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‫In this lesson, we are going to cover hash tables and we're also going to look at dictionaries later

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‫on.

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‫So hash tables and dictionaries are some of the most useful collections in C sharp.

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‫So we will see how to define them and how to use them and in which case scenario, we should take advantage

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‫of them.

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‫To understand the concept of hashing and tables.

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‫First, think about a real dictionary.

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‫So assume we have a German and English dictionary where every word in German corresponds to only one

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‫word in English, because that would be limiting the limitation here.

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‫So when we try and translate the word auto, which in German basically translates into car in English,

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‫we use one word as a key word that we can search for, for example, and we got one translation as a

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‫result.

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‫So there is always this 1 to 1 relationship.

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‫So we have auto and that translates into car and we can achieve exactly that also using hash tables.

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‫And we're going to use this in a very specific example here.

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‫So let's say we have a student registration system where every student has an ID and many other data

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‫such as name, age, grades and picture and so forth.

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‫For such a system to work, we will need a special ID for every single student.

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‫So it has to be unique.

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‫If we were to store this data using a hash table or a dictionary, the student ID would be the key and

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‫the student data would be an object that contains all of the other student info and that would be the

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‫value.

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‫So we always have this key value pair.

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‫So this is how you can find the actual data, so to speak, and this is the value that is behind it.

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‫So you can see it like a locker.

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‫So this is the ID of the locker or the location of the locker.

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‫It could be different kinds of types of data.

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‫It doesn't have to be an ID and this will be the value.

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‫So the dictionary, they have to be of the same type in a hash table.

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‫They can be of different types.

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‫So this could be an integer and this could be an object, for example.

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‫So we're going to look at hash tables first.

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‫Therefore, we are going to create a new class or actually I have prepared a new class for this.

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‫Let's put it down here.

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‫And it has a couple of properties.

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‫It has an ID property, a name property and the GPA.

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‫And then we have a very simple constructor simply assigning the value.

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‫So whatever ID we pass to the student object when we create it will be the ID of the student object

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‫and so forth.

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‫So nothing too fancy here.

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‫We're just using a normal class as we have covered in the class chapter.

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‫Now we could, of course, go ahead and create this class in a separate file so we could go ahead and

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‫open up our solution explorer.

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‫Go ahead and create a new file here.

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‫So add new item or a new class like so and call it student.

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‫But for this example, I'm going to keep it simple and put it into one file.

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‫As you know, it's best practice to put it in a separate file so that you're not mixing things and you

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‫can easier find files and classes and all of that.

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‫Okay, so now that we have our student class, we can go ahead and create a hash table.

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‫So inside of our program in here, in the main method, that's where I'm going to define a new hash

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‫table and it will be a like so hash table.

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‫And I'm going to call this one table, which will be new hash table.

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‫The hash table class is inside of system collections, so we need to add this namespace up here using

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‫system collections.

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‫So hash table and it's different to the hash tables namespace because that's how I called my, which

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‫might confuse our ID here if we want to enter hash table without having this namespace added to our

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‫project.

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‫But now that we have it inside of our programs class, we can use it.

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‫So we have this new hash table which will now be a table of data, and we can put the data that we want

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‫in there.

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‫So this is how you can create such a hash table, how you can initialize it with an empty hash table

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‫object like we did here.

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‫So now what we can do is we can assign values to this hash table, and we could even call it student's

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‫table to make our code a little more readable.

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‫So now we just need to create a bunch of students that we want to have in our table.

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‫So let me add those in here as well.

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‫I'm going to call them stud one stat, 2 to 3, step four, and they will all be new student objects.

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‫So here this will be student with the ID, one will be Maria.

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‫She has a GPA of 98.

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‫Then we have JSON which will have the ID of one.

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‫This is this ID here, the unique identifier, so to speak, and then the GPA that the students have.

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‫Okay.

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‫So now we have all of those students and we can assign them to our students table using the students

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‫ID.

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‫And the good practice would be to go ahead and stay students table dot add and here we use the student

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‫one ID so this one here the property as the key.

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‫So you can see here a hash table allows us to have a key of type object so it can be any type of object

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‫as the key and it can be an object optional or knowable as the value.

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‫So we can assign a value or it can be null.

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‫Okay, but we're going to assign a value and that will be the student one itself like so.

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‫So now we added our student one to our students table and we can do the same thing with the other students

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‫and we're going to use their ID for each of them.

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‫So let's use student's table here for all of them.

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‫Okay.

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‫So that's our hash table now, but how can we fetch entries?

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‫So how can we retrieve entries from our hash table?

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‫Well, let's look at different ways for it.

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‫So the first thing that we can do is we can just create a student object here.

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‫So student and this object will be called stored student.

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‫So I'm going to get the student value from my table.

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‫So in this case, it's the student table.

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‫Students table like so.

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‫And here similar to how I access it in an array.

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‫How X is a value in an array, I can access it by an ID.

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‫So here for example, let's say I want to get the one at ID one like so.

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‫So here the problem is that this hash table can have values that are of type object, right, as we

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‫saw earlier.

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‫So here, if we go ahead and call the add method, you can see we can assign a key, but then the value

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‫will be of type object.

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‫And that means that this here will be of type object.

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‫But we cannot assign just an object type to the student.

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‫It has to be off type student because this stored student one is in fact of type student.

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‫So we need to make sure that we cast it into a student here and then this will work.

