C# 分組統計效能比較（二）：迴圈、迭代器與 LINQ 寫法優劣分析

注意，上述兩種方法都假設輸入資料已排序。若非，需先對輸入資料排序。

若使用 LINQ，寫法更簡潔：

IEnumerable<Tuple<int, int, double>> Linq(IEnumerable<Tuple<int, double>> tuples)
{
    var result = new List<Tuple<int, int, double>>();
    var q = from k in tuples group k by k.Item1;
    foreach (var g in q) result.Add(new Tuple<int, int, double>(g.Key, g.Count(), g.Average(v => v.Item2)));
    return result;
}

但需注意 LINQ 方法無論執行時間或記憶體佔用都較大。

Main 方法

static void Main(string[] args)
{
    try { new Program().Run(Console.Out, int.Parse(args[0])); }
    catch (Exception ex) { Console.WriteLine(ex); }
}

void Run(TextWriter writer, int n)
{
    var tuples = GetTuples(n * 1024 * 1024);
    Write("ForEach", writer, ForEach(tuples));
    Write("Iterate", writer, Iterate(tuples));
    Write("Linq", writer, Linq(tuples));
}

Write 輸出方法

void Write(string title, TextWriter writer, IEnumerable<Tuple<int, int, double>> tuples)
{
    writer.WriteLine("==========> " + title + " <============");
    writer.WriteLine("Key ------Count Average----------");
    var count = 0; var sum = 0.0;
    foreach (var t in tuples)
    {
        writer.WriteLine(" {0,3} {1,11:N0} {2}", t.Item1, t.Item2, t.Item3);
        count += t.Item2; sum += t.Item2 * t.Item3;
    }
    writer.WriteLine("--- ----------- -----------------");
    writer.WriteLine(" {0,3} {1,11:N0} {2}", tuples.Count(), count, sum / count);
    writer.WriteLine();
}