A mutable dictionary can be changed, i.e. you can add and remove objects. An immutable is fixed once it is created.
create and add:
NSMutableDictionary *dict = [[NSMutableDictionary alloc]initWithCapacity:10];
[dict setObject:[NSNumber numberWithInt:42] forKey:@"A cool number"];
and retrieve:
int myNumber = [[dict objectForKey:@"A cool number"] intValue];
alternative:
NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
[dict setValue:[NSNumber numberWithInt:5] forKey:@"age"];
int myAge = [[dict objectForKey:@"age"] intValue];
