Objective-C NSMutableArray: Problem with sorting objects?

Question

I currently have an object called Station defined as:

@interface RFStation : NSObject {

    NSString *stationID; 
    NSString *callsign; 
    NSString *logo; 
    NSString *frequency;
@end

I also have an NSMutableArray containing a list of 'Station' objects. I need to have a method to sort these objects by the 'stationState' attribute.

I implemented this method:

NSComparisonResult compareCallSign(RFStation *firstStation, RFStation *secondStation, void *context) {
    if ([firstStation.stationState compare:secondStation.stationState]){
        return NSOrderedAscending;
    }else{
        return NSOrderedDescending;
    }

}

and call it using:

[stationList sortedArrayUsingFunction:compareState context:nil];
[self.tableView reloadData] 

(the data is loaded into a UITableViewController)

Can someone please tell me why my NSMutableArray is not getting sorted?

Answer 1

well something obvious, you've named your sort method compareCallSign and you've passed compareState to sortedArrayUsingFunction

Answer 2

-sortedArrayUsingFunction:… does not modify the array. It just copies the array, sort it, and return the new sorted one.

To sort the mutable array in-place, use the -sortUsingFunction:context: method.

[stationList sortUsingFunction:compareState context:nil];
[self.tableView reloadData];

---

BTW,

1. The -compare: method already returns a NSComparisonResult. Your function should be implemented as

   NSComparisonResult compareCallSign(...) {
       return [firstStation.stationState compare:secondStation.stationState];
   }
   

2. you could just use an NSSortDescriptor.

   NSSortDescriptor* sd = [NSSortDescriptor sortDescriptorWithKey:@"stationState" 
                                                        ascending:YES];
   [stationList sortUsingDescriptors:[NSArray arrayWithObject:sd]];
   [self.tableView reloadData];