Rust Book - Programming a guessing game

To obtain user input we need to bring the io library into scope. This lib comes out of the standard library called std:

use std::io

by default, rust has a set of items defined in the std that it brings into the scope of every program, these can be found here: std::prelude and it is called the prelude.

If a type you want to use is not in the prelude, you have to bring that type into scope explicitly with a use statement.

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);
}

• String - is a string type provided by the standard lib that is a growable, UTF-8 encoded bit of text.
• The :: syntax in the ::new line indicated that new is an associated function of the String type. An “associated function” is a fucntion that’s implemented on a type, in this case, String.

Useful command cargo doc --open builds a documentation website that consists of all the packages the project is using.

Generating a Random Number

use rand::Rng;

let number = rand::thread_rng().gen_range(1..=100);

the first line, brings the package to scope, (after adding it to the dependencies under the cargo.toml file)

the Rng is a trait.

the second line, consists of several things:

1. rand::thread_rng() - this is a function that gives us the particular random generator that is local to the current thread, and seeded by the os.
2. gen_range - a method that takes a range expression as an arguemnt and generates a random number in that range.

Comparing

use std::cmp::Ordering - this line brings a type called Ordering into scope from the std lib. Its another enum that has the variants Less, Greater, and Equal.

match guess.cmp(&secret_number) {
	Ordering::Less => println!("Too small!")
	Ordering::Greater => println!("Too big!")
	Ordering::Equal => println!("You win!")
}

A match expression is made out of ‘arms’ an arm consists of a pattern to match against, and the code that should be run if the value given to match each arm’s pattern in turn.

let guess: u32 = guess.trim().parse().expect(“Please type a number!”);

this line serves as a shadowing of the guess variable we declared earlier, it takes the guess, trims the whitespace around it and parses it to be a u32 integer. if an error occurs during this process the executable will crash with an error. “Please type a number”. (this means parse, return a Result type, which has 2 variants, Err and Ok)