rust - 无法从 `i32` 构建类型为 `std::iter::Iterator<Item=_>` 的集合

Question

今天我尝试在 LeetCode 上解决一个问题.这是我的代码(Playground):

#[test]
fn basic_test() {
    assert_eq!(day_of_year("2019-01-09".to_string()), 9);
    assert_eq!(day_of_year("2019-02-10".to_string()), 41);
    assert_eq!(day_of_year("2003-03-01".to_string()), 60);
    assert_eq!(day_of_year("2004-03-01".to_string()), 61);
}

pub fn day_of_year(date: String) -> i32 {
    let vec: Vec<&str> = date.split("-").collect();
    [(vec[0],vec[1],vec[2])].iter().map(|(year,month,day)|
        match month {
            &"01" => day.parse().unwrap(),
            &"02" => day.parse().unwrap() + 31,
            _ => match year.parse().unwrap(){
                y if y%4==0&&y%100!=0 
                    ||y%400==0&&y%3200!=0 
                    ||y%172800==0=>
                        match month {
                            &"03" => day.parse().unwrap()+31+29,
                            &"04" => day.parse().unwrap()+31+29+31,
                            &"05" => day.parse().unwrap()+31+29+31+30,
                            &"06" => day.parse().unwrap()+31+29+31+30+31,
                            &"07" => day.parse().unwrap()+31+29+31+30+31+30,
                            &"08" => day.parse().unwrap()+31+29+31+30+31+30+31,
                            &"09" => day.parse().unwrap()+31+29+31+30+31+30+31+31,
                            &"10" => day.parse().unwrap()+31+29+31+30+31+30+31+31+30,
                            &"11" => day.parse().unwrap()+31+29+31+30+31+30+31+31+30+31,
                            &"12" => day.parse().unwrap()+31+29+31+30+31+30+31+31+30+31+30
                        },
                _ => match month{
                        &"03" => day.parse().unwrap()+31+28,
                        &"04" => day.parse().unwrap()+31+28+31,
                        &"05" => day.parse().unwrap()+31+28+31+30,
                        &"06" => day.parse().unwrap()+31+28+31+30+31,
                        &"07" => day.parse().unwrap()+31+28+31+30+31+30,
                        &"08" => day.parse().unwrap()+31+28+31+30+31+30+31,
                        &"09" => day.parse().unwrap()+31+28+31+30+31+30+31+31,
                        &"10" => day.parse().unwrap()+31+28+31+30+31+30+31+31+30,
                        &"11" => day.parse().unwrap()+31+28+31+30+31+30+31+31+30+31,
                        &"12" => day.parse().unwrap()+31+28+31+30+31+30+31+31+30+31+30
                }
            }
        }
    ).collect()
}

我认为代码可以 self 解释。我收到此错误消息:

error[E0277]: a collection of type `i32` cannot be built from an iterator over elements of type `_`
  --> src/lib.rs:45:7
   |
45 |     ).collect()
   |       ^^^^^^^ a collection of type `i32` cannot be built from `std::iter::Iterator<Item=_>`
   |
   = help: the trait `std::iter::FromIterator<_>` is not implemented for `i32`

我尝试将其更改为 collect::<Vec<i32>>[0] .但仍然出现编译错误。让我知道如何更改代码以使其编译。

Answer 1

您根本不需要遍历元组和调用 collect。它创建了一个集合，但您的目标只是一个 i32 值。有固定密码:Playground

我还提前解析了值，并在matchees中添加了_分支，因为它应该是详尽无遗的。理想情况下，您也不需要 match。

更新:同一代码的更短版本:https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=a9a344c64f42332eb26f2a68fa260f72