parsing JSON of webservice on objective c array

If you are doing it through NSJSONSerialization you can get array of assignment_title using this simple method ;)

NSError *error = nil;
NSData *jsonData = [NSData dataWithContentsOfURL:apiURL]; 
id jsonObjectFound = [NSJSONSerialization JSONObjectWithData:jsonData options:NSJSONReadingMutableContainers error:&error];
NSArray* assignmentTitles = [jsonObjectFound valueForKey:@"assignment_title"];

If performance matters, you might consider using an ASIHTTPRequest to fetch the json asynchronously, then inside the requestFinished: you might do something like:

- (void)requestFinished:(ASIHTTPRequest *)request
{
   // Use when fetching text data
   NSString *responseString = [request responseString];

   //assuming you created a property instance variable NSArray *myArrAssignmentTitles
   NSArray *tempArray = [responseString JSONValue];
   //making an array of assignment_title
   NSMutableArray *tempMutArray = [[NSMutableArray alloc] init];
   int i;
   for(i = 0;i < [tempArray count];i++){
       [tempMutArray addObject:[[tempArray objectAtIndex:i] objectForKey:@"assignment_title"]];
   }
   //assign the data to the instance variable NSArray *myArrAssignmentTitles
   self.myArrAssignmentTitles = tempMutArray;
   //release tempMutArray since the instance variable has it
   [tempMutArray release];

   //call the reload table
   [self.tableView reloadData];//i think this is how to reload the table
}

- (void)requestFailed:(ASIHTTPRequest *)request
{
    NSError *error = [request error];
}

So, your myArrAssignmentTitles has all the values assignment_title from json all you do is just apply the array data for the cell e.g.

  cell.textLabel.text = [self.myArrAssignmentTitles objectAtIndex:indexPath.row];

its a long code sorry about that. But, thats works for me xD; it fetches the json asynchronously after that it creates an array of assignment_title hopes it helps.