如何有效地查找和插入HashMap? [英] How to lookup from and insert into a HashMap efficiently?
问题描述
- 查找
Vec
如果它不存在,为密钥创建一个空的Vec
,并将其存储以备后用。 ,但仍然保留在变量中。
如何有效地执行此操作?自然我以为我可以用 match
:
使用std :: collections :: HashMap的;
//此代码不能编译。
let mut map = HashMap :: new();
let key =foo;
let values:& Vec< isize> = match map.get(key){
Some(v)=> v,
None => {
让预设:Vec< isize> = Vec :: new();
map.insert(key,default);
& amp; default
}
};
当我尝试它时,它给了我错误:
错误[E0502]:无法借用`map`作为可变因为它也被借用为不可变
- > src / main.rs:11:13
|
7 | let values:& Vec< isize> =匹配map.get(key){
| ---不变的借款发生在这里
...
11 | map.insert(key,default);
| ^^^可变借款发生在这里
...
15 | }
| - 不变的借款在这里结束
我最终做了这样的事情,但我不喜欢事实上它执行了两次查找( map.contains_key
和 map.get
):
//此代码编译。
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 => {
恐慌!(impossiburu!);
}
};
有没有一种安全的方法可以只用一个 match
?
entry
API 就是为此而设计的。在手动形式下,它可能看起来像
使用std :: collections :: hash_map :: Entry;
让值:& Vec< isize> = match map.entry(key){
Entry :: Occupied(o)=> o.into_mut(),
Entry :: Vacant(v)=> v.insert(默认)
};
或者可以使用简单的形式:
< pre $ map.entry(key).or_insert_with(||默认)
如果默认
即使未插入,也可以计算出来,但它也可以是:
map.entry(key).or_insert(默认)
I'd like to do the following:
- Lookup a
Vec
for a certain key, and store it for later use. - If it doesn't exist, create an empty
Vec
for the key, but still keep it in the variable.
How to do this efficiently? Naturally I thought I could use 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:
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
):
// 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
?
The entry
API is designed for this. In manual form, it might look like
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:
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:
map.entry(key).or_insert(default)
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