Not all control paths return a value?

Question

Here's my code to find the indices of two numbers such that they add up to a specific target:

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]);
}

Which throws this error:

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

How can I solve this problem?

You can make all return paths return a value, in this case via removing the semicolon after `[0usize, 0usize]` — набиячлэвэли, Apr 19 '16 at 21:20
Did you "run `rustc --explain E0269` to see a detailed explanation" as suggested by the compiler? — Shepmaster, Apr 19 '16 at 22:13
You would probably be interested in [How to lookup from and insert into a HashMap efficiently?](http://stackoverflow.com/q/28512394/155423). — Shepmaster, Apr 19 '16 at 22:19
The errors are also listed online: [E0269](https://doc.rust-lang.org/error-index.html#E0269) — Shepmaster, Apr 19 '16 at 22:23

score 3 · Answer 1 · edited Apr 19 '16 at 22:14

You can remove the semicolon after [0usize, 0usize] (this is idiomatic) or add a return [0usize, 0usize].

To improve your code you can return an Option. Also, in this case, it is better to return a tuple.

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);
}

Not all control paths return a value?

1 Answers1