Creating NSDictionary with keys that have multiple values - objective-c

Question

I have a mutable array that contains NSDictionary dic1 objects,
each dictionary has a key called contactId, more than one dictionary can have the same value for contactId.
What I want to do is to create an NSDictionary with unique contactIds as the keys and an array value that contains a list of all NSDictionary dic1 objects that have the value contactId equal to the key.
How can I do this?
My data looks like this:
**myArray**:[ **dic1** {contactId = x1 , name = name1 }, **dic2**{contactId = x2, name =
name2 }, **dic3**{contactId = x1, name = name3} ]
I want it to become like this:
**NSDictionary**: { **x1**:[dic1, dic3], **x2**:[dic2] }

Answer 1

Use fast enumeration:
NSMutableDictionary *result = [NSMutableDictionary dictionary];
for (id obj in myArray)
{
NSString *contactId = [obj objectForKey:#"contactId"];
NSMutableSet *contacts = [result objectForKey:contactId];
if (!contacts)
{
contacts = [NSMutableSet set]
[result setObject:contacts forKey:contactId];
}
[contacts addObject:obj];
}
You could use blocks for no real added benefit:
__block NSMutableDictionary *result = [NSMutableDictionary dictionary];
[myArray enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop){
NSString *contactId = [obj objectForKey:#"contactId"];
NSMutableSet *contacts = [result objectForKey:contactId];
if (!contacts)
{
contacts = [NSMutableSet set]
[result setObject:contacts forKey:contactId];
}
[contacts addObject:obj];
}];

Answer 2

How about the classic way?
NSMutableDictionary* Result;
NSEnumerator* Enumerator;
NSDictionary* Dict;
Result=[[NSMutableDictionary alloc] init];
Enumerator=[YourArray objectEnumerator];
while ((Dict=[Enumerator nextObject])!=nil)
{
NSString* ContactID;
NSMutableSet* Contacts;
ContactID=[Dict objectForKey:#"contactID"];
Contacts=[Result objectForKey:ContactID];
if (Contacts==nil)
{
Contacts=[[NSMutableSet alloc] init];
[Result setObject:Contacts forKey:ContactID];
[Contacts release];
}
[Contacts addObject:Dict];
}
This should create a Result dictionary. I haven't tested (or even compiled) this, though.