## Improving our I/O Project

In our I/O project implementing `grep` in the last chapter, there are some places where the code could be made clearer and more concise using iterators. Let's take a look at how iterators can improve our implementation of the `Config::new` function and the `grep` function.

### Removing a `clone` by Using an Iterator

Back in listing 12-8, we had this code that took a slice of `String` values and created an instance of the `Config` struct by checking for the right number of arguments, indexing into the slice, and cloning the values so that the `Config` struct could own those values:

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

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

            Ok(Config {
                search: search,
                filename: filename,
            })
        }
    }

At the time, we said not to worry about the `clone` calls here, and that we could remove them in the future. Well, that time is now! So, why do we need `clone` here? The issue is that we have a slice with `String` elements in the parameter `args`, and the `new` function does not own `args`. In order to be able to return ownership of a `Config` instance, we need to clone the values that we put in the `search` and `filename` fields of `Config`, so that the `Config` instance can own its values.

Now that we know more about iterators, we can change the `new` function to instead take ownership of an iterator as its argument. We'll use the iterator functionality instead of having to check the length of the slice and index into specific locations. Since we've taken ownership of the iterator, and we won't be using indexing operations that borrow anymore, we can move the `String` values from the iterator into `Config` instead of calling `clone` and making a new allocation.

First, let's take `main` as it was in Listing 12-6, and change it to pass the return value of `env::args` to `Config::new`, instead of calling `collect` and passing a slice:

    fn main() {
        let config = Config::new(env::args());
        // ...snip...

If we look in the standard library documentation for the `env::args` function, we'll see that its return type is `std::env::Args`. So next we'll update the signature of the `Config::new` function so that the parameter `args` has the type `std::env::Args` instead of `&[String]`:

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

Next, we'll fix the body of `Config::new`. As we can also see in the standard library documentation, `std::env::Args` implements the `Iterator` trait, so we know we can call the `next` method on it! Here's the new code:

    # #![allow(unused_variables)]
    #
    #fn main() {
    # struct Config {
    #     search: String,
    #     filename: String,
    # }
    #
    impl Config {
        fn new(mut args: std::env::Args) -> Result<Config, &'static str> {
            args.next();

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

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

            Ok(Config {
                search: search,
                filename: filename,
            })
        }
    }

    #}

Remember that the first value in the return value of `env::args` is the name of the program. We want to ignore that, so first we'll call `next` and not do anything with the return value. The second time we call `next` should be the value we want to put in the `search` field of `Config`. We use a `match` to extract the value if `next` returns a `Some`, and we return early with an `Err` value if there weren't enough arguments (which would cause this call to `next` to return `None`).

We do the same thing for the `filename` value. It's slightly unfortunate that the `match` expressions for `search` and `filename` are so similar. It would be nice if we could use `?` on the `Option` returned from `next`, but `?` only works with `Result` values currently. Even if we could use `?` on `Option` like we can on `Result`, the value we would get would be borrowed, and we want to move the `String` from the iterator into `Config`.

### Making Code Clearer with Iterator Adaptors

The other bit of code where we could take advantage of iterators was in the `grep` function as implemented in Listing 12-15:

    # #![allow(unused_variables)]
    #
    #fn main() {
    fn grep<'a>(search: &str, contents: &'a str) -> Vec<&'a str> {
        let mut results = Vec::new();

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

        results
    }

    #}

We can write this code in a much shorter way, and avoiding having to have a mutable intermediate `results` vector, by using iterator adaptor methods like this instead:

    # #![allow(unused_variables)]
    #
    #fn main() {
    fn grep<'a>(search: &str, contents: &'a str) -> Vec<&'a str> {
        contents.lines()
            .filter(|line| line.contains(search))
            .collect()
    }

    #}

Here, we use the `filter` adaptor to only keep the lines that `line.contains(search)` returns true for. We then collect them up into another vector with `collect`. Much simpler!

We can use the same technique in the `grep_case_insensitive` function that we defined in Listing 12-16 as follows:

    # #![allow(unused_variables)]
    #
    #fn main() {
    fn grep_case_insensitive<'a>(search: &str, contents: &'a str) -> Vec<&'a str> {
        let search = search.to_lowercase();

        contents.lines()
            .filter(|line| {
                line.to_lowercase().contains(&search)
            }).collect()
    }

    #}

Not too bad! So which style should you choose? Most Rust programmers prefer to use the iterator style. It's a bit tougher to understand at first, but once you gain an intuition for what the various iterator adaptors do, this is much easier to understand. Instead of fiddling with the various bits of looping and building a new vector, the code focuses on the high-level objective of the loop, abstracting some of the commonplace code so that it's easier to see the concepts that are unique to this usage of the code, like the condition on which the code is filtering each element in the iterator.

But are they truly equivalent? Surely the more low-level loop will be faster. Let's talk about performance.