Introduction to. A revised course with - Concise concepts - Hands on class work. - Plenty of examples - Programming exercises

Transcription

1 Introduction to 1 A revised course with - Concise concepts - Hands on class work TextBox1 - Plenty of examples - Programming exercises Copyright HKTA Tang Hin Memorial Secondary School 2012

2 Contents Chapter 1 Ideas of Programming What is a program? What is programming? What is a programming language? Compilers Names of some programming languages... 6 Chapter 2 Visual Basic 2010 Express Download Visual Basic 2010 Express Register Visual Basic 2010 Express within 30 days Your first project Create and save a new project Introduction to Visual Basic 2010 IDE Adding controls and code Open an existing project Structure of a Visual Basic 2010 project Microsoft Developer Network (MSDN) Chapter 3 Working with Controls Introduction to GUI programming in Visual Basic Controls Properties window Changing properties at runtime Assigning values to properties Add a comment to your source code Exercise Chapter 4 Events and Methods Introduction to events and methods Events Introduction to event procedures Adding an event procedure in Visual Basic Names and types of events The structure of an event procedure TextChanged events

3 4.3 Methods Exercise Chapter 5 Variables, Input and Output Variables Declaring variables with Dim statement Keywords and names of variables Giving values to variables in the Dim statement Declaring several variables in a single line Using variables to store values Scope of variables Introduction to InputBox and MsgBox InputBox MsgBox Miscellaneous examples Exercise Chapter 6 Operators and Expressions Arithmetic Operators Parentheses (singular: parenthesis) Concatenation Operators Operator Precedence Increment and Decrement Val function Mathematical Constants and Functions Miscellaneous Examples Exercise Chapter 7 Flow Control (1) Decision Making Relational Operators If...Then...Else Statement Multiple conditions with keyword ElseIf Official Syntax of If...Then...Else Statement Exercise Alphabetical Index

4 Introduction to Visual Basic - Book 1 Chapter 1 Ideas of Programming 1.1 What is a program? A program is a set of instructions that tells a computer how to do a task. With programs, computers can perform tasks and. 1.2 What is programming? Programming is the process of designing, writing, testing and debugging (remove mistakes) a program. To write a program, you need to know and solve the problem first. After solving the problem, you tell the computer how to solve the problem using a programming language. For example, if you need to write a program that calculates percentage changes, you should first solve the problem by obtaining the formula of percentage change. After that, you will write a program to do the task. In this course, Visual Basic.net the programming language you use. To get the idea of how to create a computer program, you may refer to the diagram at the right. Define the Problem (Note: after having a finished program, you can do an evaluation to look for possible improvements.) Evaluation Solve the Problem Testing and Debugging Write a Program 4

5 Chapter 1 Ideas of Programming 1.3 What is a programming language? In computers, instructions are stored in machine code, that each instruction (e.g. ADD, SUBTRACT, MULTIPLY, etc.) is given a unique number. Computers can understand machine code only! However, machine codes are very difficult to be understood by human. Therefore, programming languages, that are understandable by human, are created to bridge the gap. A programming language consists of a specification and an Trivia: The earliest programmers write programs in machine code, because programming languages are not available yet! implementation. The specification provides a definition of the programming language, and programmers write programs according to the specification. And the implementation is a translator: something that converts the program from the programming language into machine code. 1.4 Compilers Compilers are a type of translator that converts source code into an object program. The object program is stored in an executable file (.exe), which can be executed directly in a computer. The usage of a compiler can be summarized in the diagram below: Source Code (written in programming language) Compiler Object Program (written in machine code) Execution 5

