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## 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::build` 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 13-17, we’ve reproduced the implementation of the `Config::build` function as it was in Listing 12-23.

Filename: src/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

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

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

        if let Err(e) = run(config) {
            println!("Application error: {e}");
            process::exit(1);
        }
    }

    pub struct Config {
        pub query: String,
        pub file_path: String,
        pub ignore_case: bool,
    }

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

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

            let ignore_case = env::var("IGNORE_CASE").is_ok();

            Ok(Config {
                query,
                file_path,
                ignore_case,
            })
        }
    }

    fn run(config: Config) -> Result<(), Box<dyn Error>> {
        let contents = fs::read_to_string(config.file_path)?;

        let results = if config.ignore_case {
            search_case_insensitive(&config.query, &contents)
        } else {
            search(&config.query, &contents)
        };

        for line in results {
            println!("{line}");
        }

        Ok(())
    }

Listing 13-17: Reproduction of the `Config::build` 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 `build` function doesn’t own `args`. To return ownership of a `Config` instance, we had to clone the values from the `query` and `file_path` fields of `Config` so the `Config` instance can own its values.

With our new knowledge about iterators, we can change the `build` 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::build` function is doing because the iterator will access the values.

Once `Config::build` 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

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

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

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

        // --snip--

        if let Err(e) = run(config) {
            eprintln!("Application error: {e}");
            process::exit(1);
        }
    }

    pub struct Config {
        pub query: String,
        pub file_path: String,
        pub ignore_case: bool,
    }

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

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

            let ignore_case = env::var("IGNORE_CASE").is_ok();

            Ok(Config {
                query,
                file_path,
                ignore_case,
            })
        }
    }

    fn run(config: Config) -> Result<(), Box<dyn Error>> {
        let contents = fs::read_to_string(config.file_path)?;

        let results = if config.ignore_case {
            search_case_insensitive(&config.query, &contents)
        } else {
            search(&config.query, &contents)
        };

        for line in results {
            println!("{line}");
        }

        Ok(())
    }

We’ll first change the start of the `main` function that we had in Listing 12-24 to the code in Listing 13-18, which this time uses an iterator. This won’t compile until we update `Config::build` as well.

Filename: src/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

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

        // --snip--

        if let Err(e) = run(config) {
            eprintln!("Application error: {e}");
            process::exit(1);
        }
    }

    pub struct Config {
        pub query: String,
        pub file_path: String,
        pub ignore_case: bool,
    }

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

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

            let ignore_case = env::var("IGNORE_CASE").is_ok();

            Ok(Config {
                query,
                file_path,
                ignore_case,
            })
        }
    }

    fn run(config: Config) -> Result<(), Box<dyn Error>> {
        let contents = fs::read_to_string(config.file_path)?;

        let results = if config.ignore_case {
            search_case_insensitive(&config.query, &contents)
        } else {
            search(&config.query, &contents)
        };

        for line in results {
            println!("{line}");
        }

        Ok(())
    }

Listing 13-18: Passing the return value of `env::args` to `Config::build`

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

Next, we need to update the definition of `Config::build`. Let’s change the signature of `Config::build` to look like Listing 13-19. This still won’t compile, because we need to update the function body.

Filename: src/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

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

        if let Err(e) = run(config) {
            eprintln!("Application error: {e}");
            process::exit(1);
        }
    }

    pub struct Config {
        pub query: String,
        pub file_path: String,
        pub ignore_case: bool,
    }

    impl Config {
        fn build(
            mut args: impl Iterator<Item = String>,
        ) -> Result<Config, &'static str> {
            // --snip--
            if args.len() < 3 {
                return Err("not enough arguments");
            }

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

            let ignore_case = env::var("IGNORE_CASE").is_ok();

            Ok(Config {
                query,
                file_path,
                ignore_case,
            })
        }
    }

    fn run(config: Config) -> Result<(), Box<dyn Error>> {
        let contents = fs::read_to_string(config.file_path)?;

        let results = if config.ignore_case {
            search_case_insensitive(&config.query, &contents)
        } else {
            search(&config.query, &contents)
        };

        for line in results {
            println!("{line}");
        }

        Ok(())
    }

Listing 13-19: Updating the signature of `Config::build` 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`, and that type implements the `Iterator` trait and returns `String` values.

