有机会使用Linq(C#)获得唯一记录吗? [英] Any chance to get unique records using Linq (C#)?

问题描述

我得到了list<list<string>>

是我要从中选择唯一记录的记录，因此，当我选择该记录时，我希望选择整行list[x].我在Linq中找不到适合的示例，请帮助:(

in list[x][0] are records from which I want to choose unique records thus such record wouldn't be in any other list[x][0], when I choose it, i'd like whole row list[x] to be chosen. I haven't found the appropriate exapmple for this in Linq, please help :(

编辑

当乔恩·斯基特(Jon Skeet)要求我澄清时，我不能否认;-)

When Jon Skeet asks me to clarify, I can't deny ;-)

list<list<string>>

包含字符串表列表.每个字符串"table"都包含多个键list[x][several_items]，我想从list->获取唯一记录，这意味着该"table"中的FIRST项目.

contains list of string table . Each of the string "table" contains several keys list[x][several_items] and I want to get unique records from list-> meaning FIRST item in that "table".

因此:

item[0] = "2","3","1","3"
item[1] = "2","3","4","2"
item[3] = "10","2"
item[4]= "1","2"

-> unique意味着我可以派生item[3] and item[4]行为唯一.因为数字/字符串的首次出现很重要.

-> unique would mean that I can derive rows item[3] and item[4] as unique. because first occurence of number/string is important.

如果有2个或更多记录/行(item[x] of which first item (item[x][0])在列表中存在多次，则不是唯一的.

If there are 2 or more records/rows (item[x] of which first item (item[x][0]) exists more than once in the list, it's not unique.

每个列表的第一个元素对于确定唯一性都很重要.如果有人可以帮助找到一种查找非唯一性的方法，也许会更容易->因此，从上面的示例中，列表中我只会看到item [0]和item [1]

First element of each list is important to determine uniqueness. Maybe it'd be easier if someone can help to find a way to find non-unique -> so from the above example the list I'd get only item[0] and item[1]

推荐答案

我已经更新了底部的UniqueBy实现，使其效率大大提高，并且仅遍历了源代码一次.

I've updated the UniqueBy implementation at the bottom to be significantly more efficient, and only iterate through the source once.

如果我对您的理解正确(问题尚不清楚-如果您能提供一个示例，它将非常有帮助)，这就是您想要的:

If I've understood you correctly (the question is pretty unclear - it would really help if you could provide an example) this is what you want:

public static IEnumerable<T> OnlyUnique<T>(this IEnumerable<T> source)
{
    // No error checking :)

    HashSet<T> toReturn = new HashSet<T>();
    HashSet<T> seen = new HashSet<T>();

    foreach (T element in source)
    {
        if (seen.Add(element))
        {
            toReturn.Add(element);
        }
        else
        {
            toReturn.Remove(element);
        }
    }
    // yield to get deferred execution
    foreach (T element in toReturn)
    {
        yield return element;
    }
}

好的，如果您只关心列表的第一个元素的唯一性，我们需要对其进行一些更改:

Okay, if you only care about the first element of the list for uniqueness, we need to change it somewhat:

public static IEnumerable<TElement> UniqueBy<TElement, TKey>
    (this IEnumerable<TElement> source,
     Func<TElement, TKey> keySelector)
{
    var results = new LinkedList<TElement>();
    // If we've seen a key 0 times, it won't be in here.
    // If we've seen it once, it will be in as a node.
    // If we've seen it more than once, it will be in as null.
    var nodeMap = new Dictionary<TKey, LinkedListNode<TElement>>();

    foreach (TElement element in source)
    {
        TKey key = keySelector(element);
        LinkedListNode<TElement> currentNode;

        if (nodeMap.TryGetValue(key, out currentNode))
        {
            // Seen it before. Remove if non-null
            if (currentNode != null)
            {
                results.Remove(currentNode);
                nodeMap[key] = null;
            }
            // Otherwise no action needed
        }
        else
        {
            LinkedListNode<TElement> node = results.AddLast(element);
            nodeMap[key] = node;
        }
    }
    foreach (TElement element in results)
    {
        yield return element;
    }
}

您可以通过以下方式调用它:

You'd call it with:

list.UniqueBy(row => row[0])

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