NSMutableArray 添加NSArray问题

不知道怎么使用NSMutableArray和NSArray。

代码：

for (int i=0; i<[DateSortArry2 count]; i++) {
        for (int j=0; j<[DateSortArry2Copy count]; j++) { 
            NSString *sectiondateStr2 = [NSString stringWithFormat:@"%@",[DateSortArry2Copy objectAtIndex:j]];    
            if ([[DateSortArry2 objectAtIndex:i] isEqualToString:sectiondateStr2]) {  
                [Arry addObject:sectiondateStr2];   
            }
        }
        [SumArry addObjectsFromArray:Arry];
        [Arry removeAllObjects];
    }

代码得到的结果：

SumArry:(
"20130227",
"20130227",
"20130227",
"20130226",
"20130226",
"20130226",
"20130225",
"20130225")

要求的结果：

SumArry:((
    "20130227",
    "20130227",
    "20130227",
    ),
    (
    "20130226",
    "20130226",
    "20130226",
    ),
    (
    "20130225",
    "20130225"
    ))

不知道应该怎么实现。

NSString *str = @"";
for (int i=0; i<[DateSortArry2 count]; i++)
 {
       if (str isEqualToString:[DateSortArry2 objectAtIndex:i])
       {
            return;
       }
       else
       {
            NSMutableArray * Arry = [[NSMutableArray alloc] init];
            str = [DateSortArry2 objectAtIndex:i]
            for (int j=0; j<[DateSortArry2Copy count]; j++)
            {
                if ([[DateSortArry2 objectAtIndex:i] isEqualToString:str])
                {
                    [Arry addObject:str];
                }
            }
            [SumArry addObject:Arry];
            [Arry removeAllObjects];
        }
}