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];