Rust functions

last modified February 19, 2025

In this article we show how to use functions in Rust.

Function

A function is a block of code that can be called from other parts of the program. Functions are used to organize the code and make it more readable.

In Rust, a function is defined with the fn keyword.

A function can take parameters and return values.

Simple function

The following is a simple Rust function.

fn main() {
    print_msg();
}

fn print_msg() {
    println!("Hello there!");
}

In the program, we define two functions. The main function calls the print_msg function.

fn print_msg() {
    println!("Hello there!");
}

The print_msg function prints a message to the console.

$ cargo run -q
Hello there!

Function with parameters

A function can take parameters.

fn main() {
    let name = String::from("John Doe");
    let age = 34;

    print_msg(name, age);
}

fn print_msg(name: String, age: i32) {
    println!("{} is {} years old", name, age);
}

In the program, we define a print_msg function that takes two parameters.

fn print_msg(name: String, age: i32) {
    println!("{} is {} years old", name, age);
}

The function prints the name and the age of a person.

$ cargo run -q
John Doe is 34 years old

Function with return value

A function can return a value.

fn main() {
    let name = String::from("John Doe");
    let age = 34;

    let msg = print_msg(name, age);
    println!("{}", msg);
}

fn print_msg(name: String, age: i32) -> String {
    return format!("{} is {} years old", name, age);
}

In the program, we define a print_msg function that returns a string.

fn print_msg(name: String, age: i32) -> String {
    return format!("{} is {} years old", name, age);
}

The function builds a message, which is returned from the function.

$ cargo run -q
John Doe is 34 years old

The closure

A closure is a function that can capture variables from the environment.

fn main() {
    let num = 3;

    let add_num = |x| x + num;

    println!("{}", add_num(5));
}

In the program, we define a closure that adds a number to another number.

let num = 3;

let add_num = |x| x + num;

The closure captures the num variable from the environment.

$ cargo run -q
8

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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