Grouping of Custom Objects in Objective-C

Question

I have and array of custom objects of Person Class

Person : NSObject{
    NSString *firstName;
    NSString *lastName; 
    NSString *age;
}

NSMutableArray          


        

Answer 1

I have created a small library called Linq-to-ObjectiveC which provides a number of methods which make is easier to query arrays. In order to perform the group operation you require you can simply do the following:

NSDictionary* groupedByName = [personsArray groupBy:^id(id person) {
   return [person firstName];
}];

This would return a dictionary where the keys are the distinct first names, and each value is an array of 'person' objects that have the given first name.

Answer 2

You can try this is custom object sorting for an array

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"Firstname" ascending:NO selector:@selector(compare:)];

[Array sortUsingDescriptors:[NSArray arrayWithObjects:sortDescriptor, nil]];

Answer 3

Checkout this answers it is very similar to what you want

https://stackoverflow.com/a/15375756/1190861

The code in your case will be as the following:

NSMutableDictionary *result = [NSMutableDictionary new];

NSArray *distinctNames;
distinctNames = [personsArray valueForKeyPath:@"@distinctUnionOfObjects.firstName"];
for (NSString *name in distinctNames) {
    NSPredicate predicate = [NSPredicate predicateWithFormat:@"firstName = %@", name]; 
    NSArray *persons = [personsArray filteredArrayUsingPredicate:predicate];
    [result setObject:persons forKey:name];
}

NSLog(@"%@", result);

Answer 4

- (NSDictionary *)groupObjectsInArray:(NSArray *)array byKey:(id <NSCopying> (^)(id item))keyForItemBlock
{
    NSMutableDictionary *groupedItems = [NSMutableDictionary new];
    for (id item in array)
    {
        id <NSCopying> key = keyForItemBlock(item);
        NSParameterAssert(key);

        NSMutableArray *arrayForKey = groupedItems[key];
        if (arrayForKey == nil)
        {
            arrayForKey = [NSMutableArray new];
            groupedItems[key] = arrayForKey;
        }
        [arrayForKey addObject:item];
    }
    return groupedItems;
}

Answer 5

A Drop in code to help you doing it,

    -(NSMutableArray *)sortArray:(NSArray *)arrayTosort withKey:(NSString *)key ascending:(BOOL)_ascending
{
    NSSortDescriptor *sortDescriptor;
    sortDescriptor = [[NSSortDescriptor alloc] initWithKey:key
                                                 ascending:_ascending] ;
    NSArray *sortDescriptors = [NSArray arrayWithObject:sortDescriptor];

    return [[arrayTosort sortedArrayUsingDescriptors:sortDescriptors] mutableCopy];
}

here key is key or keypath.

Answer 6

There isn't a built in group by method in NSArray. Your example implementation could be added as a category.

Unfortunately, the @distinctUnionOfObjects operation will only return the strings of the firstName property: John, David. It won't do a group by operation, nor is there such an operation.

Collection Operations