[英]How to lookup from and insert into a HashMap efficiently?
I'd like to do the following:我想做以下事情:
Vec
for a certain key, and store it for later use.Vec
,并将其存储以备后用。Vec
for the key, but still keep it in the variable.Vec
,但仍将其保留在变量中。 How to do this efficiently?如何有效地做到这一点? Naturally I thought I could use
match
:当然,我认为我可以使用
match
:
use std::collections::HashMap;
// This code doesn't compile.
let mut map = HashMap::new();
let key = "foo";
let values: &Vec<isize> = match map.get(key) {
Some(v) => v,
None => {
let default: Vec<isize> = Vec::new();
map.insert(key, default);
&default
}
};
When I tried it, it gave me errors like:当我尝试它时,它给了我以下错误:
error[E0502]: cannot borrow `map` as mutable because it is also borrowed as immutable
--> src/main.rs:11:13
|
7 | let values: &Vec<isize> = match map.get(key) {
| --- immutable borrow occurs here
...
11 | map.insert(key, default);
| ^^^ mutable borrow occurs here
...
15 | }
| - immutable borrow ends here
I ended up with doing something like this, but I don't like the fact that it performs the lookup twice ( map.contains_key
and map.get
):我最终做了这样的事情,但我不喜欢它执行两次查找(
map.contains_key
和map.get
)的事实:
// This code does compile.
let mut map = HashMap::new();
let key = "foo";
if !map.contains_key(key) {
let default: Vec<isize> = Vec::new();
map.insert(key, default);
}
let values: &Vec<isize> = match map.get(key) {
Some(v) => v,
None => {
panic!("impossiburu!");
}
};
Is there a safe way to do this with just one match
?有没有一种安全的方法可以只用一场
match
来做到这一点?
The entry
API is designed for this. entry
API就是为此而设计的。 In manual form, it might look like在手动形式中,它可能看起来像
use std::collections::hash_map::Entry;
let values: &Vec<isize> = match map.entry(key) {
Entry::Occupied(o) => o.into_mut(),
Entry::Vacant(v) => v.insert(default)
};
Or one can use the briefer form:或者可以使用更简短的形式:
map.entry(key).or_insert_with(|| default)
If default
is OK/cheap to compute even when it isn't inserted, it can also just be:如果即使没有插入
default
也可以计算/便宜,它也可以是:
map.entry(key).or_insert(default)
I've used @huon and @Shepmaster's answer and just implemented it as a trait:我使用了@huon 和@Shepmaster 的答案,并将其作为一个特征来实现:
use std::collections::HashMap;
use std::hash::Hash;
pub trait InsertOrGet<K: Eq + Hash, V: Default> {
fn insert_or_get(&mut self, item: K) -> &mut V;
}
impl<K: Eq + Hash, V: Default> InsertOrGet<K, V> for HashMap<K, V> {
fn insert_or_get(&mut self, item: K) -> &mut V {
return match self.entry(item) {
std::collections::hash_map::Entry::Occupied(o) => o.into_mut(),
std::collections::hash_map::Entry::Vacant(v) => v.insert(V::default()),
};
}
}
Then somewhere else I can just do:然后在其他地方我可以做:
use crate::utils::hashmap::InsertOrGet;
let new_or_existing_value: &mut ValueType = my_map.insert_or_get(my_key.clone());
声明:本站的技术帖子网页,遵循CC BY-SA 4.0协议,如果您需要转载,请注明本站网址或者原文地址。任何问题请咨询:yoyou2525@163.com.