NSDictionary Key For Value/Object?

Question

Can we get the key for an object in an NSDictionary by passing a particular value or object?

Solution 1

-[NSDictionary allKeysForObject:] returns an NSArray of all the keys whose objects match the passed object, where "match" is determined by isEqual:.

Solution 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 lastObject];

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

Solution 3

This is how you can get a first key for object (in case it is an NSString):

NSArray *keys = [yourDic allKeysForObject:yourObject];
NSString *yourKey;
if ([keys count] > 0) {
   yourKey = keys[0];
}

This handles if the object is not found in in the dictionary.

Comments

[Kai Huppmann]

but remember: it gives all keys for objects matching 'isEqual' not '=='

[jscs]

@Kai: True. I should perhaps also add a note about it returning an NSArray although that should be clear from the method name.

[jscs]

If this isn't an FGITW question and answer, I don't know what is. Five upvotes just because I have a search shortcut in my browser.

[AddisDev]

In the case the object is not a string, replace NSString with the object class. This is self explanatory.

[Max Steinmeyer]

You might prefer lastObject to objectAtIndex:0 to avoid throwing an exception if there are no such keys.

[Abizern]

Or even firstObject if you are using the latest SDK