Why can't I cast this NSArray to an NSMutableArray to sort it?

Question

I am serialising some JSON data into a dictionary. The dictionary contains a "status" object, and an array of dictionaries (one key of which is "time_dropped", by which I want to sort the array).

However, I get an exception claiming that a "mutating method [was] sent to [an] immutable object". Here's the relevant piece of code. I did declare pins as a NSMutableArray, by the way.

NSDictionary *dataDictionary = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingMutableLeaves error:&localError];
NSString *status = [dataDictionary objectForKey:@"status"];
NSLog(@"status: %@", status);

if ([status isEqualToString:@"Okay"]) {
    self.pins = (NSMutableArray *) [dataDictionary objectForKey:@"pins"];

    NSSortDescriptor *timeDescriptor = [NSSortDescriptor
                                           sortDescriptorWithKey:@"time_dropped"
                                           ascending:YES selector:@selector(compare:)];

    NSSortDescriptor *titleDescriptor = [NSSortDescriptor
                                            sortDescriptorWithKey:@"title"
                                            ascending:YES selector:@selector(caseInsensitiveCompare:)];

    NSArray *descriptors = @[timeDescriptor, titleDescriptor];
    [self.pins sortUsingDescriptors:descriptors];

Rather than apologize for the bad formatting, please just fix it, as rmaddy has kindly done for you. — jscs, Jun 03 '14 at 19:55
or you could use the `NSArray` method `sortedArrayUsingDescriptors:`. — CrimsonChris, Jun 03 '14 at 19:56
Just because you said the `pins` reference points to an `NSMutableArray` will NOT stop you from pointing it at WHATEVER you want. I could do this... `self.pins = (id)[[UIViewController alloc] init];` and the compiler won't do anything to stop me. — CrimsonChris, Jun 03 '14 at 19:58
Note that if you need mutable arrays/dictionaries in a JSON representation, NSJSONSerialization has an option to provide that, so you don't need to make copies. — Hot Licks, Jun 03 '14 at 20:37
@JoshCaswell, I tried to fix it, but couldn't figure out how. Relax. — Rory Byrne, Jun 04 '14 at 08:11

score 2 · Answer 1 · answered Jun 03 '14 at 19:54

You can't just turn a non-mutable array to a mutable array by casting. You need to create a new NSMutableArray:

NSMutableArray *array2 = [NSMutableArray arrayWithArray:array];

Or in your case:

self.pins = [NSMutableArray 
                 arrayWithArray:(NSArray *)[dataDictionary objectForKey:@"pins"]];

Why can't I cast this NSArray to an NSMutableArray to sort it?

1 Answers1