Error(s):

source_file.m: In function ‘main’:
source_file.m:14:14: error: expected ‘]’ before ‘addObject’
     [friends addObject:@"bff"];
              ^
source_file.m:14:5: error: array subscript is not an integer
     [friends addObject:@"bff"];
     ^
source_file.m:15:23: error: ‘subject’ undeclared (first use in this function)
 NSLog(@"friends:%@", [subject objectAtIndex:0]);       
                       ^
source_file.m:15:23: note: each undeclared identifier is reported only once for each function it appears in