淺談利用LINQ進行分組統計
讓我們來生成要統計的數據，如下所示：
- IEnumerable<Tuple<int, double>> GetTuples(int n)
- {
- var tuples = new Tuple<int, double>[n];
- var rand = new Random();
- for (int k = 1, i = 0; i < n; i++)
- {
- var r = rand.Next(n);
- k += (r >= n - 3) ? 2 : ((r >= n - 9) ? 1 : 0);
- tuples[i] = new Tuple<int, double>(k, rand.NextDouble());
- }
- return tuples;
- }
該方法生成 n 項已經排好序的數據。
現在，讓我們來按關鍵字分組，并統計每組的個數和平均值。
首先，使用 C# 的 foreach 循環，如下所示：
- IEnumerable<Tuple<int, int, double>> ForEach(IEnumerable<Tuple<int, double>> tuples)
- {
- var result = new List<Tuple<int, int, double>>();
- var count = 0;
- var sum = 0.0;
- int? key = null;
- foreach (var v in tuples)
- {
- if (key != v.Item1)
- {
- if (key != null) result.Add(new Tuple<int, int, double>(key.Value, count, sum / count));
- sum = count = 0;
- key = v.Item1;
- }
- count++;
- sum += v.Item2;
- }
- if (key != null) result.Add(new Tuple<int, int, double>(key.Value, count, sum / count));
- return result;
- }
這種方法有個***的缺點就是在 foreach 循環結束之后還要進行一次統計，聞到了代碼的“壞味道”。
那么，就讓我們來重構吧，這次，使用迭代器進行循環：
- IEnumerable<Tuple<int, int, double>> Iterate(IEnumerable<Tuple<int, double>> tuples)
- {
- var result = new List<Tuple<int, int, double>>();
- var count = 0;
- var sum = 0.0;
- int? key = null;
- for (var iter = tuples.GetEnumerator(); ; count++, sum += iter.Current.Item2)
- {
- var hasValue = iter.MoveNext();
- if (!hasValue || key != iter.Current.Item1)
- {
- if (key != null) result.Add(new Tuple<int, int, double>(key.Value, count, sum / count));
- if (!hasValue) break;
- sum = count = 0;
- key = iter.Current.Item1;
- }
- }
- return result;
- }
這樣，就消滅了“壞味道”。
注意，以上兩種方法都假設輸入數據已經排好序。如若不然，就要先對輸入數據進行一次排序。
***，如果使用LINQ的話，還可以更簡單：
- 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;
- }
要注意LINQ 方法無論是運行時間還是占用的內存都更大。
我們來看看 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));
- }
其中的 Write 方法如下所示：
- 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();
- }
***，這個程序的輸出如下所示：
==========> ForEach <============
Key ------Count Average----------
1 10,476 0.492122426354162
2 1,633,289 0.499917991099794
3 981,345 0.500446307804579
5 1,542,377 0.500567888024527
6 478,158 0.499376479287702
8 62,325 0.501552373474687
9 1,463,104 0.500270067230854
11 802,680 0.500518684820775
13 367,798 0.499572390413821
14 492,947 0.500767958524
16 2,403,053 0.500023199420802
17 248,208 0.499988049057847
--- ----------- -----------------
12 10,485,760 0.50018897689056
==========> Iterate <============
Key ------Count Average----------
1 10,476 0.492122426354162
2 1,633,289 0.499917991099794
3 981,345 0.500446307804579
5 1,542,377 0.500567888024527
6 478,158 0.499376479287702
8 62,325 0.501552373474687
9 1,463,104 0.500270067230854
11 802,680 0.500518684820775
13 367,798 0.499572390413821
14 492,947 0.500767958524
16 2,403,053 0.500023199420802
17 248,208 0.499988049057847
--- ----------- -----------------
12 10,485,760 0.50018897689056
==========> Linq <============
Key ------Count Average----------
1 10,476 0.492122426354162
2 1,633,289 0.499917991099794
3 981,345 0.500446307804579
5 1,542,377 0.500567888024527
6 478,158 0.499376479287702
8 62,325 0.501552373474687
9 1,463,104 0.500270067230854
11 802,680 0.500518684820775
13 367,798 0.499572390413821
14 492,947 0.500767958524
16 2,403,053 0.500023199420802
17 248,208 0.499988049057847
--- ----------- -----------------
12 10,485,760 0.50018897689056
這個程序中用到的 Tuple 類如下所示：
- class Tuple<T1, T2>
- {
- public T1 Item1 { get; private set; }
- public T2 Item2 { get; private set; }
- public Tuple(T1 item1, T2 item2) { Item1 = item1; Item2 = item2; }
- }
- class Tuple<T1, T2, T3> : Tuple<T1, T2>
- {
- public T3 Item3 { get; private set; }
- public Tuple(T1 item1, T2 item2, T3 item3) : base(item1, item2) { Item3 = item3; }
- }
其實 .NET Framework 4.0 Base Class Library 中已經有 Tuple 類了。
本文中的全部源程序代碼可以在這里下載。
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