錯誤：並非所有的控制路徑都返回一個值[E0269]

Question

我是Rust的新手。以下是查找這兩個數字的索引的代碼，以便它們合計到一個特定的目標。

use std::collections::HashMap; 

fn two_sum(nums: &[i32], target: i32) -> [usize;2] { 
    let mut map: HashMap<i32, usize> = HashMap::new(); 

    for i in 0..nums.len() { 
     let want = target - nums[i]; 
     match map.get(&nums[i]) { 
      Some(&seen) => return [seen, i], 
      _ => map.insert(want, i), 
      }; 
    } 

    [0usize, 0usize]; 
} 

fn main() { 
    let nums = [1,3,7,4]; 
    let res = two_sum(&nums, 10); 
    println! ("{},{}", res[0], res[1]); 
} 

我喜歡下面

src/bin/2sum.rs:3:1: 15:2 error: not all control paths return a value [E0269] 
src/bin/2sum.rs:3 fn two_sum(nums: &[i32], target: i32) -> [usize;2] { 
src/bin/2sum.rs:4  let mut map: HashMap<i32, usize> = HashMap::new(); 
src/bin/2sum.rs:5 
src/bin/2sum.rs:6  for i in 0..nums.len() { 
src/bin/2sum.rs:7   let want = target - nums[i]; 
src/bin/2sum.rs:8   match map.get(&nums[i]) { 
       ... 
src/bin/2sum.rs:3:1: 15:2 help: run `rustc --explain E0269` to see a detailed  explanation 
error: aborting due to previous error 

是如何發生的有沒有錯誤？我該如何解決這個問題？

感謝

Answer 1

可以[0usize, 0usize]（this is idiomatic）後刪除分號或添加return [0usize, 0usize]。

要改善您的代碼，您可以返回Option。另外，在這種情況下，最好返回一個元組。

use std::collections::HashMap; 

fn two_sum(nums: &[i32], target: i32) -> Option<(usize, usize)> { 
    let mut map: HashMap<i32, usize> = HashMap::new(); 

    for i in 0..nums.len() { 
     let want = target - nums[i]; 
     match map.get(&nums[i]) { 
      Some(&seen) => return Some((seen, i)), 
      _ => map.insert(want, i), 
     }; 
    } 

    None 
} 

fn main() { 
    let nums = [1, 3, 7, 4]; 
    let res = two_sum(&nums, 10); 
    println!("{:?}", res); 
}