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Introduction

Updated
17 Sept 2026
2 min read
6 sections
Page 2 of 18
Rust
rust
fn main() {
    println!("Guess the number!");
    println!("Please input your guess.");

    let guess = String::new();
}
  • new(){:rust} is an associative function, other languages call it static methods
  • In Rust, variable are immutable by default
rust
fn main() {
    println!("Guess the number!");
    println!("Please input your guess.");

    let mut guess = String::new();
}
  • mut{:rust} makes this variable mutable

Using IO

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);
}
  • pub fn read_line(&self, buf: &mut String) -> io::Result<usize>{:rust} will take user's input, append it to the specified buffer
  • Result -> enum Result<T, E> { Ok(T), Err(E) }{:rust}
Out · output
$ cargo run

Guess the number!
Please input your guess.
22
You guessed: 22

Using rand Package

  • Using the rand package to generate random numbers
rust
use std::io;
use rand::Rng;

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

   let secret_number = rand::thread_rng().gen_range(1..=101);

    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);
}
  • use rand::Rng{:rust} , imports the Rng{:rust} trait from the rand{:rust} crate into the current scope.

Comparing Variables

Comparing variables

  • pub enum Ordering { Less, Equal, Greater }{:rust}
rust
use std::io;
use std::cmp::Ordering;
use rand::Rng;

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

    let secret_number = rand::thread_rng().gen_range(1..=101);

    println!("The secret number is: {}", secret_number);

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

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

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

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

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

    match guess.cmp(&secret_number) {
        Ordering::Less => println!("Too small!"),
        Ordering::Greater => println!("Too big!"),
        Ordering::Equal => println!("You win!"),
    };
}
  • guess{:rust} variable is declared twice, i.e. called Shadowing (specially used in converting type of variables, but preserving the same variable name)

Match Expression

  • fn cmp(&self, other: &u32) -> Ordering{:rust} , handling the return type of Ordering using Match Expression
Out · output
$ cargo run

Guess the number!
The secret number is: 71
Please input your guess.
3 more lines
51
You guessed: 51
Too small!

Loops

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

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

    let secret_number = rand::thread_rng().gen_range(1..=101);

    println!("The secret number is: {}", secret_number);

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

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

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

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

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

		match guess.cmp(&secret_number) {
            Ordering::Less => println!("Too small!"),
            Ordering::Greater => println!("Too big!"),
            Ordering::Equal => {
                println!("You win!");
                break;
            },
        };
    }
}
Out · error
Bug: Error when user inputs string type
Out · error
$ cargo run

Guess the number!
The secret number is: 67
Please input your guess.
5 more lines
drip
thread 'main' panicked at src/main.rs:21:47:
Please type a number!: ParseIntError { kind: InvalidDigit }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `targetdebugguessing-game.exe` (exit code: 101)

Exception Handling

  • parse{:rust} function looks something like fn parse<F>(&self) -> Result<F, F::Err>{:rust}
  • Using match expression to catch the error
rust
use rand::Rng;
use std::cmp::Ordering;
use std::io;

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

    let secret_number = rand::thread_rng().gen_range(1..=101);

    println!("The secret number is: {}", secret_number);

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

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

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

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

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

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