C#之LINQ——方法语法与查询语法（贰）

{"1.AAA", "3.BBB", "invalidWord", "4.DDD"}

{"1.AAA", "4.DDD"}

words.Where((word,index) => word.StartsWith($"{index+1}."));

IEnumerable<PetType> allPetsTypes = Enum.GetValues(typeof(PetType)).Cast<PetType>();

// 宠物类
public class Pet
{
    public int Id { get; }
    public string Name { get; }
    public PetType PetType { get; }
    public float Weight { get; }
}

// 主人类
public class PetOwner
{
    public int Id { get; }
    public string Name { get; }
    public IEnumerable<Pet> Pets; // 一个主人有多个宠物
}

IEnumerable<Pet> pets = new[]
{
    new Pet(1, "Hannibal", PetType.Fish, 1.1f),
    new Pet(2, "Anthony", PetType.Cat, 2f),
    new Pet(3, "Ed", PetType.Cat, 0.7f),
    new Pet(4, "Taiga", PetType.Dog, 35f),
    new Pet(5, "Rex", PetType.Dog, 40f),
    new Pet(6, "Lucky", PetType.Dog, 5f),
    new Pet(7, "Storm", PetType.Cat, 0.9f),
    new Pet(8, "Nyan", PetType.Cat, 2.2f)
};

IEnumerable<PetOwner> people = new[]
{
   new PetOwner(1, "John", new [] {
       pets.ElementAt(0),
       pets.ElementAt(1),
   }),
   new PetOwner(2, "Jack", new [] {
       pets.ElementAt(2)
   }),
   new PetOwner(3, "Stephanie", new [] {
       pets.ElementAt(3),
       pets.ElementAt(4),
       pets.ElementAt(5)
   })
};

IEnumerable<Pet> petsOfOwners = people.SelectMany(owner => owner.Pets);

Id: 1, Name: Hannibal, Type: Fish, Weight: 1.1
Id: 2, Name: Anthony, Type: Cat, Weight: 2
Id: 3, Name: Ed, Type: Cat, Weight: 0.7
Id: 4, Name: Taiga, Type: Dog, Weight: 35
Id: 5, Name: Rex, Type: Dog, Weight: 40
Id: 6, Name: Lucky, Type: Dog, Weight: 5

var nestedListOfNumbers = new List<List<int>>
{
    new List<int> {1, 2, 3},
    new List<int> {4, 5, 6},
    new List<int> {5, 6},
};

var allNumbersFromNestedList = nestedListOfNumbers.SelectMany(list => list);

IEnumerable<string> ownerPetPairsInfo = people.SelectMany(
    person => person.Pets,
    (person, pet) => $"{person.Name} owns {pet.Name}");

John owns Hannibal
John owns Anthony
Jack owns Ed
Stephanie owns Taiga
Stephanie owns Rex
Stephanie owns Lucky

var numbers = new[] { 1, 2, 3 };
var letters = new[] { 'A', 'B', 'C' };
var carthesianProduct = numbers.SelectMany(
    number => letters,
    (number, letter) => $"{number},{letter}");

var weightGroups = pets.GroupBy(
    pet => Math.Floor(pet.Weight),
    (floorOfWeight, allPetsInThisGroup) => new
    {
        FloorOfWeight = floorOfWeight,
        MinWeightInThisGroup = allPetsInThisGroup.Min(pet => pet.Weight),
        MaxWeightInThisGroup = allPetsInThisGroup.Max(pet => pet.Weight)
    })
    .OrderBy(weightGroup => weightGroup.FloorOfWeight);

outer.Join(inner, outerKeySelector, innerKeySelector, resultSelector);

IEnumerable<Pet> pets = new[]
{
    new Pet(1, "Hannibal", PetType.Fish, 1.1f),
    new Pet(2, "Anthony", PetType.Cat, 2f),
    new Pet(3, "Ed", PetType.Cat, 0.7f),
    new Pet(4, "Taiga", PetType.Dog, 35f),
    new Pet(5, "Rex", PetType.Dog, 40f),
    new Pet(6, "Lucky", PetType.Dog, 5f),
    new Pet(7, "Storm", PetType.Cat, 0.9f),
    new Pet(8, "Nyan", PetType.Cat, 2.2f)
};

