hashmap

问题 I have two structs, A and B , and I want to use a HashMap<A, B> . I have a piece of code like this: use std::collections::HashMap; pub struct A { x: i32, y: i32, title: String, } pub struct B { a: u32, b: u32, } fn main() { let map = HashMap::new(); map.insert( A { x: 10, y: 20, title: "test".to_string(), }, B { a: 1, b: 2 }, ); } But the compiler gives me these errors: error[E0277]: the trait bound `A: std::cmp::Eq` is not satisfied --> src/main.rs:16:9 | 16 | map.insert( | ^^^^^^ the trait

问题 Please describe the reason if you know. I Googled it, but didn't find well explained answers. Is it for making index of bucket positive when your hashCode is negative? 回答1: For HashMap , the index in the array that stores the entries of the Map is calculated this way (where h is calculated from the hashCode of the key): static int indexFor(int h, int length) { return h & (length-1); } Where length is the length of the array. This only works when length is a power of 2. If length wasn't power

问题 I have a java.util.HashMap object. I guarantee that writing to HashMap is done by single dedicated thread. However, reading from the same HashMap object can be done from more that one thread at the time. Can I run in any troubles with such implementation? 回答1: Yes, you can run into big troubles with such an implementation! Adding a value to the HashMap is not an atomic operation. So if you read the map from another thread you might see an inconsistent state when another thread is adding a

问题 I have a java.util.HashMap object. I guarantee that writing to HashMap is done by single dedicated thread. However, reading from the same HashMap object can be done from more that one thread at the time. Can I run in any troubles with such implementation? 回答1: Yes, you can run into big troubles with such an implementation! Adding a value to the HashMap is not an atomic operation. So if you read the map from another thread you might see an inconsistent state when another thread is adding a

问题 I've observed that HashMap has a different order of elements even with the same data on the next program start. It looks like HashMap uses some absolute addresses to sort elements. Is there any other HashMap implementation, which has the same behaviour if the same data was inserted? 回答1: I believe linked-hash-map is the de facto crate for this. 回答2: I've observed that HashMap has a different order of elements even with the same data on the next program start. You don't have to observe

问题 I've observed that HashMap has a different order of elements even with the same data on the next program start. It looks like HashMap uses some absolute addresses to sort elements. Is there any other HashMap implementation, which has the same behaviour if the same data was inserted? 回答1: I believe linked-hash-map is the de facto crate for this. 回答2: I've observed that HashMap has a different order of elements even with the same data on the next program start. You don't have to observe

问题 I've observed that HashMap has a different order of elements even with the same data on the next program start. It looks like HashMap uses some absolute addresses to sort elements. Is there any other HashMap implementation, which has the same behaviour if the same data was inserted? 回答1: I believe linked-hash-map is the de facto crate for this. 回答2: I've observed that HashMap has a different order of elements even with the same data on the next program start. You don't have to observe

问题 I have just started implementing data structures in Java and was wondering can we have a situation like this. Map<HashMap<String,String>,String> map = new HashMap<HashMap<String,String>,String>(); And if yes ,, please give a small example. If you don't found question relevant ,, please mention in comments, 回答1: You can do this, but you should not do so in most cases. The key to a map needs to be constant and its equals and hashcode need to be set up to give the correct behavior. If you modify

问题 I am trying to implement a Cacher as mentioned in Chapter 13 of the Rust book and running into trouble. My Cacher code looks like: use std::collections::HashMap; use std::hash::Hash; pub struct Cacher<T, K, V> where T: Fn(K) -> V, { calculation: T, values: HashMap<K, V>, } impl<T, K: Eq + Hash, V> Cacher<T, K, V> where T: Fn(K) -> V, { pub fn new(calculation: T) -> Cacher<T, K, V> { Cacher { calculation, values: HashMap::new(), } } pub fn value(&mut self, k: K) -> &V { let result = self

问题 I am trying to implement a Cacher as mentioned in Chapter 13 of the Rust book and running into trouble. My Cacher code looks like: use std::collections::HashMap; use std::hash::Hash; pub struct Cacher<T, K, V> where T: Fn(K) -> V, { calculation: T, values: HashMap<K, V>, } impl<T, K: Eq + Hash, V> Cacher<T, K, V> where T: Fn(K) -> V, { pub fn new(calculation: T) -> Cacher<T, K, V> { Cacher { calculation, values: HashMap::new(), } } pub fn value(&mut self, k: K) -> &V { let result = self