Why does this HashMap key have to be dereferenced twice?

Question

This function computes the mode of a Vec<i32> using a HashMap to keep count of the occurrence of each value. I do not understand why this will not compile unless the key is deferenced twice in this last line:

fn mode(vec: &Vec<i32>) -> i32 {
    let mut counts = HashMap::new();

    for n in vec {
        let count = counts.entry(n).or_insert(0);
        *count += 1;
    }

    **counts.iter().max_by_key(|a| a.1).unwrap().0
}

Answer 1

It has to be dereferenced twice because you've created a double reference.

1. You are iterating over &Vec<T> which produces &T.
2. You called HashMap::iter on HashMap<K, V> which produces (&K, &V).

fn mode(vec: &[i32]) -> i32 {
    let mut counts = std::collections::HashMap::new();

    for &n in vec {
        *counts.entry(n).or_insert(0) += 1;
    }

    counts.into_iter().max_by_key(|a| a.1).unwrap().0
}

See also:

• What is the difference between iter and into_iter?
• What does it mean to pass in a vector into a `for` loop versus a reference to a vector?
• Iterating over a slice's values instead of references in Rust?
• Meaning of '&variable' in arguments/patterns
• What is the difference between `e1` and `&e2` when used as the for-loop variable?
• What is the purpose of `&` before the loop variable?
• Why is it discouraged to accept a reference to a String (&String), Vec (&Vec), or Box (&Box) as a function argument?