Simple Rust examples

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

Prompt and receive user input on the same line

This function asks the user a short question without printing a newline, so the answer is provided on the same line as the query. This is natural prompt behavior for interactive CLIs.. or text adventure games.

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: prompt_user, std::f64::consts::PI, format!, println!

Output:

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