Using NSPredicate's `array[FIRST]` operator

Question

I see in Apple's documentation there's an array[FIRST] operator, which is exactly what I need to write the predicate I want. Unfortunately, the documentation doesn't provide any examples and I don't see how I can clearly put that in a predicate.

So, for instance, I'd have something like [NSPredicate predicateWithFormat:@"array[FIRST] IN %@", @[@1, @2, @3]. This, of course, throws an exception:

Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__NSCFConstantString 0x100001058> valueForUndefinedKey:]: this class is not key value coding-compliant for the key array.'

Because this predicate will eventually go in a core data model file, I need to represent this in a stringly format, so any solutions involving code won't work.

The error indicates that the predicate is being evaluated against a string, not a CoreData managed object. Can you give more detail of the model and what you are trying to achieve? — pbasdf
– pbasdf, Commented Mar 1, 2017 at 9:21
@pbasdf I understand the error. That's not my question. The question is how to correctly extract the first element out of the array given a predicate. Core Data is incidental to this problem. — Jake Bromberg
– Jake Bromberg, Commented Mar 1, 2017 at 16:28
Which array are you trying to extract elements from? An array of managed objects? — pbasdf
– pbasdf, Commented Mar 1, 2017 at 16:33
That's right. It's going to be a fetched property on an entity in an xcdatamodel file. — Jake Bromberg
– Jake Bromberg, Commented Mar 2, 2017 at 17:50
OK. So you want your fetched property to be "the first object that matches this predicate"? If so, I don't think it can be done: a) fetched properties always return an array (even if only one object matches), and b) the model editor will not let you specify a sort order for the FP, so you are taking pot luck as to which matching object is the first. — pbasdf
– pbasdf, Commented Mar 2, 2017 at 18:29

danh · Accepted Answer · 2017-03-01 00:35:41Z

The predicate needs to make sense after substituting the %@ values in the format string and substituting SELF with the object being evaluated. Your predicate says: "is the first object in an unknown symbol called 'array' contained within the literal array 1,2,3?"

Consider the following examples...

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SELF[FIRST] == %@", @1];
BOOL result = [predicate evaluateWithObject:@[@1, @2, @3]];
// result will be YES

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SELF[FIRST] == %@", @2];
BOOL result = [predicate evaluateWithObject:@[@1, @2, @3]];
// result will be NO

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%@[FIRST] == SELF", @[@1, @2, @3]];
BOOL result = [predicate evaluateWithObject:@1];
// result will be YES

NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%@[FIRST] == SELF", @[@1, @2, @3]];
BOOL result = [predicate evaluateWithObject:@2];
// result will be NO

Collectives™ on Stack Overflow

Using NSPredicate's `array[FIRST]` operator

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related