How to update the weights in WeightedIndex? - rust

Question

I want to update the weights for the key after its drawn, how can I do that.
use rand::distributions::WeightedIndex;
use rand::prelude::*;
fn main() {
let mut rng = thread_rng();
let items = [('a', 0), ('b', 3), ('c', 7)];
let dist2 = WeightedIndex::new(items.iter().map(|item| item.1)).unwrap();
for _ in 0..10 {
// 0% chance to print 'a', 30% chance to print 'b', 70% chance to print 'c'
println!("{}", items[dist2.sample(&mut rng)].0);
// dist2.update_weights(new_weights: &[(usize, &X)])
}
}
That is when "b" is drawn, I want to set the weight for b to zero, so that its no longer drawn, or set it to some other weight.
https://docs.rs/rand/0.7.3/rand/distributions/weighted/struct.WeightedIndex.html

Answer 1

use rand::distributions::WeightedIndex;
use rand::prelude::*;
fn main() {
let mut rng = thread_rng();
let items = [('a', 8), ('b', 5), ('c', 1)];
let mut dist2 = WeightedIndex::new(items.iter().map(|item| item.1)).unwrap();
for _ in 0..3 {
let index = dist2.sample(&mut rng);
println!("{}", index);
println!("{:?}", items[index].0);
dist2.update_weights(&[(index, &0)]).unwrap();
}
}
You just need to call the update_weights function on the same distribution - it will mutate the existing distribution, so there is no need to reassign. Note that the return value of update_weights is just a Result<(), WeightedError>. An Ok(()) indicates that the mutation of the distribution was succesful.

Answer 2

Here is my code:
use rand::distributions::WeightedIndex;
use rand::prelude::*;
fn main() {
let mut rng = thread_rng();
let items = [('a', 8), ('b', 5), ('c', 1)];
let mut dist2 = WeightedIndex::new(items.iter().map(|item| item.1)).unwrap();
for _ in 0..3 {
// 0% chance to print 'a', 30% chance to print 'b', 70% chance to print 'c'
let index = dist2.sample(&mut rng);
println!("{}", index);
println!("{:?}", items[index].0);
let _d = dist2.update_weights(&[(index, &0)]);
}
}