LINQ To Objects

記錄 LINQ to Objects 的學習

LINQ to Objects 是指可將Linq查詢用于繼承 IEnumerable 或 IEnumerable<T> 的集合型別，包括框架本身定義的 List、Array、Dictionary，也可以是通過實作上面列舉介面的自定義集合型別，Linq 查詢應用在字串集合上，使得處理文本檔案中的半結構化資料時非常有用，

對某個詞在字串上出現的次數統計，（ToLowerInvariant 回傳時使用固定區域性的大小寫規則，）

  string text = @"Historically, the world of data and the world of objects" +
@" have not been well integrated. Programmers work in C# or Visual Basic" +
@" and also in SQL or XQuery. On the one side are concepts such as classes," +
@" objects, fields, inheritance, and .NET Framework APIs. On the other side" +
@" are tables, columns, rows, nodes, and separate languages for dealing with" +
@" them. Data types often require translation between the two worlds; there are" +
@" different standard functions. Because the object world has no notion of query, a" +
@" query can only be represented as a string without compile-time type checking or" +
@" IntelliSense support in the IDE. Transferring data from SQL tables or XML trees to" +
@" objects in memory is often tedious and error-prone.";

  string searchTerm = "data";

  //Convert the string into an array of words  
  string[] source = text.Split(new char[] { '.', '?', '!', ' ', ';', ':', ',' }, 
      StringSplitOptions.RemoveEmptyEntries);

  // Create the query.  Use ToLowerInvariant to match "data" and "Data"   
  var matchQuery = from word in source
                   where word.ToLowerInvariant() == searchTerm.ToLowerInvariant()
                   select word;

  // Count the matches, which executes the query.  
  int wordCount = matchQuery.Count();
  Console.WriteLine("{0} occurrences(s) of the search term \"{1}\" were found.", wordCount, searchTerm);

查詢一組包含指定單詞的句子，

   string text = @"Historically, the world of data and the world of objects" +
 @" have not been well integrated. Programmers work in C# or Visual Basic" +
 @" and also in SQL or XQuery. On the one side are concepts such as classes," +
 @" objects, fields, inheritance, and .NET Framework APIs. On the other side" +
 @" are tables, columns, rows, nodes, and separate languages for dealing with" +
 @" them. Data types often require translation between the two worlds; there are" +
 @" different standard functions. Because the object world has no notion of query, a" +
 @" query can only be represented as a string without compile-time type checking or" +
 @" IntelliSense support in the IDE. Transferring data from SQL tables or XML trees to" +
 @" objects in memory is often tedious and error-prone.";

   //句子分隔
   string[] sentences = text.Split(new char[] { '.', '?', '!' });

   //句子中的單詞分隔符
   var wordSeperator = new char[] { ' ', ';', ':', ',' };

   //句子包含的單詞  
   string[] wordsToMatch = { "Historically", "data", "integrated" };

   var sentenceQuery = from sentence in sentences
                       let words = sentence.Split(wordSeperator, StringSplitOptions.RemoveEmptyEntries)
                       where words.Intersect(wordsToMatch).Count() == wordsToMatch.Count()
                       select sentence;

   foreach (string str in sentenceQuery)
   {
       Console.WriteLine(str);
    }

查詢字串中的字符（String實作了IEnumerable<char>, IEnumerable）

string aString = "ABCDE99F-J74-12-89A";

IEnumerable<char> stringQuery = from ch in aString
                                where char.IsDigit(ch)
                                select ch;

// Execute the query  
foreach (char c in stringQuery)
    Console.Write(c + " ");

// Call the Count method on the existing query.  
int count = stringQuery.Count();
Console.WriteLine("Count = {0}", count);

LINQ查詢與正則運算式合并

namespace ConsoleApp4
{
    class Program
    {
        static void Main(string[] args)
        {
            //檔案夾路徑，注意最后的斜杠
            string startFolder = @"C:\Program Files (x86)\Microsoft Visual Studio 14.0\";

            IEnumerable<System.IO.FileInfo> fileList = GetFiles(startFolder);

            Regex searchTerm = new Regex(@"Visual (Basic|C#|C\+\+|Studio)");

            var queryMatchingFiles = from file in fileList
                                     where file.Extension == ".htm"
                                     let fileText = System.IO.File.ReadAllText(file.FullName)
                                     let matches = searchTerm.Matches(fileText) //正則匹配檔案內容
                                     where matches.Count > 0
                                     select new
                                     {
                                         path = file.FullName,
                                         matcheValues = matches.Select(x => x.Value)
                                     };           
        }
        
        static IEnumerable<System.IO.FileInfo> GetFiles(string path)
        {
            if (!System.IO.Directory.Exists(path))
                throw new System.IO.DirectoryNotFoundException();

            string[] fileNames = null;
            List<System.IO.FileInfo> files = new List<System.IO.FileInfo>();

            fileNames = System.IO.Directory.GetFiles(path, "*.*", System.IO.SearchOption.AllDirectories);
            foreach (string name in fileNames)
            {
                files.Add(new System.IO.FileInfo(name));
            }
            return files;
        }
    }
}

串列求差值

string[] names1 = { "aa","bb","cc"};
string[] names2 = { "aa", "bb", "cc","dd","ee" };

IEnumerable<string> differenceQuery =
  names2.Except(names1); //names1.Except(names2)結果為空序列

foreach (string s in differenceQuery)
    Console.WriteLine(s);   

