Reading a File

Next, we’re going to add functionality to read the file that 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 that 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: The poem “I’m nobody! Who are you?” by Emily Dickinson that will make a good test case

With that 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 for dealing with files, and std::io::prelude::* contains various traits that are useful when doing I/O, including file I/O. In the same way that Rust has a general prelude that brings certain things into scope automatically, the std::io module has its own prelude of common things you’ll need when working with I/O. Unlike the default prelude, we must explicitly use the prelude from std::io.

In main, we’ve added three things: 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 out the value of contents after the file is read, so that we can check that our program is working so far.

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

$ cargo run the poem.txt
    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! Our code read in and printed out the content of the file. We’ve got a few flaws though. 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 be. While our program is still small, these flaws aren’t a big problem, but as our program grows, it will be harder to fix them cleanly. It’s good practice to begin refactoring early on when developing a program, as it’s much easier to refactor smaller amounts of code, so we’ll do that now.