Question

Example. I’ve got an array with 15 objects. I want to start enumerating from a given index. Say start at index 5 and then the index above, the index under, above, under etc… I do want it to wrap around.

So the order of indexes in my example would be. 5, 6, 4, 7, 3, 8, 2, 9, 1, 10, 0, 11, 14, 12, 13

It would be great to have a method signature similar to following line, but I don’t require that to approva an answer:

- (void)enumerateFromIndex:(NSUInteger)index wrapAroundAndGoBothWays:(void (^)(id obj, NSUInteger idx, BOOL *stop))block

How can this be done? Would like to avoid copying array etc.

At this post we do it with no wrap around: Enumerate NSArray starting at givven index searching both ways (no wrap around)

Answer 1

Borrowing from @omz, here is the wrapping variant, which is even simpler:

@implementation NSArray (Extensions)

- (void)enumerateFromIndex:(NSUInteger)index wrapAroundAndGoBothWays:(void (^)(id obj, NSUInteger idx, BOOL *stop))block
{
    BOOL stop = NO;
    NSUInteger actual = index;
    for (NSUInteger i = 0; i < self.count && !stop; i++) {
        actual += (2*(i%2)-1)*i;
        actual = (self.count + actual)%self.count;
        block([self objectAtIndex:actual], actual, &stop);
    }
}

@end