//ouput:
//dd
//ee

按任意欄位對結構化的文本資料進行排序（結構化的文本，是指資料排列有一定的規律，例如dbf、csv）

假設有一個檔案，score.csv，內容如下：

名字,語文,數學,英語

小敏,34,45,56

小希,56,65,77

小花,99,99,99

string[] scores = System.IO.File.ReadAllLines(@"../../../scores.csv"); 

Console.WriteLine("語文成績從高到底排序:");

var scoreQuery = from score in scores
           let fields = score.Split(",")
           orderby fields[1]  //語文成績在第一列
           select score;

foreach (string str in scoreQuery)
{
    Console.WriteLine(str);
}

 合并和比較字串集合（Union是數學意義上的合并，Concat是簡單的聯結）

string[] fileA = System.IO.File.ReadAllLines(@"../../../names1.txt");
string[] fileB = System.IO.File.ReadAllLines(@"../../../names2.txt");

IEnumerable<string> concatQuery =
    fileA.Concat(fileB).OrderBy(s => s); //Concat 合并          

IEnumerable<string> uniqueNamesQuery =
    fileA.Union(fileB).OrderBy(s => s); //Union 合并，使用比較器的合并，不進行重復的合并，

多個資料源填充一個集合

假設names.csv內容如下：

firstname,lastname,id

Omelchenko,Svetlana,111

mingming,chen,112

假設scores.csv內容如下：

 id,語,數,英

111,98,23,67,

112,34,90,99

namespace ConsoleApp4
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] names = System.IO.File.ReadAllLines(@"../../../names.csv");
            string[] scores = System.IO.File.ReadAllLines(@"../../../scores.csv");

            IEnumerable<Student> queryNamesScores =
                from nameLine in names
                let splitName = nameLine.Split(',')
                from scoreLine in scores
                let splitScore = scoreLine.Split(',')
                where Convert.ToInt32(splitName[2]) == Convert.ToInt32(splitScore[0])
                select new Student()
                {
                    FirstName = splitName[0],
                    LastName = splitName[1],
                    ID = Convert.ToInt32(splitName[2]),
                    ExamScores = splitScore.Skip(1).Select(x => Convert.ToInt32(x)).ToList()
                };
          
        }
    }

    class Student
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int ID { get; set; }
        public List<int> ExamScores { get; set; }
    }
}

 使用“分組”與“合并”將一個檔案拆成多個檔案

假設：names1.txt

Bankov, Peter  
Holm, Michael  
Garcia, Hugo  
Potra, Cristina  
Noriega, Fabricio  
Aw, Kam Foo  
Beebe, Ann  
Toyoshima, Tim  
Guy, Wey Yuan  
Garcia, Debra

names2.txt

Liu, Jinghao  
Bankov, Peter  
Holm, Michael  
Garcia, Hugo  
Beebe, Ann  
Gilchrist, Beth  
Myrcha, Jacek  
Giakoumakis, Leo  
McLin, Nkenge  
El Yassir, Mehdi

string[] fileA = System.IO.File.ReadAllLines(@"../../../names1.txt");
string[] fileB = System.IO.File.ReadAllLines(@"../../../names2.txt");

var mergeQuery = fileA.Union(fileA);           

var groupQuery = from name in mergeQuery
                 group name by name[0] into g
                 orderby g.Key
                 select g;

foreach (var g in groupQuery)
{
    string fileName = @"../../../testFile_" + g.Key + ".txt";

    Console.WriteLine(g.Key);

    using (System.IO.StreamWriter sw = new System.IO.StreamWriter(fileName))
    {
        foreach (var item in g)
        {
            sw.WriteLine(item);                        
            Console.WriteLine("   {0}", item);
        }
    }
}           

結果：

CSV文本檔案計算多列的值

創建score.csv,內容如下：

111, 97, 92, 81, 60  
112, 75, 84, 91, 39  
113, 88, 94, 65, 91  
114, 97, 89, 85, 82  
115, 35, 72, 91, 70  
116, 99, 86, 90, 94  
117, 93, 92, 80, 87  
118, 92, 90, 83, 78  
119, 68, 79, 88, 92  
120, 99, 82, 81, 79  
121, 96, 85, 91, 60  
122, 94, 92, 91, 91

using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;

namespace ConsoleApp4
{
    class Program
    {
        static void Main(string[] args)
        {
            string[] lines = System.IO.File.ReadAllLines(@"../../../scores.txt");
            int exam = 3;         
            MultiColumns(lines);
        }
       
        private static void MultiColumns(string[] lines)
        {
            Console.WriteLine("Multi Column Query:");

            IEnumerable<IEnumerable<int>> multiColQuery =
               from line in lines
               let elements = line.Split(',')
               let scores = elements.Skip(1)
               select (from str in scores 
                       select Convert.ToInt32(str));

            int columnCount = multiColQuery.First().Count();
            
            for (int column = 0; column < columnCount; column++)
            {
                var results2 = from row in multiColQuery
                               select row.ElementAt(column);
                double average = results2.Average();
                int max = results2.Max();
                int min = results2.Min();
                
                Console.WriteLine("Exam #{0} Average: {1:##.##} High Score: {2} Low Score: {3}",
                              column + 1, average, max, min);
            }
        }
    }   
}

標籤：其他

上一篇：pycharm永久破解方法

下一篇：深入淺出RISC-V除錯