6 Introduction to Visual Basic - Book Names of some programming languages Here are the names and characteristics of some programming languages. You are only required to remember the names of these programming languages. (The characteristics and example are provided for reference only.) Programming language BASIC (Beginner s All-purpose Symbolic Instruction Code) Logo Characteristics BASIC is designed to be easy-to-learn. It gradually evolved into modern programming languages, firstly QuickBasic, and then Visual Basic. Example program Output 10 FOR I=1 TO PRINT I*I 4 30 NEXT I Logo is known for drawing attractive graphics. It was used to teach programming in schools. Example program Output REPEAT 5 [FD 100 RT 144] (Note: FD = forward, RT = right turn. Only the star is drawn. The triangle is the cursor, pointing upwards.) Pascal Pascal was designed mainly to teach good programming style. Pascal is being taught in the HKDSE ICT syllabus. Some modern programs are also written in Pascal. Example program Output program example1; 1 var i : integer; 4 begin 9 for i := 1 to 5 do 16 begin 25 WriteLn(i*i); end; end. 6

7 Chapter 1 Ideas of Programming Programming language C++ JavaScript PHP Visual Basic.net Characteristics C++ is one of the most popular programming language (much more popular that Pascal). If you want to be a good programmer, C++ is a programming language that you must learn. C language, the predecessor of C++, is also widely used today. Example program Output #include <iostream> int main(int argc,char* argv[]) { for (int i=1; i<=5; i++) { cout << i*i << endl; } } JavaScript is the programming language available in a web browser, allowing dynamic user interaction in a web site. New web technologies, i.e. Ajax and HTML5 rely on JavaScript as the programming language. PHP is one of the most popular programming languages for a web server. With PHP you can dynamically generate web contents, and many other things, depending on user input. PHP is used in the Visual Basic.net is the programming language you are going to learn in this book. Visual Basic is the easiest programming language to learn if you want to program in Graphical User Interface (GUI). 7

8 Introduction to Visual Basic - Book 1 Chapter 2 Visual Basic 2010 Express 2.1 Download Visual Basic 2010 Express To download Visual Basic 2010 Express, you should search of Visual Basic 2010 in Google. After that, click the second link ( Visual Basic 2010 Express Microsoft Visual Studio ) (Note: in the PDF version of the file, you can simply click the link above.) Click Install Now to download the file vb_web.exe, and then follow the instructions. 2.2 Register Visual Basic 2010 Express within 30 days You must register your copy to use Visual Basic 2010 Express after the 30 days. For more information, press Help Register Product in the menu bar, and then follow the instructions (see the figure) to get a registration key. 8

9 Chapter 2 Visual Basic 2010 Express 2.3 Your first project Create and save a new project The first thing you do in Visual Basic is to start a new project. Follow the instructions below: 1. Click New Project. 2. Choose Windows Forms Application. 3. Give the project a name. 4. Press Okay. Use only common characters (letters, digits, space,. and _ ) in the project name. Otherwise, you may not be able to compile your program. 9

10 Introduction to Visual Basic - Book 1 Once you have created a project, you should immediately save the project by pressing Save All. 5. Choose File Save All in the menu bar. (Always save the project first!) 6. Choose a location for your project. (The location is only asked when you press Save All for the first time.) In Tang Hin, do not save the project to C drive. When you switch off the computer, everything in C drive will be deleted. 10

11 Chapter 2 Visual Basic 2010 Express Introduction to Visual Basic 2010 IDE After you create and save a project, you can see the screen like the figure below. This is known as an Integrated Development Environment, or simply IDE. An IDE contains different kinds of tools that help you to write programs, like source code editor, form designer, compiler, debugger and many other tools. Toolbox Form Designer Solution Explorer Properties Window If some of the above features is/are missing, you can click the buttons to show them. (See the figure above for the icons position on the screen.) 11

12 Introduction to Visual Basic - Book Adding controls and code Now we add some controls and code to the program. For your first program, follow the steps below: 1. Find Button in Toolbox, and then put it into the form. 2. Adjust the size of the form and the button. 3. Change the property Text of the button to Go. 4. Double click the button Go, you should see that some VB codes appear. 5. Insert a line of code shown in the right. 6. Run the program. (Press the green arrow or F5 in your keyboard.) Public Class Form1 Private Sub Button1_Click(...) Handles Button1.Click MsgBox("Hello") End Sub End Class Insert this line of code When you run the program, click the button Go (i.e. Button1), and then a message box containing the message Hello will be shown. Points to note: You MUST type all the words, including the punctuations, in exactly the same way. Extra spaces is NOT allowed between letters and digits. The words Public Class Form1, Private Sub, End Sub and End Class must be put in this order. It is too difficult to understand their meanings at this stage. The button is known as Button1 because you can find the words Button1 in its (Name) property. You can rename the button by setting this property. 12

