稳定排序是一种保持具有相同值的元素的相对顺序的排序。
ArrayList.Sort上的文档说,当IComparer
提供 an 时,排序是稳定的:
如果 comparer 设置为 null,则此方法执行比较排序(也称为不稳定排序);也就是说,如果两个元素相等,则可能不会保留它们的顺序。相反,稳定排序保留了相等元素的顺序。要执行稳定排序,您必须实现自定义 IComparer 接口。
除非我遗漏了什么,否则以下测试用例表明ArrayList.Sort
没有使用稳定的排序:
internal class DisplayOrdered {
public int ID { get; set; }
public int DisplayOrder { get; set; }
public override string ToString() {
return string.Format("ID: {0}, DisplayOrder: {1}", ID, DisplayOrder);
}
}
internal class DisplayOrderedComparer : IComparer {
public int Compare(object x, object y) {
return ((DisplayOrdered)x).DisplayOrder - ((DisplayOrdered)y).DisplayOrder;
}
}
[TestFixture]
public class ArrayListStableSortTest {
[Test]
public void TestWeblinkCallArrayListIsSortedUsingStableSort() {
var call1 = new DisplayOrdered {ID = 1, DisplayOrder = 0};
var call2 = new DisplayOrdered {ID = 2, DisplayOrder = 0};
var call3 = new DisplayOrdered {ID = 3, DisplayOrder = 2};
var list = new ArrayList {call1, call2, call3};
list.Sort(new DisplayOrderedComparer());
// expected order (by ID): 1, 2, 3 (because the DisplayOrder
// is equal for ID's 1 and 2, their ordering should be
// maintained for a stable sort.)
Assert.AreEqual(call1, list[0]); // Actual: ID=2 ** FAILS
Assert.AreEqual(call2, list[1]); // Actual: ID=1
Assert.AreEqual(call3, list[2]); // Actual: ID=3
}
}
我错过了什么吗?如果不是,这是文档错误还是库错误?
显然在 Linq 中使用OrderBy可以提供稳定的排序。