to benefit from other people's work in your program. //@ -//@ For sure, we are not the first to equip a Rust program with support for command-line arguments. Someone must have written a library -//@ for the job, right? Indeed, someone has. Rust has a central repository of published libraries, called [crates.io](https://crates.io/). -//@ It's a bit like [PyPI](https://pypi.python.org/pypi) or the [Ruby Gems](https://rubygems.org/): Everybody can upload their code, -//@ and there's tooling for importing that code into your project. This tooling is provided by `cargo`, the tool we are already using to -//@ build this tutorial. (`cargo` also has support for *publishing* your crate on crates.io, I refer you to [the documentation](http://doc.crates.io/crates-io.html) for more details.) -//@ In this case, we are going to use the [`docopt` crate](https://crates.io/crates/docopt), which creates a parser for command-line -//@ arguments based on the usage string. External dependencies are declared in the `Cargo.toml` file. - -//@ I already prepared that file, but the declaration of the dependency is still commented out. So please open `Cargo.toml` of your workspace -//@ now, and enable the two commented-out lines. Then do `cargo build`. Cargo will now download the crate from crates.io, compile it, -//@ and link it to your program. In the future, you can do `cargo update` to make it download new versions of crates you depend on. -//@ Note that crates.io is only the default location for dependencies, you can also give it the URL of a git repository or some local -//@ 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. -// Remove the attribute of the `rgrep` module to enable compilation. +//@ For sure, we are not the first to equip a Rust program with support for command-line arguments. +//@ Someone must have written a library for the job, right? Indeed, someone has. Rust has a central +//@ repository of published libraries, called [crates.io](https://crates.io/). +//@ It's a bit like [PyPI](https://pypi.python.org/pypi) or the [Ruby Gems](https://rubygems.org/): +//@ Everybody can upload their code, and there's tooling for importing that code into your project. +//@ This tooling is provided by `cargo`, the tool we are already using to build this tutorial. +//@ (`cargo` also has support for *publishing* your crate on crates.io, I refer you to [the +//@ documentation](http://doc.crates.io/crates-io.html) for more details.) +//@ In this case, we are going to use the [`docopt` crate](https://crates.io/crates/docopt), which +//@ creates a parser for command-line arguments based on the usage string. External dependencies +//@ are declared in the `Cargo.toml` file. + +//@ I already prepared that file, but the declaration of the dependency is still commented out. So +//@ please open `Cargo.toml` of your workspace now, and enable the two commented-out lines. Then do +//@ `cargo build`. Cargo will now download the crate from crates.io, compile it, and link it to +//@ your program. In the future, you can do `cargo update` to make it download new versions of +//@ crates you depend on. +//@ Note that crates.io is only the default location for dependencies, you can also give it the URL +//@ of a git repository or some local 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. Remove the attribute of the `rgrep` module to enable compilation. #[cfg(feature = "disabled")] pub mod rgrep { - // Now that `docopt` is linked, we can first add it to the namespace with `extern crate` and then import shorter names with `use`. - // We also import some other pieces that we will need. + // Now that `docopt` is linked, we can first add it to the namespace with `extern crate` and + // then import shorter names with `use`. We also import some other pieces that we will need. extern crate docopt; use self::docopt::Docopt; use part13::{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] ... @@ -109,12 +128,15 @@ Options: // This function extracts the rgrep options from the command-line arguments. fn get_options() -> Options { - // This parses `argv` and exit the program with an error message if it fails. The code 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. + // This parses `argv` and exit the program with an error message if it fails. The code 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"); @@ -127,12 +149,14 @@ Options: } // We need to make the strings owned to construct the `Options` instance. - //@ 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 - //@ the reference is valid for the entire program. The bytes pointed to by `pattern`, on the other hand, are owned by someone else, - //@ and we call `to_string` on it to copy the string data into a buffer on the heap that we own. + //@ 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 the reference is valid for the entire program. The + //@ bytes pointed to by `pattern`, on the other hand, are owned by someone else, and we + //@ call `to_string` on it to copy the string data into a buffer on the heap that we own. let mode = if count { OutputMode::Count } else if sort { @@ -147,16 +171,22 @@ Options: } } - // Finally, we can call the `run` function from the previous part on the options extracted using `get_options`. Edit `main.rs` to call this function. - // You can now use `cargo run -- ` to call your program, and see the argument parser and the threads we wrote previously in action! + // Finally, we can call the `run` function from the previous part on the options extracted using + // `get_options`. Edit `main.rs` to call this function. + // You can now use `cargo run -- ` to call your program, and see the argument + // parser and the threads we wrote previously in action! pub fn main() { run(get_options()); /*@*/ } } -// **Exercise 14.3**: 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.) +// **Exercise 14.3**: 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](part13.html) | [raw source](workspace/src/part14.rs) | [next](part15.html) +//@ [index](main.html) | [previous](part13.html) | [raw source](workspace/src/part14.rs) | +//@ [next](part15.html)