在C#SortedDictionary中等效于Java的SortedMap.tailMap

我有使用SortedMap.tailMap的Java代码.在我的移植代码中,我有SortedMap = Dictionary< IComparable,value>.我需要一种在C#中复制/模仿tailMap的方法.

我已经想到了以下内容:

myDictionary.Where(p => p.Key.CompareTo(value)> = 0).ToDictionary

这将返回一个Dictionary,并且我需要返回SortedDictionary.我可以从Dictionary创建一个SortedDictionary,但是我觉得应该已经有一种更优雅,更完美的方法来做到这一点.

另一个想法是做类似的事情

var newSD = new SortedDictionary<k,v>();
foreach (var p in oldDictionary.Where(p => p.Key.CompareTo(value) >= 0))
    newSD.Add(p.Key, p.Value);

那应该行得通,我不确定在构建列表时按排序顺序添加值将如何影响插入的时间.

还有其他想法吗?

> Java SortedMap
> C# SortedDictionary

最佳答案
我过去几次需要此功能,而afaik做到这一点最简单的方法是

>创建并填充索引可访问的SortedList<K,V>(与SortedDictionary相反,这就是为什么您不能在此处使用它)
>在IList<K> SortedList.Keys上使用suitable*) BinarySearch method
>通过IList<V> SortedList.Values访问值
>将2和3.包装到扩展方法IEnumerable< KeyValuePair< K,V>中.尾巴(此SortedList< K,V>列表,K fromKey,包含布尔值= true)
>将其张贴在此处作为答案,这样我就可以复制并粘贴自此以后,我刚刚实现了Head and Tail并附加了代码-参见下文.另外,我添加了TryGetCeiling / Floor / Higher / LowerValue方法-从Java的NavigableMap派生的名称,为需要它们的任何人添加TryGetKey和TryGetEntry都很简单.

*)不幸的是,.Net自己的实现仅适用于列表,不适用于ILists.真是浪费.此外,请确保使用未找到该项目的返回〜x的项目,例如我链接到的项目.

public static class SortedListEx
{
    public static IEnumerable<KeyValuePair<K, V>> Head<K, V>(
        this SortedList<K, V> list, K toKey, bool inclusive = true)
    {
        https://stackoverflow.com/a/2948872/709537 BinarySearch
        var binarySearchResult = list.Keys.BinarySearch(toKey);
        if (binarySearchResult < 0)
            binarySearchResult = ~binarySearchResult;
        else if (inclusive)
            binarySearchResult++;
        return System.Linq.Enumerable.Take(list, binarySearchResult);
    }

    public static IEnumerable<KeyValuePair<K, V>> Tail<K, V>(
        this SortedList<K, V> list, K fromKey, bool inclusive = true)
    {
        https://stackoverflow.com/a/2948872/709537 BinarySearch
        var binarySearchResult = list.Keys.BinarySearch(fromKey);
        if (binarySearchResult < 0)
            binarySearchResult = ~binarySearchResult;
        else if (!inclusive)
            binarySearchResult++;
        return new ListOffsetEnumerable<K, V>(list, binarySearchResult);
    }

    public static bool TryGetCeilingValue<K, V>(
        this SortedList<K, V> list, K key, out V value)
    {
        var binarySearchResult = list.Keys.BinarySearch(key);
        if (binarySearchResult < 0)
            binarySearchResult = ~binarySearchResult;
        if (binarySearchResult >= list.Count)
        {
            value = default(V);
            return false;
        }
        value = list.Values[binarySearchResult];
        return true;
    }

    public static bool TryGetHigherValue<K, V>(
        this SortedList<K, V> list, K key, out V value)
    {
        var binarySearchResult = list.Keys.BinarySearch(key);
        if (binarySearchResult < 0)
            binarySearchResult = ~binarySearchResult;
        else
            binarySearchResult++;
        if (binarySearchResult >= list.Count)
        {
            value = default(V);
            return false;
        }
        value = list.Values[binarySearchResult];
        return true;
    }

