Why HashMap's contains_key(num1 - num2) gives an error?

help
1

The following code works:

pub fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
 let mut map = std::collections::HashMap::new();    
for (i, x) in nums.iter().enumerate(){
    if map.contains_key(target -x ) {
        return vec![map[x] as i32, i as i32];
    } else {
        map.insert(x, i);
    }
}

}

But the following does not work:

pub fn two_sum(nums: Vec<i32>, target: i32) -> Vec<i32> {
let mut map = std::collections::HashMap::new();    
for (i, x) in nums.iter().enumerate(){
    if map.contains_key(x) {
        return vec![map[x] as i32, i as i32];
    } else {
        map.insert(target-x, i);
    }
}

}

Why does map.contains_key(x) work but map.contains_key(target-x) gives an error?

Update: added the function signature.

3

Because contains_key() takes a reference parameter. I'm guessing nums is a vector or slice, so x is a reference because it came from iter(), vs. something like into_iter(). But target-x creates a new value, so you'll need to wrap it like contains_key(&(target - x)).

4

What @cuviper said was pretty clear also from the rustc compilation error message:

error[E0308]: mismatched types
 --> src/lib.rs:4:29
  |
4 |         if map.contains_key(target - x) {
  |                             ^^^^^^^^^^
  |                             |
  |                             expected reference, found `i32`
  |                             help: consider borrowing here: `&(target - x)`
  |
  = note: expected reference `&_`
                  found type `i32`
5

It's easier to make sense of the error after understanding the difference between iter() and into_iter(). Thanks.

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6

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