Objective C: Create arrays from first array based on value

You can use a NSMutableDictionary of NSMutableArrays - if the state encountered isn't yet in the dictionary, add a new array.

NSMutableArray* arr = [states objectForKey:state];
if (arr == nil) {
    arr = [NSMutableArray array];
    [states setObject:arr forKey:state];
}

Then you can insert values into the array, preferably as objects though as Dave DeLong mentions.

You shouldn't be maintaining this data as CSV. That's asking for a world of hurt if you ever need to manipulate this data programmatically (such as what you're trying to do).

You can naïvely break this data up into an array using NSArray * portions = [line componentsSeparatedByString:@","];. Then create a custom object to store each portion (for an example, see this post), and then you can manipulate those objects almost effortlessly.

Naively: (assuming array of strings called strings)

NSMutableDictionary *states = [NSMutableDictionary dictionary];
for (NSString *string in strings) {
  NSString *state = [[string componentsSeparatedByString:@", "] lastObject];  
  NSMutableArray *values = [states objectForKey:state];
  if (values == nil) {
     values = [NSMutableArray array];
     [states setObject:value forKey:state];
  }
  [values addObject:string];
}

Number of things about this -- first of all, I'm not at my computer, so there is a high chance of typos and or things that I missed. Second, you probably want to adapt the components separated by string line to handle whitespace better.