13 Chapter 2 Visual Basic 2010 Express Open an existing project After you save a project, you can open the project later. To open the project, please follow the instructions here: 1. Click Open Project 2. Browse into the folder and you should see a Solution File. Choose this file and then click Open. Alternatively, you can simply use Windows Explorer to find the same Solution File. However, if you have installed two different versions of Visual Basic, this method may not work well. 13

14 Introduction to Visual Basic - Book Structure of a Visual Basic 2010 project The screenshot below illustrates the folder structure of a Visual Basic project. When you need to copy the project to another computer, or submit your homework or test in eclass, remember to ZIP all the contents of the base folder! The base folder (ZIP all contents of this folder when you need to submit your program to eclass.) Do not ZIP this one!!! bin: Find the executable file (.exe) here! My Project: Configuration and other files obj: Temporary file created when we compile the program. Safe to delete. Also, when you create a Form in Visual Basic IDE, three files are created. You can open the.designer.vb file with a text editor, in order to see the source code. However, no apparent action can be done in the.resx file. The three files related to Form1.vb. Do not delete any of them! 2.5 Microsoft Developer Network (MSDN) If you have a question, you can go to Microsoft Developer Network (MSDN) to find the information. For example, if you want to know the meaning of the keyword Then, just Google then vb msdn and you will find the information in MSDN. 14

15 Chapter 3 Working with Controls Chapter 3 Working with Controls 3.1 Introduction to GUI programming in Visual Basic When we create a new program, one of the first things to do is to create a user interface. The most common type of interface is called graphical user interface (GUI). You create a GUI program in Visual Basic by creating a new Windows Form Application. When you create a new project (see for details), you see an empty dialog box, which is known as a form, and also a Toolbox with a lot of controls (see the figure). You can add controls by dragging the controls into the form. When you finish, press F5 (the function button at the top of the keyboard) to run the program. 3.2 Controls In Visual Basic, users interact with the program using the controls in the form. Each control has its own use. Here are the types of controls we learn in this chapter: Type of control TextBox Label Button Functions A text box allows users to enter text (input). It can also be used to display results (output). A label provides visual cues to users, e.g. describe the meaning of the text box next to it. It can also used to display results (output). The program performs some actions when the user press or click the button. Forms and controls have properties, events, and methods. Together they make the forms and controls useful for programmers. 15

16 Introduction to Visual Basic - Book Properties window You can change the appearance of the controls (and form) by setting their properties in the properties window (see the figure). Put some controls into a form and change their properties. Don t forget the form! Here is a shortlist of the properties we use in the course: Property (Name) (design time only) * Text ForeColor BackColor Left Top Visible Enabled TextAlign (design time only) Font Use You give a name to the control, and use this name to refer to the control in your own source code. The text on the control, or the title of the form. The foreground colour (i.e. colour of the text) inside the control. The background colour (i.e. colour of the empty space) of the control/form. Horizontal position of the object, counted by the number of pixel relative to the left side of its parent. Vertical position of the object, counted by the number of pixel relative to the top side of its parent. Whether the control is visible to the user. Whether users can interact with the control. If not, the control is usually dimmed. The horizontal and vertical alignment of the text inside the control. The font used by the text in the control. (design time only) * Even though we say design time only, it is possible to change these properties during runtime. However, how to changing these properties in runtime is too difficult to be included in this course. For labels and buttons, there is a subtle difference with the Text property. Search UseMnemonic in Google for details. 16