IEnumerable<VeterinaryClinicAppointment> veterinaryClinicAppointments = new[]
{
    new VeterinaryClinicAppointment(clinicId:2, petId:1, new DateTime(2021, 10, 1, 12, 0, 0)),
    new VeterinaryClinicAppointment(clinicId:3, petId:3, new DateTime(2021, 10, 1, 12, 30, 0)),
    new VeterinaryClinicAppointment(clinicId:1, petId:4, new DateTime(2021, 10, 2, 13, 30, 0)),
    new VeterinaryClinicAppointment(clinicId:2, petId:1, new DateTime(2021, 11, 1, 12, 0, 0))
};

IEnumerable<VeterinaryClinic> veterinaryClinics = new[]
{
    new VeterinaryClinic(id: 1, name: "Happy Paws Clinic"),
    new VeterinaryClinic(id: 2, name: "Fish Doctor"),
    new VeterinaryClinic(id: 3, name: "Pure Purr Clinic")
};

var petsAppointmentsFullInfo = pets
    .Join(veterinaryClinicAppointments,
   pet => pet.Id,
   appointmet => appointmet.PetId,
   (pet, appointment) => new { Pet = pet, Appointment = appointment })
   .Join(veterinaryClinics,
   petAppointmentPair => petAppointmentPair.Appointment.ClinicId,
   clinic => clinic.Id,
   (petAppointmentPair, clinic) => $"Pet name: {petAppointmentPair.Pet.Name}," +
   $" appointment date: {petAppointmentPair.Appointment.Date}, " +
   $"at clinic {clinic.Name}");

Pet name: Hannibal, appointment date: 2021/10/1 12:00:00, at clinic Fish Doctor
Pet name: Hannibal, appointment date: 2021/11/1 12:00:00, at clinic Fish Doctor
Pet name: Ed, appointment date: 2021/10/1 12:30:00, at clinic Pure Purr Clinic
Pet name: Taiga, appointment date: 2021/10/2 13:30:00, at clinic Happy Paws Clinic

Hannibal: appointments on 01/01/2021 12:00, 01/11/2021 12:00 
Ed: appointments on 01/10/2021 12:30
Storm: no appointments

var petsGroupedAppointments = pets
    .GroupJoin(clinicAppointments,
        pet => pet.Id,
        appointmet => appointmet.PetId,
        (pet, appointments) => new { Pet = pet, Appointments = appointments });

var leftJoin = pets
    .GroupJoin(clinicAppointments,
        pet => pet.Id,
        appointmet => appointmet.PetId,
        (pet, appointments) => new { Pet = pet, Appointments = appointments })
    .SelectMany(
        petAppointmentsPair => petAppointmentsPair.Appointments.DefaultIfEmpty(),
        (petAppointmentsPair, appointment) =>
         $"Pet name: {petAppointmentsPair.Pet.Name}, appointment date: {appointment?.Date}");

Pet name: Hannibal, appointment date: 2021/10/1 12:00:00
Pet name: Hannibal, appointment date: 2021/11/1 12:00:00
Pet name: Anthony, appointment date:
Pet name: Ed, appointment date: 2021/10/1 12:30:00
Pet name: Taiga, appointment date: 2021/10/2 13:30:00
Pet name: Rex, appointment date:
Pet name: Lucky, appointment date:
Pet name: Storm, appointment date:
Pet name: Nyan, appointment date:

var numbers = new[] { 10, 1, 4, 17, 122 };
var sum = numbers.Aggregate((sum, nextNumber) => sum + nextNumber);
Console.WriteLine($"sum is {sum}"); // sum is 154

var sentence = "The quick brown fox jumps over the lazy dog";
var longestWord = sentence.Split(" ")
    .Aggregate((longestSoFar, nextWord) =>
    nextWord.Length > longestSoFar.Length ? nextWord : longestSoFar);
Console.WriteLine($"longest word is '{longestWord}'"); // longest word is 'quick'

var chars = new[] { "a", "b", "c", "d" };
var withSeparator = chars.Aggregate((a, b) => a + ',' + b);
Console.WriteLine($"withSeparator: {withSeparator}"); // withSeparator: a,b,c,d

var sentence = "The quick brown fox jumps over the lazy dog";
var countOfWords = sentence.Split(" ")
    .Aggregate(
    0,
    (totalCount, nextWord) => totalCount + 1);
Console.WriteLine($"countOfWords: {countOfWords}"); // countOfWords: 9

 var numbers = new[] { 1, 2, 3, 4, 5 };
 var words = new[] { "The", "quick", "brown", "fox", "jumps" };
 var zipped = numbers.Zip(words,
     (number, word) => $"{number}. {word}");

