Using max_by_key on a vector of floats

Question

I want to use max_by_key to get the maximum value from a vector of f64s based on a certain key. This is a simple example, with a small vector and <

Answer 1

If you do not want to create a wrapper type you can use the ordered_float or ord_subset crate. For example

extern crate ordered_float;
extern crate ord_subset;

#[test]
fn test_example_() {
    use ordered_float::OrderedFloat;
    // OrderedFloat -> NaN is greater than all other values and equal to itself.
    // NotNaN -> NotNaN::new panics if called with NaN.

    let mut a: Vec<f64> = vec![-3.0, 0.2, 1.4];

    let max = *a.iter().max_by_key(|n| OrderedFloat(n.abs())).unwrap();
    assert_eq!(-3.0, max);

    a.sort_by_key(|n| OrderedFloat(n.abs()));
    assert_eq!(vec![0.2, 1.4, -3.0], a);
}

#[test]
fn test_example_ord_subset() {
    use ord_subset::OrdSubsetIterExt;

    let a: Vec<f64> = vec![-3.0, 0.2, 1.4];

    // For f64, NaN is ignored.
    let max = *a.iter().ord_subset_max_by_key(|n| n.abs()).unwrap();
    assert_eq!(-3.0, max);

    // ord_subset does not help with the sorting problem in the question
}