Merge pull request #41 from bzindovic/bzindovic-part13-clarity-fix

[rust-101.git] / src / part14.rs

diff --git a/src/part14.rs b/src/part14.rs
index b2d32f9d4ce07bd62eb32987742537ae432c0eac..059d1b50c9e49ef541ac07941abba060211cd228 100644 (file)
--- a/src/part14.rs
+++ b/src/part14.rs
@@ -1,104 +1,123 @@
 // Rust-101, Part 14: Slices, Arrays, External Dependencies
 // ========================================================
 
 // 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
 
 // ## 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<T>`. Of course, sorting does not actually consume the argument, so we should make that a `&mut Vec<T>`.
-//@ 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. 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.
+//@ 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<T>`. Of course, sorting does not actually consume the argument, so
+//@ we should make that a `&mut Vec<T>`. 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<T: PartialOrd>(data: &mut [T]) {
     if data.len() < 2 { return; }
 
 pub fn sort<T: PartialOrd>(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 {
     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);
 
         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 them both mutably borrowed.
+    // 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);
     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);                                                    /*@*/
 }
 
     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<i32>) {
 
 // Now, we can sort, e.g., an vector of numbers.
 fn sort_nums(data: &mut Vec<i32>) {

-    //@ 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
     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
 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 {
 #[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;
     extern crate docopt;
     use self::docopt::Docopt;

-    use part12::{run, Options, OutputMode};
+    use part13::{run, Options, OutputMode};

     use std::process;
 
     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] <pattern> <file>...
 
     static USAGE: &'static str = "
 Usage: rgrep [-c] [-s] <pattern> <file>...
 


@@ -109,12 +128,15 @@ Options:
 
     // This function extracts the rgrep options from the command-line arguments.
     fn get_options() -> 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/). <br/>
-        //@ The function `and_then` takes a closure from `T` to `Result<U, E>`, and uses it to transform a `Result<T, E>` to a
-        //@ `Result<U, E>`. 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/). <br/>
+        //@ The function `and_then` takes a closure from `T` to `Result<U, E>`, and uses it to
+        //@ transform a `Result<T, E>` to a `Result<U, E>`. 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");
         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.
         }
 
         // 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 
-        //@ 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  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 {
         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 -- <pattern> <files>` 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 -- <pattern> <files>` to call your program, and see the argument
+    // parser and the threads we wrote previously in action!

     pub fn main() {
         run(get_options());                                         /*@*/
     }
 }
 
     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](https://www.ralfj.de/git/rust-101.git/blob_plain/HEAD:/workspace/src/part14.rs) | [next](part15.html)
+//@ [index](main.html) | [previous](part13.html) | [raw source](workspace/src/part14.rs) |
+//@ [next](part15.html)