CODEWITHSUNDEEP
rustiteratormax
Max Chr.
Max Chr.

Reputation: 13

Is there a Rust function that returns all the maximum values of an iterator?

I have a specific use case where I need to choose the maximum element from an Iterator<Item = (i8, f64)>. If there are multiple maximum elements in the Iterator I want to randomly select a maximum element uniformly distributed.

I tried the max_by function for Iterators but it doesn't work as I need it because it always returns the last element if there a multiple maximum elements. Here the example:

fn main() {
    let v = [(-1i8, 0.4f64), (0, 0.2), (1, 0.4)];
    let max = v.into_iter().max_by(|(_, r), (_, s)| r.total_cmp(s)).unwrap(); //always returns (1, 0.4)
    println!("{:?}", max);
}

playground

I need a function which returns another Iterator over the maximum elements. Then I could choose a random element from that Iterator.

Upvotes: 1

Views: 1161

Answers (2)

Jmb
Jmb

Reputation: 23414

You could have your comparison function return Less or Greater at random when the compared items are equal, with a suitable weighting to get uniform sampling:

use rand::{ self, Rng };
use std::cmp::Ordering;

let mut rng = rand::thread_rng();
let mut count = 0;
let max = v.into_iter().max_by(move |(_, r), (_, s)| {
    match r.total_cmp(s) {
        Ordering::Equal => {
            count = count + 1;
            if rng.gen_range (0..=count) == 0 {
                Ordering::Less
            } else {
                Ordering::Greater
            }
        },
        Ordering::Less => {
            count = 0;
            Ordering::Less
        },
        Ordering::Greater => Ordering::Greater,
    }
}).unwrap();

Playground

Note however that this depends on the implementation of Iter::max_by (specifically on the order it passes arguments to the comparison closure), so you might need to rewrite it yourself (e.g. using Iter::fold or Iter::reduce) to make sure of the order.

Upvotes: 0

Chayim Friedman
Chayim Friedman

Reputation: 71350

Not in std, but in itertools there is:

iter.max_set();

Upvotes: 2

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