filtering NSArray into a new NSArray in Objective-C

NSArray and NSMutableArray provide methods to filter array contents. NSArray provides filteredArrayUsingPredicate: which returns a new array containing objects in the receiver that match the specified predicate. NSMutableArray adds filterUsingPredicate: which evaluates the receiver’s content against the specified predicate and leaves only objects that match. These methods are illustrated in the following example.

NSMutableArray *array =
    [NSMutableArray arrayWithObjects:@"Bill", @"Ben", @"Chris", @"Melissa", nil];

NSPredicate *bPredicate =
    [NSPredicate predicateWithFormat:@"SELF beginswith[c] 'b'"];
NSArray *beginWithB =
    [array filteredArrayUsingPredicate:bPredicate];
// beginWithB contains { @"Bill", @"Ben" }.

NSPredicate *sPredicate =
    [NSPredicate predicateWithFormat:@"SELF contains[c] 's'"];
[array filteredArrayUsingPredicate:sPredicate];
// array now contains { @"Chris", @"Melissa" }

There are loads of ways to do this, but by far the neatest is surely using [NSPredicate predicateWithBlock:]:

NSArray *filteredArray = [array filteredArrayUsingPredicate:[NSPredicate predicateWithBlock:^BOOL(id object, NSDictionary *bindings) {
    return [object shouldIKeepYou];  // Return YES for each object you want in filteredArray.
}]];

I think that's about as concise as it gets.

Swift:

For those working with NSArrays in Swift, you may prefer this even more concise version:

let filteredArray = array.filter { $0.shouldIKeepYou() }

filter is just a method on Array (NSArray is implicitly bridged to Swift’s Array). It takes one argument: a closure that takes one object in the array and returns a Bool. In your closure, just return true for any objects you want in the filtered array.