.net 中的快速并集和相交

发布于 2024-12-27 12:23:38 字数 350 浏览 0 评论 0原文

假设我有几个对象列表，需要将它们合并/相交。目前我只是做这样的事情：

List result = lists[0];
for( int i = 1; i < lists.Count; i++ )
  result = (op == 'and') ? result.Union(lists[i]).ToList() 
    : result.Intersect(lists[i]).ToList();

我相信这工作得非常慢，即使列表是按其字段之一排序的。我怎样才能让它更快？哈希集、树等？最好由.NET 4提供。

这些对象实际上是缓存的DataRows，因为依赖DB来进行这些操作似乎要慢得多。

原文

Let's say I've got several lists of objects and need to union/intersect them. Currently I simply do something like this:

List result = lists[0];
for( int i = 1; i < lists.Count; i++ )
  result = (op == 'and') ? result.Union(lists[i]).ToList() 
    : result.Intersect(lists[i]).ToList();

I believe this is working very slow, even though lists are sorted by one of their fields. How would I make it faster? Hashsets, trees, etc.? Preferable provided by .NET 4.

These objects are actually cached DataRows, as relying on DB to make these operations seems to be much slower.

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浅笑轻吟梦一曲 2025-01-03 12:23:38

好吧，使用 HashSet 似乎是有意义的，是的 - 假设您真的不关心顺序，那么将所有内容来回转换为列表是没有意义的：

var result = new HashSet<Foo>(lists[0]);
foreach (var list in lists.Skip(1))
{
    if (op == "and")
    {
        result.UnionWith(list);
    }
    else
    {
        result.IntersectWith(list);
    }
}

拥有请注意，这里有“如果”。您可能想要：

var result = new HashSet<Foo>(lists[0]);
Action<IEnumerable<Foo>> action = op == "and"
     ? result.UnionWith : result.IntersectWith;
foreach (var list in lists.Skip(1))
{
    action(list);
}

Well using a HashSet<T> would seem to make sense, yes - there's no point converting everything to a list back and forth, assuming you really don't care about the order:

var result = new HashSet<Foo>(lists[0]);
foreach (var list in lists.Skip(1))
{
    if (op == "and")
    {
        result.UnionWith(list);
    }
    else
    {
        result.IntersectWith(list);
    }
}

It's somewhat ugly having the "if" there, mind. You might want:

var result = new HashSet<Foo>(lists[0]);
Action<IEnumerable<Foo>> action = op == "and"
     ? result.UnionWith : result.IntersectWith;
foreach (var list in lists.Skip(1))
{
    action(list);
}

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