CODEWITHSUNDEEP
iosarraysnsarrayenumeration
hfossli
hfossli

Reputation: 22972

Enumerate NSArray starting at givven index searching both ways (no wrap around)

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 don't want it to wrap around, but rather stop and continue in unexplored direction.

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

How can this be done?

Upvotes: 8

Views: 367

Answers (3)

Basem Saadawy
Basem Saadawy

Reputation: 1818

-(void)enumerateArray:(NSArray *)array inBothDirectionsFromIndex:(int)startIndex
{
    for (int i=0; i<array.count; i++)
    {
        int index = startIndex;
        int indexAfter = startIndex + round(i/2.f) + (i%2 ? 0 : 1);
        int indexBefore = startIndex - round(i/2.f);

        if ((i%2 && indexAfter < array.count) || indexBefore < 0)
        {
            index = indexAfter;
            if (indexBefore < 0)
                index -= indexBefore + 1;
        }
        else if ((i > 0 && indexBefore > -1) || indexAfter > array.count-1)
        {
            index = indexBefore;
            if (indexAfter > array.count-1)
                index -= indexAfter - array.count;;
        }

        id item = [array objectAtIndex:index];

        //Do what you want with the item here

    }
}

Upvotes: 1

omz
omz

Reputation: 53561

Here's a more compact implementation that doesn't require creating subarrays:

@implementation NSArray (Extensions)

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

@end

Upvotes: 7

hfossli
hfossli

Reputation: 22972

It answers the question, but is not especially DRY and creating subarrays makes it less efficient than what it should be. 

@implementation NSArray (Extensions)

- (void)enumerateFromIndex:(NSUInteger)index goBothWaysUsingBlock:(void (^)(id obj, NSUInteger idx, BOOL *stop))block
{
    NSArray *lastObjects = [self subarrayFromIndex:index];
    NSArray *firstObjects = [self subarrayToIndex:index];
    int currentIndex = 0;

    NSEnumerator *firstObjectsEnumerator = [firstObjects reverseObjectEnumerator];
    NSEnumerator *lastObjectsEnumerator = [lastObjects objectEnumerator];
    BOOL shouldStop = NO;
    NSUInteger numberOfIndexesEnumerated = 0;
    id obj = nil;

    while(numberOfIndexesEnumerated < self.count)
    {
        if (obj = [lastObjectsEnumerator nextObject])
        {
            NSInteger objIndex = [self indexOfObject:obj];
            block(obj, objIndex, &shouldStop);
            currentIndex++;
            numberOfIndexesEnumerated++;
        }
        if (shouldStop)
        {
            return;
        }

        if (obj = [firstObjectsEnumerator nextObject])
        {
            NSInteger objIndex = [self indexOfObject:obj];
            block(obj, objIndex, &shouldStop);
            currentIndex++;
            numberOfIndexesEnumerated++;
        }
        if (shouldStop)
        {
            return;
        }
    }
}

@end

Credits to @jalmaas for this answer

Upvotes: 0

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