Loading data from a dictionary of dictionaries into an array in Objective C for an iphone app

Question

I have a UINavigationController consisting of a tableview I want to load some data into. I have a dictionary plist containing Dictionaries for each Train Line which in turn have dictionaries for each station with the relevant information along with one string lineName. I need to collect the station Names keys and add them to an array to populate my table (This is working).

The line names are stored as a string in my lines dictionary with the key being "lineName"

Root->|
      |
      |->TrainLine1(Dictionary)->|
      |                          |-> lineName (String)
      |                          |-> Station1 (Dictionary)
      |                          |-> Station2 (Dictionary)
      |
      |
      |->TrainLine2(Dictionary)->|
      |                          |-> lineName (String)
      |                          |-> Station1 (Dictionary)
      |                          |-> Station2 (Dictionary)

Am I going about this the wrong way? Should I reorganise my plist? The code below crashes the app.

- (void)viewDidLoad {
     NSString *path = [[NSBundle mainBundle] pathForResource:@"lineDetails" ofType:@"plist"];
     NSDictionary *dictionary = [[NSDictionary alloc] initWithContentsOfFile:path];
     NSDictionary *lineDictionary = [[NSDictionary alloc] initWithDictionary:[dictionary objectForKey:stationNameKey]];
     NSMutableArray *stationsOnLine = [[NSArray alloc] init];

     NSString *key;

     for (key in lineDictionary) {

         NSLog(@"Adding this in array:%@", key);
         //NSString *key2;
         NSString *nameToTry = [NSString stringWithString:key];
         NSLog(@"nameToTry : %@", nameToTry);    
         //NSMutableDictionary *stationDictionary = [[NSDictionary alloc] init];
         if (![key isEqualToString: @"lineName"]) 
         {
             //NSMutableDictionary *stationDictionary = [[NSDictionary alloc] init];
            // NSLog(@"Yes");
             //NSMutableDictionary *tempDict = [[NSDictionary alloc] initWithDictionary:[lineDictionary objectForKey:key]];
             NSMutableDictionary *stationDictionary = [[NSDictionary alloc] initWithDictionary:[lineDictionary objectForKey:key]];
             //stationDictionary = tempDict;
             NSLog(@"Object for key--  %@",[stationDictionary objectForKey:kStationName]);
             [stationsOnLine addObject:[stationDictionary objectForKey:kStationName]];
             [stationDictionary release];
             //[tempDict release];
         }
         /*
         for (key2 in stationDictionary)
         {
             NSLog(@"Adding this in station array:%@", key);
         }
          */

     }
     stationNames = stationsOnLine;
     //[stationDictionary release];

     [stationsOnLine release];
     [lineDictionary release];
     [dictionary release];
 }

Debugger console output:

2010-03-31 00:42:39.842 AMT_Schedule[8395:207] did SelectRow Array contents:deux-montagnes
2010-03-31 00:42:39.844 AMT_Schedule[8395:207] Adding this in array:sunnybrooke
2010-03-31 00:42:39.844 AMT_Schedule[8395:207] nameToTry : sunnybrooke
2010-03-31 00:42:39.845 AMT_Schedule[8395:207] Object for key--  Sunnybrooke
2010-03-31 00:42:39.846 AMT_Schedule[8395:207] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: '*** -[NSCFArray insertObject:atIndex:]: mutating method sent to immutable object'
2010-03-31 00:42:39.847 AMT_Schedule[8395:207] Stack: (
    29320283,
    2521638153,
    29404219,
    29404058,
    107345,
    107124,
    17393,
    3270466,
    3263806,
    3306080,
    3302106,
    3308563,
    3289798,
    3310951,
    3289447,
    15819,
    3066438,
    3049604,
    303530,
    29104832,
    29101128,
    37410325,
    37410522,
    2793391,
    8628,
    8482
)

Answer 1

Your code all looks fine to me (the key variable declared outside the for statement is a little unusual, but I don't think it would blow up). You don't say what kind of crash you're seeing, but the only reason I can think of why that code would crash is if one of the methods is returning nil somewhere along the line and then nil is getting passed as an argument to a method that won't take it. Maybe you're not getting a valid path back from NSBundle, maybe the NSDictionary isn't being properly created from the path, something like that.

Answer 2

 NSMutableArray *stationsOnLine = [[NSArray alloc] init];

Should be

 NSMutableArray *stationsOnLine = [[NSMutableArray alloc] init];

It compiles fine because the Obj-C compiler thinks this an NSMutableArray, but the pointer is actually to an NSArray instance, so at runtime that's what the "mutating method sent to immutable type" error is all about. Normally this would show up as an "unrecognized selector" error, but I suppose the way that arrays are handled internally causes this other, more cryptic error message.