Itertools::group_by
是一种迭代方法,每次键函数都会改变新组。提供的示例显示了如何使用for
循环使用它,但是在迭代链中使用输出GroupBy
结构似乎很麻烦,除非我误解了某些东西:
let data = vec![1, 3, -2, -2, 1, 0, 1, 2];
// example from docs
for (key, group) in &data.into_iter().group_by(|elt| *elt >= 0) {
assert_eq!(4, group.sum::<i32>().abs());
}
// usage in an iterator method chain
data.iter()
.group_by(|elt| **elt >= 0)
.into_iter()
.map(|bool, group| (bool, group.collect::<Vec<i32>>()))
.collect::<Vec<(bool, Vec<i32>)>>();
第二个示例未能编译:
error[E0619]: the type of this value must be known in this context
--> src/main.rs:16:35
|
16 | .map(|bool, group| (bool, group.collect::<Vec<i32>>()))
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
error[E0599]: no method named `collect` found for type `std::iter::Map<itertools::Groups<'_, bool, std::slice::Iter<'_, i32>, [closure@src/main.rs:14:19: 14:35]>, [closure@src/main.rs:16:14: 16:63]>` in the current scope
--> src/main.rs:17:10
|
17 | .collect::<Vec<(bool, Vec<i32>)>>();
| ^^^^^^^
|
= note: the method `collect` exists but the following trait bounds were not satisfied:
`std::iter::Map<itertools::Groups<'_, bool, std::slice::Iter<'_, i32>, [closure@src/main.rs:14:19: 14:35]>, [closure@src/main.rs:16:14: 16:63]> : std::iter::Iterator`
`&mut std::iter::Map<itertools::Groups<'_, bool, std::slice::Iter<'_, i32>, [closure@src/main.rs:14:19: 14:35]>, [closure@src/main.rs:16:14: 16:63]> : std::iter::Iterator`
error[E0593]: closure takes 2 arguments but 1 argument is required
--> src/main.rs:16:10
|
16 | .map(|bool, group| (bool, group.collect::<Vec<i32>>()))
| ^^^ ------------------------------------------------- takes 2 arguments
| |
| expected closure that takes 1 argument
我不信心我理解此错误,但是我认为编译器希望CLOSURE的group
参数具有这种显式类型:
itertools::Group<'_, bool, std::slice::Iter<'_, i32>, [closure@srcmain.rs:26:15: 26:31]>
没人想要在他们的代码中,所以我希望我误解了。
问题是,是否可能以及如何使用 group_by
而不a循环?
传递给map
的闭合的参数数错误(两个而不是一个:使用元组);当然,错误消息不是很有帮助:)
另外,我也不会使用bool
作为变量名称,并且您需要在收集整数引用之前(由于iter()
vs.into_iter()
)。
游乐场
extern crate itertools;
use itertools::Itertools;
fn main() {
let data = vec![1, 3, -2, -2, 1, 0, 1, 2];
let groups = data.iter()
.group_by(|elt| **elt >= 0)
.into_iter()
.map(|(ge0, group)| (ge0, group.cloned().collect()))
.collect::<Vec<(bool, Vec<i32>)>>();
println!("{:?}", groups);
}