C#中的Collection 1

Collection定義

Collection是個關於一些變量的集合，按功能可分為Lists，Dictionaries，Sets三個大類。

1. Lists：是一個有序的集合，支持index look up。
   • Array
   • List<T>
2. Dictionaries:同TKey store 一些TValue集合，沒有特定order
3. Sets：用來form collections into onther collection，比如HashSet<T>

Collection提供的一些公共方法

是Collection就至少會有Enumerat，但是不一定都有Look up（比如Sets就沒有，Statcks和Queues也是有自己的固定排列順序，不提供look up的方法），注意List的look up用index，而Dictionary的look up用Key。

For adding和removing的位置可能也不一樣，list可以指定加和減的位置，而Statck是先進先出

Array

1: 有Fixed size並且是固定從上到下的順序儲存，index從0開始。

 static void Main(string[] args)
        {
            //Declare
            string[] daysOfWeek ={
                                    "Monday",
                                    "Tuesday",
                                    "Wednesday",
                                    "Thurday",
                                    "Friday",
                                    "Saturday",
                                    "Sunday"
                                };
            Console.WriteLine("Type in index of day of look up>");
            int index = int.Parse(Console.ReadLine());

            //look up using index
            Console.WriteLine(daysOfWeek[index]);

            //Enumerates
            foreach (string day in daysOfWeek)
            {
                Console.WriteLine(day);
            }

            //replace
            daysOfWeek[5] = "PartyDay";
        }

• 由於Array是固定size，所以沒有Add和Remove item

 2: Array是reference type，有兩種initialize

int[] x4 = new int[5];　　//這個5不可以省
int[] x5 = new int[5] {1, 4, 9, 16, 25};　　//可以全部寫出來

int[] x5 = new int[] {1, 4, 9, 16, 25};　　//讓compiler來認有幾個元素

var x5 = {1, 4, 9, 16, 25};　　//int[]可以換成var，后面賦值都可以都不寫int[]

 　　　　　　  int eight = 8;
            int[] square = new int[]{
                1,
                2 * 2,
                eight + 1,
                int.Parse("16"),
                (int)Math.Sqrt(625)
            };

3: Enumerating

這里出現錯誤，foreach 的interation plceholder是只讀的reference type，for不是

比較上面array[i]會改變原array里的元素，下面則不會

4: For VS Foreach

For:Can replace an array element,must directly access it using the index not using foreach

 

Foreach: Can't replace element(只讀) but can modify value of elements，對其他readonly collection也一樣

 class Program
    {
        static void Main(string[] args)
        {
            Person[] student = new Person[]
           {
                new Person{Name = "Shawn", Age = 33},
                new Person{Name = "Yawei", Age = 30}
           };
            //foreach的iteration placeholder是只讀不可以改
            //我們這里沒有直接改這個reference
            //指定的每個person object，比如指向別的object
            //而是修改這個object里面的值
            foreach (Person person in student)
            {
                person.Age = 20;
            }

            foreach (Person person in student)
            {
                Console.WriteLine(person);
            }
            Console.ReadLine();
        }

    }
    public class Person
    {
        public string Name{get; set;}
        public int Age{get; set;}
        public override string ToString()
        {
            return string.Format("{0} 's age = {1}", Name, Age);
        }
    }

 

 在collection里包括array == 是判斷reference是否相等的，而不是值。

Derived reference can used in place of base reference(this can apply to all collections)

static void Main(string[] args)
        {
            Human[] human = new Human[]
            {
                new Human{Name = "Shawn", Age = 33},
                new Human{Name = "Yawei", Age = 30}
            };
            human[0] = new Employee { Name = "Tutu", Age=12, Salary=120.2f};
        }
        public class Employee : Human
        {
            public float Salary { get; set; }
        }
        public class Human{
            public string Name { get; set; }
            public int Age { get; set; }
        }

 

5: Convariance

 

 

static void Main(string[] args)
        {
            object[] obj1 = new object[3];
            string[] daysOfWeeks = {
                                       "Monday",
                                       "Tuesday",
                                       "Wendesday"
                                   };
            //this can work
            obj1[0] = 3;

            //這個是reference type，obj2 point to daysOfWeeks's location
            object[] obj2 = daysOfWeeks;
            //這句有問題，因為refer的地址是一個string[] type
            //compile time的時候VS發現不了問題，會直接給runtime 帶來break的后果
            //same result as daysOfWeeks[0] = 3;
            obj2[0] = 3;
            daysOfWeeks[0] = 3;
        }

 解釋：

一般Convariance不允許用在Collection(List<T>, Dictionial<TKey, TValue>)上即Collection of derived type to a collection of base type,即使像上面用在了array上也會有或許被broke掉的危險。

但是enumerator，enumerable可以做convariance，因為他們是只讀的type保證是安全的，不會修改collection。

var strList = new List<string>{"Monday","Tuesday"};

List<object> objList = (List<Object>)strList;
//compiler will show error, can not do this kind of covariance

IEnumerable<object> objEnum = strList;
//this can do

 

6: Array的方法

一般有普通Array方法或者LINQ方法，Array方法tend to work in-place，而LINQ總是tend to return new object

Copy:

• Array.CopyTo(普通)
• ToArray(LINQ)

Reverse:

• Array.Reverse（普通，static方法）
• var reversed = daysOfWeek.Reverse(); (LINQ方法)

注意LINQ方法經常return IEnumerable 而不是Array，如果需要變為Array要在后面加一個.ToArray

 

 Sort:

Array.Sort

Array.Sort([T], comparer)

IEnumerable<T>.OrderBy();

Binary Search:

Array.BinarySearch([T], "Sunday")

Search

Find item in array: Array.IndexOf(), Array.LastIndexOf()

Find item with condition in array: Array.FindIndex([T], x=> x[0] == 'W'), Array.FindAll([T], x=> x.length == 6)