1. The
2. quick
3. brown
4. fox
5. jumps

var points = new[]
{
    new Point(10, 10),
    new Point(10, 11),
    new Point(11, 12),
    new Point(11, 14),
    new Point(12, 16)
};

var distances = points.Zip(points.Skip(1),
    (point1, point2) => GetDistance(point1, point2));

var numbers = new[] { 9, 3, 7, 1, 2 };
var orderedNumbers = from number in numbers 
                     orderby number 
                     select number;

var numbers = new[] { 9, 3, 7, 1, 2 };
var orderedNumbers = from number in numbers 
                     let floorOfSquare = MathF.Floor(MathF.Sqrt(number))
                     orderby floorOfSquare
                     select floorOfSquare;

orderby field

orderby field descending

orderby field1 [descending], field2 [descending], ...

var nestedListOfNumbers = new List<List<int>>
{
    new List<int> { 1, 2, 3},
    new List<int> { 4,5,6},
    new List<int> { 5,6},
};

var allNumbersFromNestedList = from list in nestedListOfNumbers
                               from number in list
                               select number;

// 方法语法
var allPetsOfJPeople = people
    .Where(person => person.Name.StartsWith("J"))
    .SelectMany(person => person.Pets);
// 查询语法
var allPetsOfJPeople = from person in people
                       where person.Name.StartsWith("J")
                       from pet in person.Pets
                       select pet;

var numbers = new[] { 1, 2, 3 };
var letters = new[] { 'A', 'B', 'C' };
// 方法语法
var carthesianProduct = numbers.SelectMany(
    _ => letters,
    (number, letter) => $"{number},{letter}");
// 查询语法
var carthesianProduct = from number in numbers
                        from letter in letters
                        select $"{number},{letter}";

var petsGroupedByType = from pet in pets
                        group pet by pet.PetType;

// 方法语法
var weightGroups = pets.GroupBy(
    pet => Math.Floor(pet.Weight),
    (floorOfWeight, allPetsInThisGroup) => new
    {
        FloorOfWeight = floorOfWeight,
        MinWeightInThisGroup = allPetsInThisGroup.Min(pet => pet.Weight),
        MaxWeightInThisGroup = allPetsInThisGroup.Max(pet => pet.Weight)
    })
    .OrderBy(weightGroup => weightGroup.FloorOfWeight);

// 查询语法
var weightGroups = from pet in pets
                   group pet by Math.Floor(pet.Weight) into weightGroup
                   orderby weightGroup.Key
                   let weightsInThisGroup = from pet in weightGroup
                                             orderby pet.Weight
                                             select pet.Weight
                   select new
                   {
                       FloorOfWeight = weightGroup.Key,
                       MinWeightInThisGroup = weightsInThisGroup.First(),
                       MaxWeightInThisGroup = weightsInThisGroup.Last()
                   };

var petsAppointmentsFullInfo = from pet in pets
                               join appointment in veterinaryClinicAppointments 
                                    on pet.Id equals appointment.PetId
                               join clinic in veterinaryClinics
                                    on appointment.ClinicId equals clinic.Id
                               select $"Pet name: {pet.Name}," +
                                      $" appointment date: {appointment.Date}," +
                                      $" at clinic {clinic.Name}";

var petsGroupedAppointments = pets
    .GroupJoin(clinicAppointments,
        pet => pet.Id,
        appointmet => appointmet.PetId,
        (pet, appointments) => new { Pet = pet, Appointments = appointments });

var petsGroupedAppointments = 
    from pet in pets
    join appointment in clinicAppointments
    	on pet.Id equals appointment.PetId into appointments
    select new { Pet = pet, Appointments = appointments };

var leftJoin = from pet in pets
               join appointment in clinicAppointments
                 on pet.Id equals appointment.PetId into appointments
               from appointment in appointments.DefaultIfEmpty()
               select $"Pet name: {pet.Name}, appointment date: {appointment?.Date}";