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Hashing6:48 with Jeremy McLain
HashSet relies on each item's hash code. Let's learn about hash codes.
Let's go ahead and change the students list back here and 0:00 main from a list to a hash set as well. 0:02 So change this list to the HashSet and also over here. 0:05 Notice that we can initialize a hash set the same way we did with lists. 0:12 Probably the most common way to initialize a hash set 0:16 is by passing another collection into its constructor. 0:19 The constructors of both list and hash set accept any innumerable type. 0:22 So we can pass in any collection type we want because 0:27 they all implement innumerable. 0:30 I'll change this so that we're creating a list and 0:32 then passing it into the hash set constructor. 0:35 So add a parenthesis here and say new List<student>. 0:37 Now these curly braces become the initialization list for 0:42 the list</student> and then close the parenthesis. 0:45 So this is essentially creates a set from the list. 0:48 It will automatically take out any duplicate items in the list. 0:51 This can be pretty handy. 0:55 For now I'll change this back to a simple collection initializer. 0:57 Let's play around a bit with hash sets by working with the student set directly. 1:01 We can go ahead and delete this other code here, And we'll change this for 1:06 each loop so that it's looping through the students that are declared in main. 1:14 Let's create a new student and add it to the list. 1:20 So I'll say student Joe Equals new student 1:23 with the name of Joe, and 1:30 grade level is second grade. 1:35 We can add Joe to the set using the add method. 1:41 So say Students.Add(joe). 1:45 Now it's compile and run this. 1:50 We can see that Joe is now part of our set of students. 1:57 We've seen what happens when we try to add an object that's already been 2:00 added to the set. 2:04 It doesn't get added because it would be a duplicate and sets don't allow duplicate. 2:05 Let's do something a little different. 2:11 Let's try adding Joe again. 2:13 But let's create a new object and pass that in instead. 2:15 So here, I'll create another student object called duplicate Joe. 2:20 And I'll just copy this instantiation from up here. 2:28 Now I'll add duplicate Joe to the students set. 2:35 So I'll say, students.add(duplicateJoe). 2:37 Both of these objects contain the same information. 2:45 They both represent the student Joe. 2:48 Let's see what happened when we run this and print out the contents of the set now. 2:51 Now it appears that we have a duplicate student in our set 2:56 that isn't supposed to allow duplicates. 2:58 What happened? 3:01 Although, these two objects contain the same data, 3:02 they're still two distinct objects. 3:05 HashSet uses hash codes to determine if two objects are the same. 3:07 A hash code is an integer that can be used to identify an object. 3:12 Every class in C# inherits from the System.Object class 3:16 which contains a GetHashCode method. 3:20 Let's print the result of calling 3:22 GetHashCode on each of these two objects. 3:27 So here I'll say Console.Writeline(joe.GetHashCode).. 3:32 And also print out the hash code for duplicateJoe. 3:40 Just compile that and run them, A little more space here, 3:48 when we run this we see that it prints out two different hashCodes. 3:56 By default the gethashCode method returns a unique number for every object. 4:00 If we want two objects to return the same hash code, 4:06 we need to override the get hash code method. 4:09 We should override the get hash code method so that it returns the same hash 4:12 code when the properties of the student objects are the same. 4:15 So back in student, let's create another method and 4:18 it will override the GetHashCode method. 4:24 Now and here, we need to return a number that unique to the contents of the object. 4:34 To do that we need to return an integer that's a representation of the information 4:38 in the object. 4:43 In this case it's the values contained in the name and grade level properties. 4:44 We can simplify this task by just combining the hash codes 4:50 of these properties. 4:53 So we'll put Name.GetHashCode() [BLANK AUDIO] and 4:56 we also have GradeLevel.GetHashcode. 5:03 The problem is these are two integers, but we can only return one. 5:12 If we only return the hash code for the name property then 5:16 any student object with the same name would have the same hash code. 5:20 Identical strings have the same hash code. 5:24 We'll have a similar problem if we only returned the hash code of the grade 5:27 level property. 5:30 The hash code of an integer is just the value of the integer itself. 5:32 Instead, we should return an integer that's a combination of the two hash 5:38 codes. 5:43 There are a number of ways we could do this. 5:43 Probably the simplest is just to add the hash code of the grade level to the hash 5:46 code of the name property like this. 5:50 This will create a sufficiently unique hash code for student objects. 5:59 This works because the hash code for any string is, on average, 6:02 significantly different than any other string. 6:06 However, grade level can only take on one of relatively few values. 6:10 So adding this to the hash code of the name property produces a resulting hash 6:14 code that's unlikely to be the same for any two student objects. 6:19 Had our student object come from a database, 6:23 it would most likely have had a unique ID assigned to it. 6:26 In that case we could return this ID as the hash code. 6:29 The way we implement the GetHashCode method depends on the number of types and 6:32 properties who want to compute the hash code from. 6:36 See the teacher's notes for 6:39 some additional suggestions on how to compute hash codes. 6:40 I suggest you take a look at those suggestions before you attempt to write 6:43 your own GetHashCode methods. 6:46
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