You can use NSPredicate
instead:
NSString *formul = @"15 < 9";
NSPredicate *predicate = [NSPredicate predicateWithFormat:formul];
BOOL b = [predicate evaluateWithObject:nil];
In Swift:
let formula = "15 < 9"
let predicate = NSPredicate(format: formula)
let b = predicate.evaluate(with: nil)
print(b) // false