NSArray+enumerableExtras.h

@interface NSArray (EnumerableExtras)
/**
 * similar to a map
 */
- (NSArray *) objectsTransformedByBlock:(id (^)(id obj, NSUInteger idx, BOOL *stop))block;

/**
 * filter the array
 */
- (NSArray *) objectsPassingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate;

/**
 * takes the return value of the block as the key and maps it to the object for it
 */

- (NSDictionary *) dictionaryOfObjectsWithKeyBlock:(id (^)(id obj, NSUInteger idx, BOOL *stop))block;
@end

NSArray+enumerableExtras.m

#import "NSArray+enumerableExtras.h"


@implementation NSArray (EnumerableExtras)

- (NSArray *) objectsTransformedByBlock:(id (^)(id obj, NSUInteger idx, BOOL *stop))block {
	NSMutableArray *ret = [NSMutableArray arrayWithCapacity:[self count]];
	
	[self enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
		[ret addObject:block(obj, idx, stop)];
	}];
	
	return ret;
}


- (NSArray *) objectsPassingTest:(BOOL (^)(id obj, NSUInteger idx, BOOL *stop))predicate {
	return [self objectsAtIndexes:[self indexesOfObjectsPassingTest:predicate]];
}


- (NSDictionary *) dictionaryOfObjectsWithKeyBlock:(id (^)(id, NSUInteger, BOOL *))block {
	NSArray *keys = [self objectsTransformedByBlock:block];
	return [NSDictionary dictionaryWithObjects:self forKeys:keys];
}

@end

NSSet+enumerableExtras.h

@interface NSSet (EnumerableExtras)
/**
 * set difference. self - other
 */
-(NSSet *) setByRemovingObjectsInSet:(NSSet *)other;

/**
 * set difference. self - other
 */
-(NSSet *) setByRemovingObjectsInArray:(NSArray *)other;


/**
 * set intersection
 */
-(NSSet *) setByKeepingObjectsInSet:(NSSet *)other;

/**
 * set intersection
 */
-(NSSet *) setByKeepingObjectsInArray:(NSArray *)other;

@end

NSSet+enumerableExtras.m

#import "NSSet+enumerableExtras.h"


@implementation NSSet (EnumerableExtras)
-(NSSet *) setByRemovingObjectsInSet:(NSSet *)other {
	return [self objectsPassingTest:^(id obj, BOOL *stop) {
		return (BOOL)![other containsObject:obj];
	}];
}

-(NSSet *) setByRemovingObjectsInArray:(NSArray *)other {
	return [self setByRemovingObjectsInSet:[NSSet setWithArray:other]];
}

-(NSSet *) setByKeepingObjectsInSet:(NSSet *)other {
	return [self objectsPassingTest:^(id obj, BOOL *stop) {
		return (BOOL)[other containsObject:obj];
	}];
}

-(NSSet *) setByKeepingObjectsInArray:(NSArray *)other {
	return [NSSet setWithArray:[other objectsPassingTest:^(id obj, NSUInteger idx, BOOL *stop) {
		return [self containsObject:obj];
	}]];
}

@end