如何使用 LINQ 对通用列表中的值求和？

Question

我有这门课：

public class ItemsForMonthYear
{
    public String ItemDescription { get; set; }
    public String monthYr { get; set; }
    public int TotalPackages { get; set; }
    public Decimal TotalPurchases { get; set; }
    public Decimal AveragePrice { get; set; }
    public Double PercentOfTotal { get; set; }
}

...我存储数据的地方。我有一个用于存储其中 N 个的通用列表：

List<ItemsForMonthYear> itemsForMonthYearList;

...然后我从该列表中读取以填充电子表格，这非常简单：

totPackagesCell.Value2 = ifmy.TotalPackages;
totPurchasesCell.Value2 = ifmy.TotalPurchases;
avgPriceCell.Value2 = ifmy.AveragePrice;

...但所需的下一个单元格 val 是根据给定月份的所有 TotalPurchases 值计算得出的。因此，我需要为具有相同月值的所有 ItemsForMonthYear “记录”计算“总百分比”值。在 SQL 中，我可以这样做：

SELECT SUM(TotalPurchases) FROM ItemsForMonthYear WHERE monthYr = 'Apr 14'

...然后将该结果除以每个 ItemDescription 的 TotalPurchases 值

IOW，如果“4 月 14 日”月份的所有 TotalPurchases 总和为 100 万美元，并且 ItemDescription“Cocoa Puffs”的 TotalPurchases 为 10 万美元，我可以计算“Cocoa Puffs”的 PercentOfTotal 为为 10%。

我可以“暴力破解”并遍历整个通用列表：

Decimal allTotalPurchases;
foreach (ItemsForMonthYear ifmy in itemsForMonthYearList)
{
    if (ifmy.monthYr.Equals("Apr 14") 
    {
        allTotalPurchases = allTotalPurchases + ifmy.TotalPurchases;
    }
}

...但我认为有一种更优雅/性能更友好的方式可以使用 LINQ 完成此任务。我想做这样的事情：

percentOfTotalCell.Value2 = GetPercentForPurchaseForMonthYearAsStr(ifmy.TotalPurchases, curMonth);

private String GetPercentForPurchaseForMonthYearAsStr(decimal totPurchasesForItemMonthYear, string totPurchasesForMonthYear)
{    
    percentOfTotal = // What LINQ magic goes here?
    percentOfTotalStr = percentOfTotal.ToString("P", CultureInfo.InvariantCulture);
    return percentOfTotalStr;
}

...其中 arg "totPurchasesForItemMonthYear" 将是上面示例中的十万美元等值，而 arg "totPurchasesForMonthYear" 将是值例如“4 月 14 日”。

有谁知道这个聪明的 LINQ 技巧？

Answer 1

所需的下一个单元格 val 是根据给定月份的所有 TotalPurchases 值计算得出的。所以我需要为所有具有相同月值的 ItemsForMonthYear “记录”计算“总百分比”值。

嗯，首先，您需要获得具有相同monthYr 的那些：

var allMonthYr = itemsForMonthYearList.Where(x => x.monthYr == ifmy.monthYr);

那么就是简单的Sum：

var totalPurchasesOfMonthYr = allMonthYr.Sum(x => x.TotalPurchases);
var percentOfTotal = ifmy.TotalPurchases / totalPurchasesOfMonthYr;

现在，这被证明是非常低效的——因为您不必要地重复这个列表很多次。最好先分组然后求和一次：

var groupedItemsForMonthYr = itemsForMonthYearList.ToLookup(x => x.monthYr);
var totalsForMonthYr = groupedItemsForMonthYr.ToDictionary(
    x => x.Key, 
    x => x.Sum(x => x.TotalPurchases)
);

var percentOfTotal = ifmy.TotalPurchases / totalsForMonthYr[ifmy.monthYr];