    public static bool TryGetFloorValue<K, V>(
        this SortedList<K, V> list, K key, out V value)
    {
        var binarySearchResult = list.Keys.BinarySearch(key);
        if (binarySearchResult < 0)
            binarySearchResult = ~binarySearchResult;
        else
            binarySearchResult++;
        if (binarySearchResult >= list.Count)
        {
            value = default(V);
            return false;
        }
        value = list.Values[binarySearchResult];
        return true;
    }

    public static bool TryGetLowerValue<K, V>(
        this SortedList<K, V> list, K key, out V value)
    {
        var binarySearchResult = list.Keys.BinarySearch(key);
        if (binarySearchResult < 0)
            binarySearchResult = ~binarySearchResult;
        if (binarySearchResult >= list.Count)
        {
            value = default(V);
            return false;
        }
        value = list.Values[binarySearchResult];
        return true;
    }

    class ListOffsetEnumerable<K, V> : IEnumerable<KeyValuePair<K, V>>
    {
        private readonly SortedList<K, V> _sortedList;
        private readonly int _offset;

        public ListOffsetEnumerable(SortedList<K, V> sortedList, int offset)
        {
            _sortedList = sortedList;
            _offset = offset;
        }

        public IEnumerator<KeyValuePair<K, V>> GetEnumerator()
        {
            return new ListOffsetEnumerator<K, V>(_sortedList, _offset);
        }

        IEnumerator IEnumerable.GetEnumerator() { return GetEnumerator(); }
    }

    class ListOffsetEnumerator<K, V> : IEnumerator<KeyValuePair<K, V>>
    {
        private readonly SortedList<K, V> _sortedList;
        private int _index;

        public ListOffsetEnumerator(SortedList<K, V> sortedList, int offset)
        {
            _sortedList = sortedList;
            _index = offset - 1;
        }

        public bool MoveNext()
        {
            if (_index >= _sortedList.Count)
                return false;
            _index++;
            return _index < _sortedList.Count;
        }

        public KeyValuePair<K, V> Current
        { 
            get
            {
                return new KeyValuePair<K, V>(
                    _sortedList.Keys[_index],
                    _sortedList.Values[_index]);
            }
        }
        object IEnumerator.Current { get { return Current; } }

        public void Dispose() { }
        public void Reset() { throw new NotSupportedException(); }
    }
}

这是一个简单的测试

SortedList<int, int> l =
    new SortedList<int, int> { { 1, 1 }, { 3, 3 }, { 4, 4 } };
for (int i = 0; i <= 5; i++)
{
    Console.WriteLine("{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( " +i+ ",  true): "
        + string.Join(", ", l.Head(i, true)));
    Console.WriteLine("{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( " +i+ ", false): "
        + string.Join(", ", l.Head(i, false)));
}
for (int i = 0; i <= 5; i++)
{
    Console.WriteLine("{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( " +i+ ",  true): "
        + string.Join(", ", l.Tail(i, true)));
    Console.WriteLine("{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( " +i+ ", false): "
        + string.Join(", ", l.Tail(i, false)));
}

打印以下内容:

{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 0,  true):
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 0, false):
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 1,  true): [1, 1]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 1, false):
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 2,  true): [1, 1]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 2, false): [1, 1]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 3,  true): [1, 1], [3, 3]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 3, false): [1, 1]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 4,  true): [1, 1], [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 4, false): [1, 1], [3, 3]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 5,  true): [1, 1], [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Head( 5, false): [1, 1], [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 0,  true): [1, 1], [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 0, false): [1, 1], [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 1,  true): [1, 1], [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 1, false): [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 2,  true): [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 2, false): [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 3,  true): [3, 3], [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 3, false): [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 4,  true): [4, 4]
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 4, false):
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 5,  true):
{ { 1, 1 }, { 3, 3 }, { 4, 4 } }.Tail( 5, false):

转载注明原文:在C#SortedDictionary中等效于Java的SortedMap.tailMap - 代码日志