Process Array in parallel using GCD

Question

I have a large array that I would like to process by handing slices of it to a few asynchronous tasks. As a proof of concept, I have the written the following code:

Answer 1

When you use the += operator, the LHS is an inout parameter -- I think you're getting race conditions when, as you mention in your update, Swift moves around the array for optimization. I was able to get it to work by summing the chunk in a local variable, then simply assigning to the right index in summary:

func calcSummary() {
    let group =  dispatch_group_create()
    let queue = dispatch_get_global_queue(QOS_CLASS_USER_INITIATED, 0)

    for i in 0 ..< 10 {
        dispatch_group_async(group, queue, {
            let base = i * 50000
            var sum = 0
            for x in base ..< base + 50000 {
                sum += self.array[x]
            }
            self.summary[i] = sum
        })
    }

    dispatch_group_notify(group, queue, {
        println(self.summary)
    })
}