Number of elements in NSArray where (....) - objective-c

Question

I am used to working in C# using the linq extensions (list.select(...).where(...) ext), and I was wondering if there was some way of doing the same sort of thing in Objective-C. This would save me from building a number of rather complicated queries using Core Data, which is great for some things, but perhaps not the best for complex queries (or maybe I'm just uninformed).
Is there some kind of equivalent for linq in Objective-C/Core Data?
EDIT: More specifically, I would like to count the number of elements that fit some criteria. Say my model has a field called date. I am trying to select the distinct dates, and then calculate how many of each date there are. In SQL this would be like a group by, and a COUNT aggregate.

Answer 1

Your question goes from very general ("linq equivalent?") to very specific (computing count by date). I'll just answer your specific question.
Unfortunately, NSArray doesn't have a built-in map or select method, but it does offer NSCountedSet, which will compute what you want:
NSCountedSet *dateSet = [NSCountedSet set];
for (id thing in array) {
[dateSet addObject:[thing date]];
}
for (NSDate *date in dateSet) {
NSLog(#"There are %d instances of date %#", [dateSet countForObject:date], date);
}

Answer 2

Change predicate , and "Date" keys with your props
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"child" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
[fetchRequest setReturnsDistinctResults:YES];
[fetchRequest setPropertiesToFetch:[NSArray arrayWithObject:#"Date"]];
NSPredicate *predicate =[NSPredicate predicateWithFormat:#"(start <= %# ) and (completion < 100)",sda ];
[fetchRequest setPredicate:predicate];
int c = [self.managedObjectContext countForFetchRequest:fetchRequest error:nil];

Answer 3

Here was something posted with comes close
filtering NSArray into a new NSArray in objective-c
Anyway AFAIK you don't have some sort of linq in Objective-C but you have. Arrays and Blocks. And Blocks are functions. So you can really filter on anything in there.

Answer 4

Of course, Cora Data has many functions to make complex queries:
In example to get sum of elements, you have two major ways:
first - get your data to NSSet or NSArray and use #sum operator:
//assume that `childs` are NSArray of your child entities and ammount is attribute to sum, and has attributes start (date) and completion (integer)
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"child" inManagedObjectContext:managedObjectContext];
[fetchRequest setEntity:entity];
NSPredicate *predicate =[NSPredicate predicateWithFormat:#"(start <= %# ) and (completion < 100)", dzis];
[fetchRequest setPredicate:predicate];
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:managedObjectContext sectionNameKeyPath:nil cacheName:#"Root"];
NSError *error = nil;
if ([afetchedResultsController performFetch:&error]) {
NSArray *childs = afetchedResultsController.fetchedObjects;
NSNumber *sum=[childs valueForKeyPath:"#sum.ammount"];
}
second is using specific fetch for specific value with added NSExpressionDescription with a sum. This way is harder but better for larger db's

Answer 5

suppose if you have an array of your model objects, you could that with the following statement,
NSArray *distintDatesArray = [array valueForKeyPath:#"#distinctUnionOfObjects.date"];
for ( NSDate *date in distintDatesArray)
{
NSLog (#"Date :%# ,count : %d",date,[[array filteredArrayUsingPredicate:[NSPredicate predicateWithFormat:#"date = %#",date]] count]);
}
This will have same effect as the group by query.