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‫Okay, so we're casting this object into a student because we know that inside of the student's table

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‫at position one.

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‫So at the ID one, the value will be in fact of type student.

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‫So here you can get the student at ID one, you could get the student at ID two and so forth.

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‫But that only works because we know that we are using in fact these numbers.

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‫So if you want to get it, otherwise you could use third one by ID.

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‫So this will now give you the value behind the student one ID.

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‫So this will basically give you this value here or this student here.

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‫And it's this one, to be precise.

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‫So this one here, because that's what we are storing at this ID one.

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‫So now we can go ahead and do something like this where we now instead of fried hello world we are going

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‫to actually write something useful where we write the student ID, the name of the student, as well

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‫as his GPA.

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‫So we're going to use stored student one dot ID.

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‫So we get his ID, we get his name and we get his GPA.

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‫And in this case, it's her GPA and so forth, because it's Maria at position one at ID one.

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‫Okay, so let's run this and you can see student ID is one name is Maria and GPA is 98.

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‫So here we saw how we can create a hash table.

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‫This is how we can store data in the hash table.

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‫Actually, this is where we store the data in a hash table.

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‫And then this is how we can retrieve data from a hash table.

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‫So now let's assume we want to actually print all the data inside of our hash table and we don't have

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‫a particular key at our disposal.

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‫We can simply do that by getting all the keys from our hash table to solve this.

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‫First, we have to discuss a struct called dictionary entry.

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‫Whenever we add a new entry to our hash table.

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‫So our key value pair, a new dictionary entry object will be created for us and it will get inserted

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‫into our hash table, which means that a hash table is basically a collection of dictionary entries.

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‫So using a temporary object of dictionary entry, we can go through our hash table using a for each

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‫loop.

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‫So let's look at this in this particular example here where I'm going to go ahead and use a for each

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‫loop here.

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‫And this for each loop type will be the dictionary entry.

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‫That's the struct that I was talking about.

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‫You can see here struct dictionary entry.

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‫So what is the item name that I'm going to use?

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‫I'm going to use entry.

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‫Of course, you could call this student or whatever because you know that the entries will be of type

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‫student, but let's say you don't know it and you can also call it entry.

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‫And then what is the collection that we want to run this through or that we want to get the values from,

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‫and that will be our students table.

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‫So this one retrieve.

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‫An individual item with known ID.

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‫And here.

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‫Retrieve all values from a hash table.

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‫Okay.

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‫So an entry inside of our hash table is of type dictionary entry, as I just said.

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‫So this allows us now to go through this.

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‫Table, the students table and basically display the values.

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‫So here, because entry value will return an object, you can see here it says object.

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‫But what we want in order to get the student details is going to be a student object.

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‫What we'll need to do is we will need to do the following.

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‫So we create a temporary value, which will be our entry value.

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‫Now we have the same problem that we had up here where we need to cast this into a student because it's

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‫just an object, right?

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‫But we want it to be of type student so that we can access the student properties.

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‫So because otherwise entry value dot, it doesn't have ID here, I can't access its ID, I can't access

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‫its name, all of that stuff even though it exists in there.

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‫But our ID just doesn't allow that because we first need to make sure that we convert or cast the value

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‫into the right type, which is going to be our type student.

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‫So now we're casting it, we're storing it temporarily, and now we can go ahead and write the individual

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‫values to our console, for example, or using our program however we will want.

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‫So here we can use the student ID by using template ID because temp now is of type student and student

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‫has ID name GPA and we can access all of those to now iterate through the entire list of all of our

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‫students.

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‫So let's do that real quick.

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‫Let's run our code and see what this for each loop does for us so you can see it goes through it.

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‫It says.

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‫STUDENT four Steve.

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‫STUDENT three Sclera.

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‫STUDENT two is Jason and one is Maria.

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‫So you see it goes from back to well, it starts at the very last entry and goes up to the first entry.

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‫Now, that is if at this point we don't know if it's a student, even though we are converting it into

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‫a student already.

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‫This is really just to show you that a hash table is really just a collection of dictionary entries.

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‫So let's look at how we can simplify this for each loop.

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‫We can simplify it by actually saying for each where the variable type is going to be a student.

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‫The item name, well, you could call it student.

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‫I'm just going to call it value like.

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‫So that's fine.

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‫And the collection that I want to go through is going to be table while students tables dot values.

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‫So there is this AI collection of hash table values.

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‫So gets an AI collection containing the values in the hash table.

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‫So this allows us to go through the values directly without having to.

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‫Basically do what we did here, where we converted it or where we cast it into a student object.

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‫So now we can just go ahead and.

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‫Do the same thing as we did here.

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‫But now, of course, it's going to be value ID and so forth because it's not entry and it's not temp,

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‫but it's going to be in fact value because that's what this for each group.

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‫Is giving us for each iteration of it, where it goes through all of the values that we have.

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‫And value is going to be the individual value that we're currently looking at for that particular iteration.

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‫So in the first iteration, it will be.

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‫Steve.

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‫Then it will be Clara, then Jason, then Maria.

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‫So if we get rid of this line here or it is for each loop and we run it again, we will see that we

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‫get the same results.

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‫So Steve, Clara, Jason and Maria.

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‫Okay.

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‫So that was a quick introduction into hash tables and the next video, I'll have a little challenge

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‫for you where you will basically extend your code a little bit to understand hash tables a little better.

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‫And then we're going to look at dictionaries.