Answer 2

您可以这样做来计算一个月的总购买量：

decimal totalPurchases = itemsForMonthYearList.Where(item => item.monthYr == "Apr 14")
                                              .Sum(item => item.TotalPurchases);

然后您可以通过以下方式计算给定项目的百分比：

decimal percentage = (itemsForMonthYearList.Single(item => item.ItemDescription == "Cocoa Puffs")
                                           .TotalPurchases / totalPurchases) * 100;

Answer 3

这是 Mark Brackett 的回答，我认为这是适当的修复：

// This is code that goes outside your loop
var groupedItemsForMonthYr=itemsForMonthYearList.GroupBy(x=>x.monthYr);
var totalsForMonthYr=groupedItemsForMonthYr.ToDictionary(
    x => x.Key, 
    x => x.Sum(y => y.TotalPurchases)
);

// This is the code that goes inside your loop
var percentOfTotal = ifmy.TotalPurchases / totalsForMonthYr[ifmy.monthYr];

这是我的答案，给出了 List<ItemsForMonthYear> 并在正确填写 PercentOfTotal 的项目中返回 List<ItemsForMonthYear>：

var items=itemsForMonthYearList.GroupBy(x=>x.monthYr)
  .Select(x=>new { total=x.Sum(y=>y.TotalPurchases), i=x })
  .Select(x=>x.i.Select(y=>new ItemsForMonthYear {
        ItemDescription=y.ItemDescription,
        monthYr=y.monthYr,
        TotalPackages=y.TotalPackages,
        TotalPurchases=y.TotalPurchases,
        AveragePrice=y.AveragePrice,
        PercentOfTotal=(double)y.TotalPurchases/(double)x.total
      }))
  .SelectMany(x=>x);

这是我的测试代码：

void Main()
{
    var itemsForMonthYearList=new[]{
        new ItemsForMonthYear { ItemDescription="A",monthYr="Aug 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Sep 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Sep 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Oct 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Oct 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
        new ItemsForMonthYear { ItemDescription="A",monthYr="Oct 2014",TotalPackages=1,TotalPurchases=1,AveragePrice=1},
    };

    var items=itemsForMonthYearList.GroupBy(x=>x.monthYr)
      .Select(x=>new { total=x.Sum(y=>y.TotalPurchases), i=x })
      .Select(x=>x.i.Select(y=>new ItemsForMonthYear {
            ItemDescription=y.ItemDescription,
            monthYr=y.monthYr,
            TotalPackages=y.TotalPackages,
            TotalPurchases=y.TotalPurchases,
            AveragePrice=y.AveragePrice,
            PercentOfTotal=(double)y.TotalPurchases/(double)x.total
          }))
      .SelectMany(x=>x);
      items.Dump();
}

public class ItemsForMonthYear
{
    public String ItemDescription { get; set; }
    public String monthYr { get; set; }
    public int TotalPackages { get; set; }
    public Decimal TotalPurchases { get; set; }
    public Decimal AveragePrice { get; set; }
    public Double PercentOfTotal { get; set; }
}

输出：

Answer 4

decimal allTotalPurchases = itemsForMonthYearList.Where(s => s.monthYr.Equals("Apr 14")).Select(s => s.TotalPurchases).Sum();

Answer 5

这段代码来自 Mark Brackett 自称“效率极低”的第一次尝试，运行良好：

private String GetPercentForPurchaseForMonthYearAsStr(decimal totPurchasesForItemMonthYear, string monthToCalculate)
{
    var allRecordsForMonthYr = itemsForMonthYearList.Where(x => x.monthYr == monthToCalculate);
    var totalPurchasesOfMonthYr = allRecordsForMonthYr.Sum(x => x.TotalPurchases);
    var percentOfTotal = totPurchasesForItemMonthYear / totalPurchasesOfMonthYr;
    return percentOfTotal.ToString("P", CultureInfo.CurrentCulture);
}

由于分组代码无法为我编译，我对上面的代码很满意。