Question

I have an NSArray of objects, which has a particular property called name (type NSString).
I have a second NSArray of NSStrings which are names.

I’d like to get an NSArray of all the objects whose .name property matches one of the names in the second NSArray.

How do I go about this, fast and efficiently as this will be required quite often.

Answer 1

With your current data structures, you can only do it in O(n^2) time by looping over the first array once for each member of the second array:

NSMutableArray * array = [NSMutableArray array];
for (NSString * name in names) {
    for (MyObject * object in objects) {
        if ([[myObject name] isEqualToString:name]) {
            [array addObject:object];
        }
    }
}

(Alternate as suggested by Stefan: loop over the objects array and ask the names array if it containsObject: for the name of each object.)

But if this really needs to be faster (really depends on the size of the arrays as much as how often you do it), you can improve this by introducing an NSDictionary that maps the names in the first array to their objects. Then each of those lookups is O(1) and the overall time is O(n). (You’d have to keep this dictionary always in sync with the array of objects, which isn’t hard with reasonable accessors. This technique also has the constraint that the same name can’t appear on more than one object.)

An alternate way of getting this result (and which doesn’t have that last constraint) is to use an NSSet for your second collection, then walk through the objects array calling containsObject: with each one on the set of names. Whether this technique is better depends on whether your two collections are roughly the same size, or if one is much larger than the other.