如何编写一个带有迭代器的 Rust 函数? [英] How to write a Rust function that takes an iterator?
问题描述
我想写一个函数,它接受一个迭代器并返回一些操作的结果.具体来说,我正在尝试迭代 HashMap
:
I'd like to write a function that accepts an iterator and returns the results of some operations on it. Specifically, I'm trying to iterate over the values of a HashMap
:
use std::collections::HashMap;
fn find_min<'a>(vals: Iterator<Item=&'a u32>) -> Option<&'a u32> {
vals.min()
}
fn main() {
let mut map = HashMap::new();
map.insert("zero", 0u32);
map.insert("one", 1u32);
println!("Min value {:?}", find_min(map.values()));
}
可惜:
error: the `min` method cannot be invoked on a trait object
--> src/main.rs:4:10
|
4 | vals.min()
| ^^^
error[E0277]: the trait bound `std::iter::Iterator<Item=&'a u32> + 'static: std::marker::Sized` is not satisfied
--> src/main.rs:3:17
|
3 | fn find_min<'a>(vals: Iterator<Item = &'a u32>) -> Option<&'a u32> {
| ^^^^ `std::iter::Iterator<Item=&'a u32> + 'static` does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `std::iter::Iterator<Item=&'a u32> + 'static`
= note: all local variables must have a statically known size
error[E0308]: mismatched types
--> src/main.rs:11:41
|
11 | println!("Min value {:?}", find_min(map.values()));
| ^^^^^^^^^^^^ expected trait std::iter::Iterator, found struct `std::collections::hash_map::Values`
|
= note: expected type `std::iter::Iterator<Item=&u32> + 'static`
found type `std::collections::hash_map::Values<'_, &str, u32>`
如果我尝试通过引用传递,我会得到同样的错误;如果我使用 Box
,我会得到终身错误.
I get the same error if I try to pass by reference; if I use a Box
, I get lifetime errors.
推荐答案
你想在这里使用泛型:
fn find_min<'a, I>(vals: I) -> Option<&'a u32>
where
I: Iterator<Item = &'a u32>,
{
vals.min()
}
Traits 可以以两种方式使用:作为类型参数的边界和作为 trait 对象.The Rust Programming Language 这本书有一章是关于 traits 和一章关于 trait 对象 解释这两个用例.
Traits can be used in two ways: as bounds on type parameters and as trait objects. The book The Rust Programming Language has a chapter on traits and a chapter on trait objects that explain these two use cases.
此外,您通常希望采用实现 IntoIterator
的东西,因为这可以使调用您的函数的代码更好:
Additionally, you often want to take something that implements IntoIterator
as this can make the code calling your function nicer:
fn find_min<'a, I>(vals: I) -> Option<&'a u32>
where
I: IntoIterator<Item = &'a u32>,
{
vals.into_iter().min()
}
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