Improving Our I/O Project - The Rust Programming Language

## Improving Our I/O Project

With this new knowledge about iterators, we can improve the I/O project in Chapter 12 by using iterators to make places in the code clearer and more concise. Let’s look at how iterators can improve our implementation of the `Config::new` function and the `search` function.

### Removing a `clone` Using an Iterator

In Listing 12-6, we added code that took a slice of `String` values and created an instance of the `Config` struct by indexing into the slice and cloning the values, allowing the `Config` struct to own those values. In Listing 13-24, we’ve reproduced the implementation of the `Config::new` function as it was in Listing 12-23:

Filename: src/lib.rs

    impl Config {
        pub fn new(args: &[String]) -> Result<Config, &'static str> {
            if args.len() < 3 {
                return Err("not enough arguments");
            }

            let query = args[1].clone();
            let filename = args[2].clone();

            let case_sensitive = env::var("CASE_INSENSITIVE").is_err();

            Ok(Config { query, filename, case_sensitive })
        }
    }

Listing 13-24: Reproduction of the `Config::new` function from Listing 12-23

At the time, we said not to worry about the inefficient `clone` calls because we would remove them in the future. Well, that time is now!

We needed `clone` here because we have a slice with `String` elements in the parameter `args`, but the `new` function doesn’t own `args`. To return ownership of a `Config` instance, we had to clone the values from the `query` and `filename` fields of `Config` so the `Config` instance can own its values.

With our new knowledge about iterators, we can change the `new` function to take ownership of an iterator as its argument instead of borrowing a slice. We’ll use the iterator functionality instead of the code that checks the length of the slice and indexes into specific locations. This will clarify what the `Config::new` function is doing because the iterator will access the values.

Once `Config::new` takes ownership of the iterator and stops using indexing operations that borrow, we can move the `String` values from the iterator into `Config` rather than calling `clone` and making a new allocation.

#### Using the Returned Iterator Directly

Open your I/O project’s src/main.rs file, which should look like this:

Filename: src/main.rs

    fn main() {
        let args: Vec<String> = env::args().collect();

        let config = Config::new(&args).unwrap_or_else(|err| {
            eprintln!("Problem parsing arguments: {}", err);
            process::exit(1);
        });

        // --snip--
    }

We’ll change the start of the `main` function that we had in Listing 12-24 at to the code in Listing 13-25. This won’t compile until we update `Config::new` as well.

Filename: src/main.rs

    fn main() {
        let config = Config::new(env::args()).unwrap_or_else(|err| {
            eprintln!("Problem parsing arguments: {}", err);
            process::exit(1);
        });

        // --snip--
    }

Listing 13-25: Passing the return value of `env::args` to `Config::new`

The `env::args` function returns an iterator! Rather than collecting the iterator values into a vector and then passing a slice to `Config::new`, now we’re passing ownership of the iterator returned from `env::args` to `Config::new` directly.

Next, we need to update the definition of `Config::new`. In your I/O project’s src/lib.rs file, let’s change the signature of `Config::new` to look like Listing 13-26. This still won’t compile because we need to update the function body.

Filename: src/lib.rs

    impl Config {
        pub fn new(mut args: std::env::Args) -> Result<Config, &'static str> {
            // --snip--

Listing 13-26: Updating the signature of `Config::new` to expect an iterator

The standard library documentation for the `env::args` function shows that the type of the iterator it returns is `std::env::Args`. We’ve updated the signature of the `Config::new` function so the parameter `args` has the type `std::env::Args` instead of `&[String]`. Because we’re taking ownership of `args` and we’ll be mutating `args` by iterating over it, we can add the `mut` keyword into the specification of the `args` parameter to make it mutable.

#### Using `Iterator` Trait Methods Instead of Indexing

Next, we’ll fix the body of `Config::new`. The standard library documentation also mentions that `std::env::Args` implements the `Iterator` trait, so we know we can call the `next` method on it! Listing 13-27 updates the code from Listing 12-23 to use the `next` method:

Filename: src/lib.rs

    # fn main() {}
    # use std::env;
    #
    # struct Config {
    #     query: String,
    #     filename: String,
    #     case_sensitive: bool,
    # }
    #
    impl Config {
        pub fn new(mut args: std::env::Args) -> Result<Config, &'static str> {
            args.next();

            let query = match args.next() {
                Some(arg) => arg,
                None => return Err("Didn't get a query string"),
            };

            let filename = match args.next() {
                Some(arg) => arg,
                None => return Err("Didn't get a file name"),
            };

            let case_sensitive = env::var("CASE_INSENSITIVE").is_err();

            Ok(Config { query, filename, case_sensitive })
        }
    }

Listing 13-27: Changing the body of `Config::new` to use iterator methods

Remember that the first value in the return value of `env::args` is the name of the program. We want to ignore that and get to the next value, so first we call `next` and do nothing with the return value. Second, we call `next` to get the value we want to put in the `query` field of `Config`. If `next` returns a `Some`, we use a `match` to extract the value. If it returns `None`, it means not enough arguments were given and we return early with an `Err` value. We do the same thing for the `filename` value.

### Making Code Clearer with Iterator Adaptors

We can also take advantage of iterators in the `search` function in our I/O project, which is reproduced here in Listing 13-28 as it was in Listing 12-19:

Filename: src/lib.rs

    pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
        let mut results = Vec::new();

        for line in contents.lines() {
            if line.contains(query) {
                results.push(line);
            }
        }

        results
    }

Listing 13-28: The implementation of the `search` function from Listing 12-19

We can write this code in a more concise way using iterator adaptor methods. Doing so also lets us avoid having a mutable intermediate `results` vector. The functional programming style prefers to minimize the amount of mutable state to make code clearer. Removing the mutable state might enable a future enhancement to make searching happen in parallel, because we wouldn’t have to manage concurrent access to the `results` vector. Listing 13-29 shows this change:

Filename: src/lib.rs

    pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
        contents.lines()
            .filter(|line| line.contains(query))
            .collect()
    }

Listing 13-29: Using iterator adaptor methods in the implementation of the `search` function

Recall that the purpose of the `search` function is to return all lines in `contents` that contain the `query`. Similar to the `filter` example in Listing 13-19, this code uses the `filter` adaptor to keep only the lines that `line.contains(query)` returns `true` for. We then collect the matching lines into another vector with `collect`. Much simpler! Feel free to make the same change to use iterator methods in the `search_case_insensitive` function as well.

The next logical question is which style you should choose in your own code and why: the original implementation in Listing 13-28 or the version using iterators in Listing 13-29. Most Rust programmers prefer to use the iterator style. It’s a bit tougher to get the hang of at first, but once you get a feel for the various iterator adaptors and what they do, iterators can be easier to understand. Instead of fiddling with the various bits of looping and building new vectors, the code focuses on the high-level objective of the loop. This abstracts away some of the commonplace code so it’s easier to see the concepts that are unique to this code, such as the filtering condition each element in the iterator must pass.

But are the two implementations truly equivalent? The intuitive assumption might be that the more low-level loop will be faster. Let’s talk about performance.