Read/Write plist on iPhone iOS 5 - ios5

Question

I am studying iPhone development and facing a problem with a reading/writing plist file. I followed an example from a iPhone development book but keep getting an error message when running.
The error message says : 2012-04-26 00:21:09.759 FileHandling[5915:207] -[__NSCFDictionary addObject:]: unrecognized selector sent to instance 0x685ac40
Here is the example code (it seems fine to me...though):
NSString *plistFileName = [[self documentPath] stringByAppendingPathComponent: #"Apps.plist"];
NSLog(#"Where is the file? => %#", plistFileName);
if ([[NSFileManager defaultManager] fileExistsAtPath:plistFileName]) {
NSDictionary *dict = [[NSDictionary alloc] initWithContentsOfFile:plistFileName];
for (NSString *category in dict) {
NSLog(#"%#", category);
NSLog(#"=========");
NSArray *titles = [dict valueForKey:category];
for (NSString *title in titles) {
NSLog(#"%#", title);
}
}
} else {
NSString *plistPath = [[NSBundle mainBundle] pathForResource:#"Apps" ofType: #"plist"];
NSLog(#"%#", plistPath);
NSDictionary *dict = [[NSDictionary alloc] initWithContentsOfFile: plistPath];
NSLog(#"Let's take a look : %#", dict);
NSMutableDictionary *copyOfDict = [dict mutableCopy];
NSLog(#"Let's look at the mutable dictationary : %#", copyOfDict);
NSArray *categoriesArray = [[copyOfDict allKeys] sortedArrayUsingSelector: #selector(compare:)];
for (NSString *cateogry in categoriesArray) {
NSArray *titles = [dict valueForKey: cateogry];
NSMutableArray *mutableTitles = [titles mutableCopy];
[mutableTitles addObject: #"New App Title"];
[copyOfDict setObject: mutableTitles forKey:cateogry];
}
NSString *fileName = [[self documentPath] stringByAppendingPathComponent: #"Apps.plist"];
[copyOfDict writeToFile: fileName atomically:YES];
}

Answer 1

According to the error message, the problem is occurring in the call to addObject: on an __NSCFDictionary. This means that, at runtime, a dictionary received a message to add an object.
However, in this code snippet, addObject: is apparently being sent to an NSMutableArray. This probably means that each object titles you're retrieving from dict in the last for-loop is not an array, but in fact another dictionary, that your code is simply referring to as an array.
Indeed, your code does seem well-formed, so check the well-formedness of your source plist; open it up in a plain text editor. Also, you use a ton of logging, so confirm this way: in the output, dictionaries (including the root entry) are denoted by {curly = braces}, where arrays are denoted by (round parentheses).