NSMutableArray替换对象

我想要在NSMutableArray替换一个数组对象，从_alertsArray获取了oldArray，然后修改值，用newArray替换oldArray，但是没实现。

  NSArray *oldArray=(NSArray*)[_alertsArray objectAtIndex:[indexPath row]];

    NSMutableArray *newArray = [[NSMutableArray alloc] initWithArray:oldArray copyItems:YES];

    [newArray replaceObjectAtIndex:3 withObject:@"YES"];

 [_alertsArray replaceObjectAtIndex:[indexPath row] withObject:newArray];

解决办法：

  NSMutableArray *newArray = [_alertsArray mutableCopy];

    [newArray replaceObjectAtIndex:3 withObject:@"YES"];

    _alertsArray = newArray;

    [_alertsArray replaceObjectAtIndex:[indexPath row] withObject:newArray];