fn main() {
    let slice = "test\ntest";
    let comments = "#";
    let num_cols = 1;

    let mut rows = 0;
    let mut these_cols = 0;
    let _data = slice
        .lines()
        .filter(|l| !l.starts_with(comments))
        .flat_map(|l| {
            rows += 1;
            these_cols = 0;
            let parsed = l.split_whitespace().map(|s| {
                these_cols += 1;
                s.parse().unwrap()
            });
            assert_eq!(num_cols, these_cols);
            parsed
        })
        .collect::<Vec<f64>>();

}