# Let's dig in!

{% code title="main.rs" %}

```rust
use std::io;

fn main() {
    println!("Guess the number!");

    println!("Please input your guess.");

    let mut guess = String::new();

    io::stdin().read_line(&mut guess)
        .expect("Failed to read line");

    println!("You guessed: {}", guess);

}
```

{% endcode %}

The first line `use std::io` brings the standard io (input/output) library into scope. We've covered `main` and `println!`, so let's go into `let`, `mut`, and `String::new`. `let` is used to defined variables, like so:

```rust
let foo = bar;
```

A standard variable is immutable (cannot be changed), but `mut` makes it mutable.

```rust
let foo = bar; // immutable
let mut foo2 = bar; // mutable
```

Here's the error message you will see if you try to change an immutable variable.

![](https://2720625490-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LBlB5eCVilPmyCc9hb0%2F-LBlSURMVV3zkjDhOHC0%2F-LBlYfITXr176dVSMH-X%2FScreen%20Shot%202018-05-05%20at%202.26.21%20PM.png?alt=media\&token=17b400cb-9fa9-40ae-ad56-2fa0787ed05b)

Rust also binds variables to a certain type when they are initialized. For example, with line :

{% code title="" %}

```rust
let mut guess = String::new();
```

{% endcode %}

`guess` is bound to an empty string. Here's an example of how this works:

![](https://2720625490-files.gitbook.io/~/files/v0/b/gitbook-legacy-files/o/assets%2F-LBlB5eCVilPmyCc9hb0%2F-LBlSURMVV3zkjDhOHC0%2F-LBlhaDycJoU3dT3dEL_%2FScreen%20Shot%202018-05-05%20at%203.09.56%20PM.png?alt=media\&token=dd76e890-dfcb-471e-9a02-eb2c41d77d06)

The next part of the code, `.read_line(&mut guess)`, calls the [`read_line`](https://doc.rust-lang.org/std/io/struct.Stdin.html#method.read_line) method on the standard input handle to get input from the user. We’re also passing one argument to `read_line`: `&mut guess`. `read_line` requires a string as a parameter and that string must be mutable. The `&` indicates that it is a reference. References are immutable by default, so the `&mut guess` is required.