Edit

I read through some articles on blocks and fast enumeration and GCD and the like. @Bbum, who's written many articles on the subject of GCD and blocks, says that the block enumeration methods are always as fast or faster than the fast enumeration equivalents. You can read his reasoning here.

While this has been a fascinating, intellectual conversation, I agree with those who said that it really depends on the task at hand.

I have some tasks to accomplish and I need them done fast, cheap, and efficiently. Apple gives us many choices for how we want to enumerate an array, but I'm not sure which to choose.

Fast Enumeration

for (id obj in array)
{
    /* Do something with |obj|. */
}

Nonconcurrent Block Enumeration

[array enumerateObjectsUsingBlock: ^(id obj, NSUInteger idx, BOOL *stop) {
    /* Do something with |obj|. */
}];

Concurrent Block Enumeration

[array enumerateObjectsWithOptions: NSEnumerationConcurrent usingBlock: ^(id obj, NSUInteger idx, BOOL *stop) {
    /* Do something with |obj|. */
}];

GCD Apply

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_apply([array count], queue, ^(size_t idx) {
    id obj = [array objectAtIndex: idx];
    /* Do something with |obj|. */
});

GCD Async Apply

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_async(queue, ^(void) {
    dispatch_apply([array count], queue, ^(size_t idx) {
        id obj = [array objectAtIndex: idx];
        /* Do something with |obj|. */
    });
});

Or perhaps something with NSBlockOperations or an NSOperationQueue?

TIA, Alex.