Reading a File - The Rust Programming Language

## Reading a File

Now we’ll add functionality to read the file that is specified in the `filename` command line argument. First, we need a sample file to test it with: the best kind of file to use to make sure `minigrep` is working is one with a small amount of text over multiple lines with some repeated words. Listing 12-3 has an Emily Dickinson poem that will work well! Create a file called poem.txt at the root level of your project, and enter the poem “I’m Nobody! Who are you?”

Filename: poem.txt

    I'm nobody! Who are you?
    Are you nobody, too?
    Then there's a pair of us - don't tell!
    They'd banish us, you know.

    How dreary to be somebody!
    How public, like a frog
    To tell your name the livelong day
    To an admiring bog!

Listing 12-3: A poem by Emily Dickinson makes a good test case

With the text in place, edit src/main.rs and add code to open the file, as shown in Listing 12-4.

Filename: src/main.rs

    use std::env;
    use std::fs::File;
    use std::io::prelude::*;

    fn main() {
    #     let args: Vec<String> = env::args().collect();
    #
    #     let query = &args[1];
    #     let filename = &args[2];
    #
    #     println!("Searching for {}", query);
        // --snip--
        println!("In file {}", filename);

        let mut f = File::open(filename).expect("file not found");

        let mut contents = String::new();
        f.read_to_string(&mut contents)
            .expect("something went wrong reading the file");

        println!("With text:\n{}", contents);
    }

Listing 12-4: Reading the contents of the file specified by the second argument

First, we add some more `use` statements to bring in relevant parts of the standard library: we need `std::fs::File` to handle files, and `std::io::prelude::*` contains various useful traits for doing I/O, including file I/O. In the same way that Rust has a general prelude that brings certain types and functions into scope automatically, the `std::io` module has its own prelude of common types and functions you’ll need when working with I/O. Unlike with the default prelude, we must explicitly add a `use` statement for the prelude from `std::io`.

In `main`, we’ve added three statements: first, we get a mutable handle to the file by calling the `File::open` function and passing it the value of the `filename` variable. Second, we create a variable called `contents` and set it to a mutable, empty `String`. This will hold the content of the file after we read it in. Third, we call `read_to_string` on our file handle and pass a mutable reference to `contents` as an argument.

After those lines, we’ve again added a temporary `println!` statement that prints the value of `contents` after the file is read, so we can check that the program is working so far.

Let’s run this code with any string as the first command line argument (because we haven’t implemented the searching part yet) and the poem.txt file as the second argument:

    $ cargo run the poem.txt
       Compiling minigrep v0.1.0 (file:///projects/minigrep)
        Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
         Running `target/debug/minigrep the poem.txt`
    Searching for the
    In file poem.txt
    With text:
    I'm nobody! Who are you?
    Are you nobody, too?
    Then there's a pair of us - don't tell!
    They'd banish us, you know.

    How dreary to be somebody!
    How public, like a frog
    To tell your name the livelong day
    To an admiring bog!

Great! The code read and then printed the contents of the file. But the code has a few flaws. The `main` function has multiple responsibilities: generally, functions are clearer and easier to maintain if each function is responsible for only one idea. The other problem is that we’re not handling errors as well as we could. The program is still small, so these flaws aren’t a big problem, but as the program grows, it will be harder to fix them cleanly. It’s good practice to begin refactoring early on when developing a program, because it’s much easier to refactor smaller amounts of code. We’ll do that next.