get frequency in rust

main.rs
use std::collections::HashMap;

fn main() {
    let mut frequency_map = HashMap::new();
    let items = vec![1, 2, 2, 3, 3, 3, 4, 4, 4, 4];
    
    for &item in &items {
        *frequency_map.entry(item).or_insert(0) += 1;
    }
    
    for (key, value) in frequency_map {
        println!("Item {} appears {} times", key, value);
    }
}
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