Your first program

Write your first Rust program with fn main and println!.

The first program

Every executable Rust program starts at the main function. The exclamation mark at the end of println! shows it is a macro; it prints and adds a newline.

fn main() {
    println!("Merhaba, dünya!");
}

Statements end with a semicolon; the body is inside curly braces. The file can also be compiled on its own with rustc main.rs.

print! and eprintln!

print! doesn't add a newline; eprintln! writes to the standard error stream (stderr) and is used for error messages:

print!("Yükleniyor");
print!("...");
println!();             // just a newline
eprintln!("uyarı: dosya bulunamadı");

When output is redirected to a file (> cikti.txt), stderr messages stay on the screen.

Exercise Loading bar

Call print!("Yükleniyor"), then print!(".") three times, and finally println!(" tamam").

Show example solution

fn main() {
    print!("Yükleniyor");
    print!(".");
    print!(".");
    print!(".");
    println!(" tamam");
}

Test yourself

Why is <code>println!</code> written with an exclamation mark?
- Because it's important
- Because it can fail
- It's optional
- It is a macro; it expands into code at compile time
Show answer

It is a macro; it expands into code at compile time — Macros can take a variable number of arguments and check the format string at compile time; so if the {} count is wrong, the code doesn't compile.

Print without a newline

____!("Yükleniyor...");

Show answer

print!("Yükleniyor...");

Macro and target

println!

stdout + newline
print!

stdout, no newline
eprintln!

stderr + newline
fn main

Entry point

Next lesson: println! and formatting

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