collatz.rs

// Prints Collatz sequence from a given integer.
// By Scott Feeney, released under CC0 Public Domain Dedication 1.0.

use std::io;

fn main() {
    let mut num = read_number();
    loop {
        println!("{:d}", num);
        match next_collatz(num) {
            Some(next) => { num = next; }
            None => { break; }
        }
    }
}

fn next_collatz(n: int) -> Option<int> {
    if n <= 1 { None }
    else if n % 2 == 0 { Some(n / 2) }
    else { Some(3*n + 1) }
}

fn chomp<'a>(s: &'a str) -> &'a str {
    s.trim_right_chars('\n')
}

fn read_number() -> int {
    loop {
        print!("Enter an integer: ");
        let val = io::stdin().read_line().ok().and_then(|s| {
            from_str(chomp(s.as_slice()))
        });
        match val {
            Some(number) => { return number },
            None => { println!("That isn't a valid integer!") }
        }
    }
}