We’ve updated the signature of the `Config::build` function so the parameter `args` has a generic type with the trait bounds `impl Iterator<Item = String>` instead of `&[String]`. This usage of the `impl Trait` syntax we discussed in the “Traits as Parameters” section of Chapter 10 means that `args` can be any type that implements the `Iterator` trait and returns `String` items.

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::build`. Because `args` implements the `Iterator` trait, we know we can call the `next` method on it! Listing 13-20 updates the code from Listing 12-23 to use the `next` method.

Filename: src/main.rs

    use std::env;
    use std::error::Error;
    use std::fs;
    use std::process;

    use minigrep::{search, search_case_insensitive};

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

        if let Err(e) = run(config) {
            eprintln!("Application error: {e}");
            process::exit(1);
        }
    }

    pub struct Config {
        pub query: String,
        pub file_path: String,
        pub ignore_case: bool,
    }

    impl Config {
        fn build(
            mut args: impl Iterator<Item = String>,
        ) -> Result<Config, &'static str> {
            args.next();

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

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

            let ignore_case = env::var("IGNORE_CASE").is_ok();

            Ok(Config {
                query,
                file_path,
                ignore_case,
            })
        }
    }

    fn run(config: Config) -> Result<(), Box<dyn Error>> {
        let contents = fs::read_to_string(config.file_path)?;

        let results = if config.ignore_case {
            search_case_insensitive(&config.query, &contents)
        } else {
            search(&config.query, &contents)
        };

        for line in results {
            println!("{line}");
        }

        Ok(())
    }

Listing 13-20: Changing the body of `Config::build` 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. Then we call `next` to get the value we want to put in the `query` field of `Config`. If `next` returns `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 `file_path` value.

### Making Code Clearer with Iterator Adapters

We can also take advantage of iterators in the `search` function in our I/O project, which is reproduced here in Listing 13-21 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
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn one_result() {
            let query = "duct";
            let contents = "\
    Rust:
    safe, fast, productive.
    Pick three.";

            assert_eq!(vec!["safe, fast, productive."], search(query, contents));
        }
    }

Listing 13-21: The implementation of the `search` function from Listing 12-19

We can write this code in a more concise way using iterator adapter 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 13-22 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()
    }

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

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

        results
    }

    #[cfg(test)]
    mod tests {
        use super::*;

        #[test]
        fn case_sensitive() {
            let query = "duct";
            let contents = "\
    Rust:
    safe, fast, productive.
    Pick three.
    Duct tape.";

            assert_eq!(vec!["safe, fast, productive."], search(query, contents));
        }

        #[test]
        fn case_insensitive() {
            let query = "rUsT";
            let contents = "\
    Rust:
    safe, fast, productive.
    Pick three.
    Trust me.";

            assert_eq!(
                vec!["Rust:", "Trust me."],
                search_case_insensitive(query, contents)
            );
        }
    }

Listing 13-22: Using iterator adapter 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 13-16, this code uses the `filter` adapter to keep only the lines for which `line.contains(query)` returns `true`. 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.

For a further improvement, return an iterator from the `search` function by removing the call to `collect` and changing the return type to `impl Iterator<Item = &'a str>` so that the function becomes an iterator adapter. Note that you’ll also need to update the tests! Search through a large file using your `minigrep` tool before and after making this change to observe the difference in behavior. Before this change, the program won’t print any results until it has collected all of the results, but after the change, the results will be printed as each matching line is found because the `for` loop in the `run` function is able to take advantage of the laziness of the iterator.

### Choosing Between Loops and Iterators

The next logical question is which style you should choose in your own code and why: the original implementation in Listing 13-21 or the version using iterators in Listing 13-22 (assuming we’re collecting all the results before returning them rather than returning the iterator). 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 adapters 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 lower-level loop will be faster. Let’s talk about performance.