Question

我试图获取IEnumerable<T>

内的一系列项目的索引

var collection = new[] { 1, 2, 3, 4, 5 };
var sequence   = new[] { 2, 3 };

// IndexOf is an extension method.
collection.IndexOf(sequence); // Should return 1

我为此编写了一个IndexOf扩展方法，它可以正常工作，除非连续的序列中第一个项目不止一个：

// There are two items that are 2, consecutively in the collection,
// which is the first item of the sequence.
var collection = new[] { 1, 2, 2, 3, 4, 5 };
var sequence   = new[] { 2, 3 };

collection.IndexOf(sequence); // Should return 2 but returns -1

以下是IndexOf方法：

public static int IndexOf<T>(this IEnumerable<T> collection,
    IEnumerable<T> sequence)
{
    var comparer = EqualityComparer<T>.Default;
    var counter = 0;
    var index = 0;
    var seqEnumerator = sequence.GetEnumerator();

    foreach (var item in collection)
        if (seqEnumerator.MoveNext())
        {
            if (!comparer.Equals(item, seqEnumerator.Current))
            {
                seqEnumerator.Dispose();
                seqEnumerator = sequence.GetEnumerator();
                counter = 0;

                // UPDATED AFTER MICHAEL'S ANSWER,
                // IT WORKS WITH THIS ADDED PART:
                seqEnumerator.MoveNext();
                if (comparer.Equals(item, seqEnumerator.Current))
                    counter++;
            }
            else counter++;
            index++;
        }
        else break;

    var done = !seqEnumerator.MoveNext();
    seqEnumerator.Dispose();
    return done ? index - counter : -1;
}

我无法弄清楚如何解决这个问题。

Answer 1

如果在第一个位置遇到错误符号，则重新启动序列迭代器，但不检查当前项是否与序列迭代器的开头匹配，因此实际上从不将第二个2从集合与序列中的2进行比较。

Answer 2

public static int IndexOf<T>(this IEnumerable<T> collection,
                                IEnumerable<T> sequence)
{
    var ccount = collection.Count();
    var scount = sequence.Count();

    if (scount > ccount) return -1;

    if (collection.Take(scount).SequenceEqual(sequence)) return 0;

    int index = Enumerable.Range(1, ccount - scount + 1)
                          .FirstOrDefault(i => collection.Skip(i).Take(scount).SequenceEqual(sequence));
    if (index == 0) return -1;
    return index;
}

获取一系列项目的索引

2 个答案: