How to get the key for a given object from an NSMutableDictionary?

Question

I have an object which is in an big NSMutableDictionary, and need to find out which key this has. So I want to look up that "table" from both columns. Not only with keys, but also with objects (to get keys). Is that possible?

Answer 1

Look to the parent class (NSDictionary)

- (NSArray *)allKeysForObject:(id)anObject

Which will return a NSArray of all the keys for a given Object Value. BUT it does this by sending an isEqual message to each object of the Dictionary so for your large dataset this may not be best performance.

Maybe you need to hold some form of additional indexing structure structure(s) to allow you to locate the objects on some critical values within them, linked to the key without direct object comparison

Answer 2

To answer you question in a more specific manner, use the following to get a key for a particular object:

NSString *knownObject = @"the object";
NSArray *temp = [dict allKeysForObject:knownObject];
NSString *key = [temp objectAtIndex:0];

//"key" is now equal to the key of the object you were looking for

Answer 3

Take a look at:

- (NSArray *)allKeysForObject:(id)anObject

Answer 4

That is definitely possible with NSDictionary's block method

- (NSSet *)keysOfEntriesPassingTest:(BOOL (^)(id key, id obj, BOOL *stop))predicate;

You need to return objects which satisfy some condition (predicate).

Use it like this:

    NSSet *keys = [myDictionary keysOfEntriesPassingTest:^BOOL(id key, id obj, BOOL *stop) {

       BOOL found = (objectForWhichIWantTheKey == obj);
       if (found) *stop = YES;
       return found;

    }];

Check out this answer for more details

How do I specify the block object / predicate required by NSDictionary's keysOfEntriesPassingTest?