17 Chapter 3 Working with Controls 3.4 Changing properties at runtime Properties of controls can be changed using the properties window (during design time) or by Visual Basic code (during runtime). Here is an example program that changes some properties of the form and the controls. To enter the code into Visual Basic IDE, you can double In Visual Basic, the word Color is spelt in American English (no U ). click Button1 in design view. Can you guess what will happen after Button1 is clicked? (Note: Me refers to the form.) Example Public Class Form1 Changing properties at runtime Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click Me.Text = "Button1 Pressed!" Me.BackColor = Color.Pink Label1.Text = "Name" Label1.BackColor = Color.Green Label1.ForeColor = Color.Yellow Label1.Top = 80 TextBox1.Text = "Sarah" TextBox1.BackColor = Color.Red TextBox1.Enabled = False TextBox1.Left = 20 End Sub End Class Button1.Visible = False 17

18 Introduction to Visual Basic - Book Assigning values to properties Different properties accept values of different data types. Please see the table below: Name of property Data type Example Text String Me.Text = "Title of the Window" ForeColor Color * Label1.ForeColor = Color.Red BackColor Color TextBox1.BackColor = Color.Green Left Integer Me.Left = 100 Top Integer Label1.Top = 234 Visible Boolean (True/False) Button1.Visible = True Enabled Boolean (True/False) TextBox1.Enabled = False Note: A string is enclosed in a pair of double quotes (""). If you want to have the " character inside a string, type that character twice. E.g. "She said ""hello"" to me." Class Work 3.5 Write down a Visual Basic statement for each of the following operation. Operation Change the background colour of the TextBox1 to blue. Make Button2 disappear. Set the title of the form to New Document Change the colour of the text in Label1 to yellow. Move Button1 that Button1 is vertically aligned with Button2. (Assume that the heights of both buttons are the same.) VB statement * For a list of the colours available, search for System.Drawing.Color in Google. 18

19 Chapter 3 Working with Controls 3.6 Add a comment to your source code After you write some source code, you should also write some comments in your program. Comments in Visual Basic start with a single quote ('), which is followed by any text you want. Please see the following example: Example Comments Public Class Form1 ' This is a comment that occupy for a whole line Private Sub Button1_Click(...) Handles Button1.Click TextBox1.Text = "Sarah" ' This is another comment TextBox1.BackColor = Color.Red End Sub End Class It is easy to understand the program if there are only a few lines of source code. However, after writing 100 or even 1000 lines of source code, it will very difficult to understand the logic behind the source code without reading the comments. Exercise 3 1. Write a program with two controls: Button1 and TextBox1. When Button1 is clicked, the following things should happen: (a) TextBox1 is disabled, (b) The background colour of TextBox1 becomes yellow, (c) Button1 becomes visible, and (d) The form s background colour becomes white. 2. Identify the mistakes in the following source code. There is one mistake in each line. (Note: There are no mistakes with the words Me, Label1, Button1 and TextBox1.) Me.Title = "Title of the form" Label1.BackColor = Colour.Green Button1.Visible = Ture TextBox1.Enable = False TextBox1.Text = Very good! 19

20 Introduction to Visual Basic - Book 1 Chapter 4 Events and Methods 4.1 Introduction to events and methods Besides working with properties, you need to use events and methods to communicate with the controls. 4.2 Events When the user does some action in the form (e.g. click a button), an event is raised or triggered. You can write a procedure in your Visual Basic program to handle the event, and that procedure will be executed every time the user does the same action. Some Action (e.g. the user clicks a button) Raise an Event Execute an Event Procedure Introduction to event procedures In the previous chapters, you have seen something like the following excerpt: Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click [Add your code here] End Sub The excerpt above is an event procedure, i.e. a procedure that handles an event. When Button1 is clicked, the event Button1.Click is handled by the procedure Button1_Click, and the code inside is executed. The user clicks Button1 Raise the event Button1.Click Execute procedure Button1_Click 20

