Having to Borrow a "0" inside an if-statment

Im currently trying to wrap my Head around this compilation error that im getting:

My code:

fn plusMinus(arr: &[i32]) {
    let mut res = (0,0,0);
    for i in arr.iter(){
        if i>0 {
            res.0 +=1;
        }else if i<0{
            res.1 +=1;
        }else{
            res.2 +=1;
        }
    }
    println!("{}", res.0 as f64/arr.len() as f64);
    println!("{}", res.1 as f64/arr.len() as f64);
    println!("{}", res.2 as f64/arr.len() as f64);
}

The error (and the fix that works):

error[E0308]: mismatched types
  --> main.rs:12:14
   |
12 |         if i>0 {
   |              ^
   |              |
   |              expected `&i32`, found integer
   |              help: consider borrowing here: `&0`

My question:

Why do i have to borrow the "0" here? Isnt it just a value to compare against and not a variable that has to be owned by something? Is it because my function has no return?