Question

I have an NSArray of NewsArticle objects:

@interface NewsArticle : NSObject

@property (nonatomic, strong) NSString *title;
@property (nonatomic, strong) NSDate *dateTime;
@property (nonatomic, strong) NSString *content;

@end

And I want to create an NSArray of the unique dates (ignoring time) stored within each article.dateTime – I know I can do this the long way around by enumerating the array and doing the checking myself, but I’m hoping for a simpler solution.

I’m from a C# background and I know in C# I could do this with a simple LINQ statement – it would seem that NSPredicate might offer this functionality, but I can’t work out how, or find any suitable examples online.

Answer 1

NSPredicate is designed for filtering a collection of objects, but you want to extract a property from each member of the collection.

You can (as CAMOBAP said in his comment) use key-value coding (KVC) to get an array of all the dateTime values. Then you can use an NSSet to eliminate the duplicates:

NSArray *allDateTimes = [articles valueForKey:@"dateTime"];
NSSet *uniqueDateTimes = [NSSet setWithArray:allDateTimes];

You can even do it all in one KVC message, using the somewhat esoteric @distinctUnionOfObjects key:

NSArray *uniqueDateTimes = [articles valueForKeyPath:@"@distinctUnionOfObjects.dateTime"];

Note that this second example uses valueForKeyPath:, not valueForKey:.

You can learn more (oh so much more!) by reading the Key-Value Coding Programming Guide.