ios - 从给定索引开始枚举 NSArray 双向搜索（无环绕）

Question

例子。我有一个包含 15 个对象的数组。我想从给定的索引开始枚举。说从索引 5 开始，然后是上面的索引，下面的索引，上面的索引，下面的等等……我不希望它环绕，而是停止并继续朝着未知的方向前进。

所以我的例子中的索引顺序是。5, 6, 4, 7, 3, 8, 2, 9, 1, 10, 0, 11, 12, 13, 14

如何才能做到这一点？

Answer 1

这是一个不需要创建子数组的更紧凑的实现：

@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

Answer 2

-(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

    }
}

Answer 3

它回答了这个问题，但并不是特别干燥，并且创建子数组使其效率低于应有的效率。

@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

此答案归功于@jalmaas