Introduction to Rust - Lesson 1

Hello World is the first program most people write when learning a new programming language. In Rust, the println! macro is used to display text on the screen. The fn main()function is the entry point of every Rust program — it is where execution begins. Writing a Hello World program introduces you to the basic structure of a Rust project: defining a function, using a macro, and producing output. This simple exercise confirms that your Rust environment is set up correctly and that you understand the fundamental syntax. From here, you can build more complex programs by adding variables, functions, and control flow.

fn main() {
    println!("Hello, world!");
}

• fn main() declares the main function, which is the starting point of every Rust program.
• The body of the function is enclosed in curly braces { and }.
• println! is a Rust macro that prints text followed by a newline.
• The text "Hello, world!" is a string literal enclosed in double quotes.
• Every statement in Rust ends with a semicolon ;.

fn main() {
    let name = "Rust";
    println!("Hello, {}!", name);
}

• let creates a new variable. Here name holds the string "Rust".
• Rust infers the type of name as &str (a string slice) automatically.
• The curly braces {} inside the string act as placeholders for values.
• println!("Hello, !", name); replaces with the value of name.
• Using variables in print statements lets you display dynamic content.

fn main() {
    println!("Hello, world!");
    println!("Welcome to Rust.");
}

• A Rust program can have multiple println! calls, each producing one line of output.
• The statements run in order from top to bottom inside main.
• The first println! prints Hello, world! and the second prints Welcome to Rust..
• Each println! automatically adds a newline at the end of its output.
• Printing multiple lines is how you display more than one piece of information to the user.

fn greet() {
    println!("Hello, world!");
}
fn main() {
    greet();
}

• You can define your own functions with fn followed by the function name and parentheses.
• fn greet() defines a function named greet that prints a message.
• main calls greet() by writing its name followed by parentheses.
• When greet() is called, execution jumps to that function, runs its code, and returns.
• Functions let you reuse code and keep your program organized.

fn main() {
    let x = 42;
    println!("The answer is {}", x);
}

• Variables can store numbers as well as text. Here x holds the integer 42.
• Rust infers x as type i32 (a 32-bit integer) automatically.
• The placeholder {} works for numbers just like it does for strings.
• println!("The answer is ", x); inserts the value 42 into the printed text.
• Combining text with variable values is a common way to display results to the user.

fn main() {
    for i in 0..3 {
        println!("Hello {}", i);
    }
}

• A for loop repeats a block of code a specific number of times.
• 0..3 creates a range that produces the numbers 0, 1, and 2.
• The loop variable i takes each value in the range, one at a time.
• println!("Hello ", i); prints Hello 0, then Hello 1, then Hello 2.
• Loops are a powerful way to repeat actions without writing the same code multiple times.