Guessing Game

use rand::Rng;
use std::cmp::Ordering;
use std::io;

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

    loop {
        println!("Please input your guess");

        let mut guess = String::new();
        let secret_number = rand::thread_rng().gen_range(1, 200);

        io::stdin()
            .read_line(&mut guess)
            .expect("Faild to Read Line");

        let guess: u32 = match guess.trim().parse() {
            Ok(num) => num,
            Err(_) => {
                println!("Invalid Input.");
                continue;
            }
        };

        match guess.cmp(&secret_number) {
            Ordering::Less => println!("Too Small!"),
            Ordering::Greater => println!("Too Big!"),
            Ordering::Equal => {
                println!("You win!");
                break;
            }
        }
    }
}

Explanation

The use statement

use rand::Rng;
use std::cmp::Ordering;
use std::io;

Rust only brings a few types into global scope, by default. The rest of the types a program needs, should be brought to scope by using the use keyword.

Here, rand is an external package, Ordering is an enum for comparing two variables of same type. Ordering provides Ordering:Less, Ordering:Greater and Ordering:Equal. io provides methods required for input and output.

Loop

loop {

}

loop is keyword for creating infinite loops. In the above program, it allows user to keep-on guessing until they make the correct choice.

let, mut and String::new()

let mut guess = String::new();

let is a keyword for defining variables and assign a value to it. By default, Rust variables are immutable. They can't be changed once declared. The mut keyword is used for mutating a variable so that it could be modified in a later stage.

The String::new() creates an empty value of type string. The new() after :: is known as an associated function of the type String.

An associated function is implemented on a type, rather than on a particular instance of a Type. Some languages call this a static method.

Getting a Random Number

let secret_number = rand::thread_rng().gen_range(1, 200);

Here, a random number is generated using an external package called rand. It is first imported using use rand::Rng;. rand::thread_rng is the perticular generator used in this example. This generator is local to the current thread and seeded by the Operating System.

Method gen_range() generates a random number between two given parameter.

Adding an external package

In order to add an external package, one needs add the package name and version number in Cargo.toml file. Cargo follows the specifications set by SemVer. match guess.cmp(&secret_number) { Ordering::Less => println!("Too Small!"), Ordering::Greater => println!("Too Big!"), Ordering::Equal => { println!("You win!"); break; } }

Getting input from stdin

io::stdin()
  .read_line(&mut guess)
  .expect("Faild to Read Line");

read_line() method gets a string from stdin. The expect() method is used to handle possible errors.

Mutating Parameters

By default, parameters in Rust does not mutate. If the mutation is needed, it has to be done using mut keyword. For rxample, someFn(&mut someVar). Here, someVar is passed by reference.

Variable Shadowing and Error Handling

let guess: u32 = match guess.trim().parse() {
  Ok(num) => num,
  Err(_) => {
    println!("Invalid Input.");
    continue;
  }
};

Variable Shadowing is the process of redeclaring a already defined variable with a new type. This is a unique thing allowed in Rust! Shadowing helps a programmer to save-up memory space by reusing a variable.

u32 (unsigned 32 bit Integer) is the data type the string is to be converted to.

trim() method (of type String) on guess variable removes extra characters like newline, trailing space etc from a String.

parse() method tries to parse a number from given string. It could result in either of two values: Ok and Err.

match is a keyword which is used to compare the value of parse() method with a given enum. So, if the parse() method returns Ok, Ok(num) => num, block would be executed. Otherwise, Err(_) => { println!("Invalid Input."); continue; } block would be executed.

Less, Greater, Equal

match guess.cmp(&secret_number) {
  Ordering::Less => println!("Too Small!"),
  Ordering::Greater => println!("Too Big!"),
  Ordering::Equal => {
    println!("You win!");
    break;
  }
}

Ordering is provided by std::cmp::Ordering. It is used to compare the results of two number. This is used along with .cmp() method which returns an enum.