Question

I have an NSArray of Object that has an interesting property that I would like to use in the following way: Given my array of objects with properties:

Object1 - Property A;
Object2 - Property A;
Object3 - Property B;
Object4 - Property D;
Object5 - Property D;
Object6 - Property D

I want these to be bucket sorted by their properties into a new array:

Array1 - Objects Object1, Object2

Array2 - Objects Object3

Array3 - Objects Object 4, Object5, Object6

And then within each array, sort by using a timeStamp property.

I have tried to accomplish this naively by creating a dictionary, adding interesting objects to the dictionary by property like if ([dictionary objectForKey:@"propertyVal"]) //add object else // create array for key, add object to array. This approach has not worked as expected because I end up needing to dekey the NSMutableDictionary using allKeysForValue, which is not reliable.

I feel that this is a fairly common problem and I would love to hear any insight into how I might go about solving this. Code is great, but even an algorithm (with the appropriate objects to use) should suffice.

Answer 1

It’s not a proper bucket sort, but should work for a set of three properties. A bit of fiddling and you should be able to adjust it for any number of properties:

Edit. I made a dynamic version (just set property type to what you need):

- (NSMutableArray *)order:(NSDictionary *)objects byProperty:(id)property {
    NSMutableSet *propertySet = [NSMutableSet setWithCapacity:5]; // so we can count the unique properties
    for (Object *obj in [objects allValues]) {
        [propertySet addObject:[obj property]];
    }

    NSMutableArray *objectCollections = [NSMutableArray arrayWithCapacity:[propertySet count]];

    // create arrays for every property
    for (int i = 0; i < [objects allValues]; i++) {
        NSMutableArray *collection = [NSMutableArray arrayWithCapacity:5];
        [objectCollections addObject:collection];
    }  
    NSArray *allProperties = [propertySet allObjects];

    // push objects into arrays according to a certain property
    for (Object *obj in [dictionary allValues]) {
        [[objectCollections objectAtIndex:[allProperties indexOfObject:[obj property]] addObject:obj];
    }

    NSMutableArray *result = [NSMutableArray arrayWithCapacity:[objectCollections count]];
    // sort arrays by timestamp
    for (int i = 0; i < [objectCollections count]; i++) {
            [result addObject:[[objectCollections objectAtIndex:i] sortedArrayUsingComparator:^(id obj1, id obj2) {
            if ([(Object *)obj1 timeStamp] > [(Object *)obj2 timeStamp]) {
                return (NSComparisonResult)NSOrderedAscending;
            }
            if ([(Object *)obj1 timeStamp] < [(Object *)obj2 timeStamp]) {
                return (NSComparisonResult)NSOrderedDescending;
            }            
            return (NSComparisonResult)NSOrderedSame;
        }];
    }
    return result;
}