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Linq to dataset 根据列的最大值选择行

发布于 2024-09-15 03:36:04 字数 122 浏览 3 评论 0原文

我有一个数据集表，我想按列MOID对其进行分组，然后在该组中我想选择具有列radi最大值的行。

谁能告诉我如何通过 LINQ to dataset 来做到这一点？

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从来不烧饼 2024-09-22 03:36:04

尽管 Barry 发布的解决方案应该可以工作（需要进行一些修复），但它并不是最优的：您不需要对集合进行排序来查找具有字段最大值的项目。我编写了一个 WithMax 扩展方法，它返回具有指定函数最大值的项目：

    public static T WithMax<T, TValue>(this IEnumerable<T> source, Func<T, TValue> selector)
    {
        var max = default(TValue);
        var withMax = default(T);
        bool first = true;
        var comparer = Comparer<TValue>.Default;
        foreach (var item in source)
        {
            var value = selector(item);
            int compare = comparer.Compare(value, max);

            if (compare > 0 || first)
            {
                max = value;
                withMax = item;
            }
            first = false;
        }
        return withMax;
    }

它仅迭代集合一次，这比仅对它进行排序以获取第一项要快得多。

然后您可以按如下方式使用它

var query =
    from row in table.AsEnumerable()
    group row by row.Field<int>("MOID") into g
    select g.WithMax(r => r.Field<int>("radi"));

Although the solution posted by Barry should work (with a few fixes), it is sub-optimal : you don't need to sort a collection to find the item with the maximum value of a field. I wrote a WithMax extension method, which returns the item with the maximum value of the specified function :

    public static T WithMax<T, TValue>(this IEnumerable<T> source, Func<T, TValue> selector)
    {
        var max = default(TValue);
        var withMax = default(T);
        bool first = true;
        var comparer = Comparer<TValue>.Default;
        foreach (var item in source)
        {
            var value = selector(item);
            int compare = comparer.Compare(value, max);

            if (compare > 0 || first)
            {
                max = value;
                withMax = item;
            }
            first = false;
        }
        return withMax;
    }

It iterates the collection only once, which is much faster than sorting it just to get the first item.

You can then use it as follows

var query =
    from row in table.AsEnumerable()
    group row by row.Field<int>("MOID") into g
    select g.WithMax(r => r.Field<int>("radi"));

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宛菡 2024-09-22 03:36:04

这是未经测试的，但我认为这样的东西应该有效：

        var qry = from m in [YourDataSource]
                      group p by m.MOID into grp
                      select grp.OrderByDescending(a => a.RADI).First();

This is untested but I think something like this should work:

        var qry = from m in [YourDataSource]
                      group p by m.MOID into grp
                      select grp.OrderByDescending(a => a.RADI).First();

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扎心 2024-09-22 03:36:04

这适用于一个查询！

public static T WithMax<T, TValue>(this IEnumerable<T> source, Func<T, TValue> keySelector)
{
    return source.OrderByDescending(keySelector).FirstOrDefault();
}

this works with one query!

public static T WithMax<T, TValue>(this IEnumerable<T> source, Func<T, TValue> keySelector)
{
    return source.OrderByDescending(keySelector).FirstOrDefault();
}

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