// this returns an Option (None or Some(value)), since the
// largest might not exist (when the vector is empty)
fn find_largest<T> (vec: &Vec<T>) -> Option<&T> where T:Ord {
    let len = vec.len();
    if len == 0 { return None };
    let mut max = &vec[0];
    for i in 1..len {
        if &vec[i] > max {
            max = &vec[i];
        }
    }
    Some(max)
}

fn main() {
    let largest;
    {
        let vec = vec![1, 3, 2];
        // unwrap tells the program to panic if it doesn't exist
        largest = find_largest(&vec).unwrap();
    }
    println!("largest is: {}", largest);
}