Simple Rust examples

Here are some simple examples of Rust syntax... If you're beginning Rust programming, you may find them useful.

Prompting for user input

This function asks the user a question, and returns the answer as a String:

fn prompt_user(prompt: String) -> String {
    // Prompt user with a String, return the result as a String.
    print!("{prompt}: ");
    // Must flush output because there was no newline.
    io::stdout().flush().unwrap();
    // Get the answer and return it...
    let mut answer = String::new();
    io::stdin().read_line(&mut answer).expect("Failed to read line");
    answer
}

Uses: print!, std::io::stdout().flush(), io::stdin().read_line()

Print an arbitrary number of digits from an f64

In this case, I just want to print a number of digits from the PI constant in Rust's standard library:

fn print_pi() {
    // Get a string representation of π. I've chosen 48 digits, which is the max for this data type.
    let pi_str: String = format!("{0:.48}", std::f64::consts::PI);
    // The number of decimal places that can be printed depends on the size of the string.
    let max_val: usize = pi_str.len()-2;
    // Prompt the user for input.
    let answer: String = prompt_user(
        format!("Decimal places of π to print? [0-{max_val}]"));
    let mut width: usize = answer.trim().parse().expect("You didn't enter a number!");
    // Constrain the result to max_val.
    if width > max_val {
        width = max_val;
    }
    // Print it!
    println!("{}", &pi_str[0..width+2]);
}

Uses: Prompting for user input, std::f64::consts::PI, format!, println!

Output:

Decimal places of π to print? [0-48]: 16
3.1415926535897931