How to pass an owned array by reference

Question

I'm just playing around with rust for the first time, implementing quicksort, and I'm stuck on references to dynamically sized arrays (I had no problem with fixed size arrays).

I would like to have an indefinitely sized array of integers to sort, which I gather I can create with something like:

let array = ~[1,2,3,4,3,2,1];

However, I am not sure how I can pass this by reference into a partition function.

partition ( a : &mut ~[uint], p: uint, i: uint) {
     // partition a, in place
}

As soon as I try to reorder any elements in a, the compiler complains:

error: cannot assign to immutable vec content a[..]

Answer 1

You should use a mutable borrow of the vector rather than a mutable borrow of a pointer to a vector, so you should get rid of that ~ pointer in the type of your partition function. For example:

fn partition(_: &mut [uint], _: uint, _: uint) { }

fn main() {
    let mut array = ~[1, 2, 3, 4, 3, 2, 1];
    partition(array, 0, 0);
}

Notice that array is automatically passed as a mutable borrowed pointer.

If you use a Vec instead, then you need to explicitly create a slice:

fn partition(_: &mut [uint], _: uint, _: uint) { }

fn main() {
    let mut array = vec!(1, 2, 3, 4, 3, 2, 1);
    partition(array.as_mut_slice(), 0, 0);
}

Both code snippets should compile on the latest Rust (from tip).

Answer 2

With 0.10 the language is undergoing some changes to array types at the moment, so things are a bit messy. Vec<T> is Rust's intended dynamically sized array type.

let vec = vec!(1u,2,3,4,3,2,1);

partition ( a : &mut Vec<uint>, p: uint, i: uint) {
     // partition a, in place
}

Note that indexing of a Vec via brackets is currently only possible by first calling .as_slice() or .as_mut_slice() on it since the respective traits are not yet implemented.