21 Chapter 4 Events and Methods Adding an event procedure in Visual Basic Method1 Double click the needed item in a form A quick method to add an event procedure is to double click the related control in the form designer. The following events will be created: Type of control / form Event When the event is raised Form Load When the form is displayed for the first time Button Click When the Button is clicked Label Click When the Label is clicked TextBox TextChanged When the Text property of the TextBox is changed Method2 Select the event in the source code window If you want to create other event procedures, then you need to select the list in the source code view. First, select the control at the left, and then select the event at the right. Note: To add events to Form1, select (Form1 Events) instead of Form1. 21

22 Introduction to Visual Basic - Book Names and types of events The most commonly used events of are Load and Click. Here is a shortlist of events we use in this course: Name of event Load Click DoubleClick * TextChanged MouseEnter MouseLeave When the event is raised Before a form is displayed the first time. In most cases, this is when the program starts running. When the user clicks a form or control (e.g. Button). When the user double clicks certain kinds of controls (e.g. Form, Label, TextBox). Note: A Button does not raise the DoubleClick event. When the content of the Text property is changed (either by user input or by Visual Basic.net code.) When the mouse enter a form or control When the mouse leaves a form or control We identify an event by connecting the name of the control and the name of the event using a dot symbol, e.g. Button1.Click, TextBox2.TextChanged, etc. Events related to the form are prefixed by Me or MyBase, e.g. MyBase.Load, Me.Click, Me.MouseEnter, etc. Class Work Identify the events raised by the actions below: Action Execute a program Click Button1 The text inside TextBox1 is changed The mouse cursor leaves Label3 Event raised * Only some types of controls raises DoubleClick events. For details, see TextChanged events are also raised when the form is created, before the Load event is raised. Open Form1.designer.vb in a text editor to see the reason behind. Careless implementation of TextChanged events always causes runtime errors. It is okay to use either Me.Load or MyBase.Load 22

23 Chapter 4 Events and Methods The structure of an event procedure You can look at the following example for the structure of an event procedure. Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click [Add your code here] End Sub Now the code is explained part by part: Part Private Sub Button1_Click (sender As System.Object, e As System.EventArgs) Handles Button1.Click End Sub Meaning Declare a procedure The name of the procedure. The default is the name of the event, with. changed to _. You can change this name by yourself. Currently unimportant. Usually abbreviated as ( ) in this book The event or events to handle. Change this to something like Handles Button1.Click, Button2.Click, Button3.Click to handle multiple events in the same procedure Marks the end of the event procedure TextChanged events A correct implementation of TextChanged events is provided here without explanation. You should read this section again after you finish Chapter 7. Public Class Form1 Dim loaded As Boolean = False Private Sub Form1_Load(...) Handles MyBase.Load loaded = True End Sub Private Sub TextBox1_TextChanged(...) Handles TextBox1.TextChanged If Not loaded Then Exit Sub ' Add your code below End Sub Private Sub TextBox2_TextChanged(...) Handles TextBox2.TextChanged If Not loaded Then Exit Sub ' Add your code below End Sub End Class 23

24 Introduction to Visual Basic - Book Methods A method is a procedure that exists inside the control. You invoke methods to do something with a control. Sometimes it is possible to the same thing using either properties or methods. For example, the statements Form2.Visible = True (setting a property) and Form2.Show() (invoke a method) do the same thing. Here is the list of methods we learn in the course: Control Method Meaning Example [All] Hide Hide a control/form. Form2.Hide() [All] Show Show a control/form. Form2.Show() Form ShowDialog Show a form as a modal dialog box. Form2.ShowDialog() (A modal form must be closed or hidden before you can continue working with the rest of the application.) Form Close Close a form. The program ends when the startup form (usually Form1) is closed. TextBox AppendText Add some text to the end of the current text in the text box. TextBox Paste Paste some text to the cursor/selection of the text box. Me.Close() TextBox1.AppendText("Hello") TextBox1.Paste("Hello") Example Public Class Form1 AppendText and Paste methods Private Sub ButtonAppendText_Click(...) Handles ButtonAppendText.Click TextBox2.AppendText(TextBox1.Text) End Sub Private Sub ButtonPaste_Click(...) Handles ButtonPaste.Click TextBox2.Paste(TextBox1.Text) End Sub End Class Note: When you run the example, you should note the way computer join strings. No extra spaces are added by the computer! 24

