1. What is collection?
   Data structure, used when we want to operate on group or set of data with vary length.
2. Table, list, loop foreach
   A)Table – special kind of collection with const length

var array = new int[10]; //rozmiar tablicy: 10
var array1 = new int[] {1,2,3,45,56 };//automatycznie ustawiona wielkosc tablicy na:5

     3. tables are indexed from 0

array1[2]=12345; //modyfikacja elementu
Console.WriteLine(array1[2]); //odczyt elementu

    4. default values for table int = 0
    5. default values for object table =NULL
    6. how to display elements of array?

Console.WriteLine(array1);

for (int i=0;i<array1.Length;i++)
{
  Console.WriteLine(array1[i]);
}

    7.  how to add additional element at the end of table (remember table has const number of elements)
-create new table on the basis of the old one with length greater of 1 and at the end add new element

     8. List
       -table of vary length (you can add/delete/sort elements)
      – better use list instead of tables!
      9. how to create a list?

using System.Collections.Generic;
var list=new List(); //<> to jest typ generyczny
var list1=new List() {1,2,3,4,5,6};

   10. adding elements

   list.Add(5);

     11. how to display element from the list?
      a)first way

for (int i=0;i< list.Count;i++)
{
Console.WriteLine(list[i]);
}

      b) second way [more profitable]

foreach(var item in list)
{
Console.WriteLine(item);
}

            c)example

public class Student
{
public int Id {get;set;}
public string Name {get; set;}
public List grades {get; set;}



public Student()
{
Grades =new List();
}
}



public class Lecture
{
private readonly List _students;



public Lecture(List students)
{

_students=students;
}



public void AddLateStudents(List lateStudents)
{
_students.AddRange(lateStudents);



}



public void MakeExam()
{
var random=new Random();



foreach (var student in _students)
 {
    student.Grades.Add(random.Next(101));
 }

}



public void DisplayNumberOfStudents()
{
Console.WriteLine($"There are {_students.Count} students.");
}



public void DisplayAverageGrades()
{
foreach (var student in _students)
{
int gradeSum=0;
foreach (var grade in student.Grades)
{
gradeSum +=grade;
}
double average =(double)gradeSum/student.Grades.Count;

     Console.WriteLine($"Student {student.Name} has an average grade of {average.ToString("#.##")}% out of {student.Count}");
}

}



public void SortGradesForEachStudent(bool ascending=true) //parametr domyslny
{
foreach (var student in _students)
{

    if (ascending)
     {
      student.Grades.Sort(); //ascending
     }
    else
     {
      student.Grades.Reverse(); //descending
     }

    Console.WriteLine($"Student {student.Name} has following grades.");

    foreach(var grade in student.Grades)
     {
       Console.Write($"{grade} ");
     }

  }

}

}

class Program
{
static void Main(string[] args)
{
var student=new List()
{
new Student()
{
Id=1,
Name="John"
},
new Student()
{
Id=2,
Name="Ann"
},
new Student()
{
Id=3,
Name="Ann1"
}

 };

var lateStudents=new List()
{
    new Student()
    {
      Id=4,
      Name="John2"
    },
   new Student()
    {
      Id=5,
      Name="Ann2"
    }
 };

var lecture=new Lecture(student);

lecture.DisplayNumberOfStudents();
lecture.MakeExam();

lecture.AddLateStudents(lateStudents);

lecture.DisplayNumberOfStudents();
lecture.MakeExam();

lecture.DisplayAverageofGrades();

Console.WriteLine("-------------------------");
lecture.SortGradesForEachStudent(ascending:true);
}
}
12. Dictionary,stack, queue

        a) Dictionary – collection with elements as pair: key->value (ie. word and its translation); Keys must be unique!!! (can be created using hash function)

class Student
{
public Dictionary PhoneNumbers {get; set;} //pierwszy typ->klucz; drugi typ->wartosc

public Student()
{
Grades=new List();
PhoneNumbers=new dictionary();
}

public void AddPhoneNumber(string name,int number)
{
PhoneNumbers.Add(name,number);
}

public void DisplayPhoneNumber(string name)
{
var number=PhoneNumbers[name];
}
}

b) how to ensure that keys are unique?

public void AddPhoneNumberSafely(string name,int number)
{
var success= PhoneNumbers.TryAdd(name,number); //ta metoda probuje dodac nr o tym kluczu i zwraca wart. czy sie udalo czy nie



if (!success)
{
Console.WriteLine("Nie udalo sie dodac tego nr-klucz istnieje");
}
}

c) what will happen when we try to use nonexisting key?

student.DisplayPhoneNumber("Fax");//bedzie exception!

//jak to obejsc?
public void DisplayPhoneNumberSafely(string name)
{
//out - to jest przekazywanie przez referencje!
var success=PhoneNumbers.TryGetValue(name,out int number);

if (!success)
{
Console.WriteLine("ojej - nie udalo sie pobrac nr-bo nie istnieje!");
}
}

d) how to display the whole dictionary?

foreach (var number in PhoneNumbers)
{
Console.WriteLine($"Name: {number.key}= {number.value}");
}

    13. Stack
     -data structure of type LIFO (as stcak of plates on the table)
     -we have access only to element, which is on the top of the stack
     -we can use it as implementation of 'Undo’ in Text editor

var stack=new Stack();

   14. Queue
     -data structure of type FIFO(like in the shop)
     -we have access to the first element 

var queue=new Queue();

     15. Naming convention

         – parameters in methods – caps
        – class and methods – camelcase
        -private parameters – _nazwa

      16.How to sort list of objects?

public void SortStudentByAverage()
{
_students.Sort(new StudentComparer());
}

   17. how to create own comparator?

using System.Diagnostics.CodeAnalysis;

public class StudentComparer:IComparer
{
private double Calculate(Student student)
{
int sum=0;
foreach(var grade in student.Grades)
{
sum +=grade;
}
double average=(double)sum/student.Grades.Count();

return Math.Round(average,2);

}

public int Compare([AllowNull] Student x,[AllowNull] Student y)
{
var averageX = CalculateAverage(x);
var averageY = CalculateAverage(y);

 if (averageX-averageY >0)
  {
     return 1; 
  }     

if (averageX-averageY ==0)
  {
     return 0; 
  }

if (averageX-averageY <0)
  {
     return -1; 
  }           

}
}