Hacker News Reader

Comment by est31 | Hacker News Reader

It will still implement that with a loop over the array where there is a (bounds) check for the integer when it's being incremented (the i < len in int i; for(i = 0; i < len; i++)). That's no different from a loop over a slice, both of which eliminate the bounds check in the body of the loop (the one in list[i]). Arrays do have an advantage however, compilers can see their size so if you have an array (or array reference) and index it with a constant, then that bounds check will be eliminated. Also, array references are cheaper than slices because slices always contain the length.

View on HN

> Arrays do have an advantage however, compilers can see their size

Wouldn’t vectors have the same advantage in Rust? If we’re iterating over a vector, it’s proveable at compile time that the length is not being modified during the iteration.

I think they mean a one-off constant index should never check.

    let arr = [1, 2, 3, 4];
    // Will not compile a bound check
    let x = arr [3];
    
    let v = vec! [1, 2, 3, 4];
    // Imagine other code separates these two lines
    // Might compile a bound check
    let x = v [3];

In this case, v is not declared mutable, so you could ignore. In fact, the compiler will probably end up assigning x = 4 at compile time.

A better comparison is:

    fn foo_arr(v: &[u8; 5]) -> u8 {
        v[2]
    }

    fn foo_vec(v: &Vec<u8>) -> u8 {
        v[2]
    }

In the foo_arr case, the index lookup can be optimized out. In the foo_vec case, it can't be, because theoretically you might pass something with only 2 elements or less to foo_vec, and access to the third element will fail.

Same goes for e.g. the slice::windows vs slice::array_windows functions. In windows, you get a slice in the closure, and while the size is guaranteed by the implementation, without there being inlining the optimizer doesn't know about this guarantee. With array_windows, this size guarantee is communicated.

Length not being modified during iteration sure, but not the exact length. It's not possible in the general case to determine the exact length of a Vec at compile time, whereas Rust arrays can never change length so their exact length is know statically (unless you've got a slice, which is different)

Reply on news.ycombinator.com