Simple 2-dimensional cartesian product using std::iter::from_fn.

cartesian.rs

fn main() {
    let xs = vec![1, 2, 3];
    let ys = vec![6, 5, 4];
    let mut i = 0;
    let mut j = 0;
    let iter = std::iter::from_fn(|| {
        if i >= xs.len() {
            return None;
        }
        let rv = Some((xs[i], ys[j]));
        j = (j + 1) % ys.len();
        if j == 0 {
            i += 1;
        }
        rv
    });
    let expected = vec![
        (1, 6),
        (1, 5),
        (1, 4),
        (2, 6),
        (2, 5),
        (2, 4),
        (3, 6),
        (3, 5),
        (3, 4),
    ];
    // alternatively you could do repeat for each element zipped
    // with each latter list. you have to work backwards to do this.
    assert_eq!(iter.collect::<Vec<_>>(), expected);

    let mut i = 0;
    let mut j = 0;
    let xslen = xs.len();
    let yslen = ys.len();
    let iter = std::iter::from_fn(move || {
        if i >= xslen {
            return None;
        }
        let rv = Some((i, j));
        j = (j + 1) % yslen;
        if j == 0 {
            i += 1;
        }
        rv
    });
    let mut results = vec![];
    for (i, j) in iter.clone() {
        results.push((xs[i], ys[j]));
    }
    assert_eq!(results, expected);
    let mut results = vec![];
    for (i, j) in iter {
        results.push((xs[i], ys[j]));
    }
    assert_eq!(results, expected);
}