为什么必须将此HashMap密钥取消引用两次？

Question

此函数使用Vec<i32>计算HashMap的模式，以计数每个值的出现次数。我不明白为什么除非在最后一行中两次引用该键，否则为什么它不会编译：

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

它必须被取消引用两次，因为您已经创建了一个双重引用。

1. 您是iterating over &Vec<T>的produces &T。
2. 您在HashMap::iter的HashMap<K, V>上致电了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
}

另请参阅：

• 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?