Using NSPredicate to filter an NSMutableArray of NSDictionaries to find if a "key" exists - objective-c

Question

I have an NSMutableArray of dictionaries. I am using NSPredicate to filter through the array to find if a dictionary with a particular key exists or not.
I have referred to various examples, one of the closest is here: Using NSPredicate to filter an NSArray based on NSDictionary keys. However, I don't wish to have a value to the key. My problem is that I want to find the key first. I tried different syntaxes, but it did not help.
What I have done so far:
NSString *key = #"open_house_updated_endhour";
NSPredicate *predicateString = [NSPredicate predicateWithFormat:#"%K", key];
//NSPredicate *predicateString = [NSPredicate predicateWithFormat:#"%K contains [cd]", key]; Doesn't work.
//NSPredicate *predicateString = [NSPredicate predicateWithFormat:#"%K == %#", key]; Won't work because it expects a value here.
NSLog(#"predicate %#",predicateString);
NSArray *filtered = [updatedDateAndTime filteredArrayUsingPredicate:predicate]; // updatedDateAndTime is the NSMutableArray

Answer 1

A dictionary delivers nil as value for absent key. So simply compare the key against nil.
NSPredicate *predicateString = [NSPredicate predicateWithFormat:#"%K==NULL", key]; // Dictionaries not having the key
NSPredicate *predicateString = [NSPredicate predicateWithFormat:#"%K!=NULL", key]; // Dictionaries having the key

Answer 2

Try it:
NSArray *Myarray = [NSArray arrayWithObject:[NSMutableDictionary dictionaryWithObject:#"my hello string" forKey:#"name"]];
NSArray *filteredarray = [Myarray filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:#"(name == %#)", #"my hello string"]];
NSLog("%#",filteredarray);
Another example :
NSString *mycategory = #"iamsomeone";
NSArray *myitems = #[#{ #"types" : #[#"novel", #"iamsomeone", #"dog"] },
#{ #"types" : #[#"cow", #"iamsomeone-iam", #"dog"] },
#{ #"types" : #[#"cow", #"bow", #"cat"] }];
NSPredicate *mypredicate = [NSPredicate predicateWithBlock:^BOOL(id evaluatedObject, NSDictionary *bindings) {
NSArray *categories = [evaluatedObject objectForKey:#"types"];
return [categories containsObject:mycategory];
}];
NSArray *outpuArray = [myitems filteredArrayUsingPredicate:mypredicate];
NSLog(#"hello output:%#",outpuArray);