What is a good way to store a persistent array? - ios

Question

I have an NSMutableArray of objects that I would like to persist between user sessions. Each object has an NSInteger property, and I would like to be able to access all of the objects for calculations using that property (averages, mins, maxes, etc.)
Would it be better to store that array into a Core Data database or just store my objects individually into the database?
EDIT: The array should hold no more than 1,000 items.

Answer 1

Depends on the size. If it's simple just archive the array to a file
Edited to add
If all you want to persist is an array you don't need to use the archiver methods, you can do it directly.
Imagine you have a file URL in your app's Documents directory called persistentURL which you want to use to save and read your array from.
All you need to do to persist the array is:
[yourArray writeToURL:persistentURL atomically:YES];
And when you want to read the data back into an array it's as simple as:
yourArray = [NSArray arrayWithContentsOfURL:persistentURL];
You can use this method here because the array contains only NSNumbers which are property list objects (i.e. objects that can be put directly into a plist).

Answer 2

NSArray and NSMutableArray can't contain integers directly. It has to contain NSNumber objects.
Anyway, I'd just use NSCoding. Try the methods archiveRootObject:toFile: (which writes directly to a file) or archivedDataWithRootObject (which converts the content to a byte-stream in NSData).
For <=1000 items it should be fast enough.
If you're reading and writing the array enough, you might want to consider using a malloc'ed C array of "long"s, and saving that as bytes to a file. That would be faster, and you could read/write values to your C array easily, instead of having to do object replacement. On the other hand resizing the array or inserting values is a pain with a C array, and you also have to be aware of data size and byte ordering issues if you will ever transfer this data between devices.
Edit: I just wrote a little test, and writing an NSArray of 1,000 NSNumbers to a file with archiveRootObject:toFile: takes about 0.0038 seconds on on old iPhone 4 I have around for testing. That's the slowest machine that supports iOS 7, and it's 3.8 THOUSANDTHS of a second, to write 1,000 items. So writing an array of 10,000 items should take 3.8 hundredths of a second or less.
Unless you are reading and writing this array many times a second in a loop, using archiveRootObject:toFile: is plenty fast enough.

Answer 3

Personally, I would use the following to store that array:
[[NSUserDefaults standardUserDefaults] setObject:myArray ForKey:#"myArray"]
and retrieve it anywhere in the app:
NSMutableArray *myArray = [[[NSUserDefaults standardUserDefaults] objectForKey:#"myArray"] mutableCopy];
That seems to me to be the easiest way to persist that data. You can store the mutable array in NSUser defaults, and I believe you'll need to create a mutable copy of it when you retrieve it later on to be able to continue to make changes to it.