25 Chapter 4 Events and Methods Example Form1.vb Public Class Form1 Multiple forms (try the buttons and text box yourself) Private Sub ButtonShow_Click(...) Handles ButtonShow.Click Form2.Show() End Sub Private Sub ButtonShowDialog_Click(...) Handles ButtonShowDialog.Click ' Note: a runtime error will occur i f Form2 is already visible Form2.ShowDialog() End Sub End Class Form2.vb Public Class Form2 Private Sub ButtonClose_Click(...) Handles ButtonClose.Click Me.Close() End Sub End Class Class Work 4.3 Write down Visual Basic statements for each of the following operation. Operation Append Hello! to TextBox1. Show Form3 as a modal dialog box. Show Form2 as a modal dialog box. The current form is hidden until Form2 is closed. VB statement(s) A hidden form is NOT closed. If the startup form is hidden, then the program may not end properly! (If it happens, you can terminate the program in the Task Manager.) 25

26 Introduction to Visual Basic - Book 1 Exercise 4 1. Write a program with two controls: Button1 and Button2. When Button1 is clicked, the following things should happen: (a) Button1 is disabled, (b) The text in Button1 becomes Disabled!!!, (c) Button2 becomes visible, and (d) The form s background colour becomes pink. And when Button2 is clicked, the following things should happen: (a) Button1 is enabled, (b) The text in Button1 goes back to Button1, (c) Button2 becomes invisible, and (d) The form s background colour becomes yellow. Make sure your source code is inputted in the correct events! Check for spelling mistakes! 2. Write a program to tease the user with a button labelled Click Me. In this program, the user cannot press a button because the following happens: When the mouse is inside the form, the button is disabled. When the mouse is outside the form, the button is enabled again. To verify that the tease works, you should also add a Click event for the button, which changes the text of the button to Clicked. (Note: I would like to replace the words form by button in the question, but the program created that way does not work properly.) 26

27 Chapter 5 Variables, Input and Output Chapter 5 Variables, Input and Output 5.1 Variables In Visual Basic, you use variables to store values. Variables have a name and a data type Declaring variables with Dim statement Before you use a variable, you should tell Visual Basic in advance, using the Dim statement: Dim variablename [As type] The data type of the variable can be one of the following: Data type Description Integer Integer (between -2,147,483,648 through 2,147,483,647) Single Single-precision floating-point (about 7 significant figures). Double Double-precision floating-point (about 16 significant figures). Boolean True or False String Text (holds your name, ID card number, etc ) Example Dim statements Dim score As Integer Dim weight As Double Dim finished As Boolean Dim name As String Class Work Determine the correct data type(s) for the following data, and write down a Dim statement, where the name of the variable is the underlined word. Description of data Data type Dim statement The height of a student (in cm). The marks of a test. The academic grade of a student. The age of your father. Whether you are wealthy. 27

28 Introduction to Visual Basic - Book Keywords and names of variables Some words have special meanings in Visual Basic, such as Dim, As, Integer, Double, Boolean, and String. These words are known as keywords, or reserved keywords. The list of keywords in Visual Basic 2010 can be found in The name of a variable cannot be any of the reserved keywords. In addition, it must obey the following rules: Visual Basic is not case sensitive, e.g. ScOrE and score are the same name in Visual Basic. It must begin with an alphabetic character or an underscore (_). It must only contain alphabetic characters, decimal digits, and underscores. It must contain at least one alphabetic character or decimal digit. It must not be more than 1023 characters long Giving values to variables in the Dim statement We can assign a value to a variable in the same Dim statement. This is a good practice in programming because it eliminates potential mistakes. Example Dim statements with initial values Dim score As Integer = 0 Dim weight As Double = Dim finished As Boolean = False Dim name As String = "Chan Tai Man" Declaring several variables in a single line We can declare several variables in a single Dim statement. However, in Visual Basic, we cannot assign a value to these variables at the same time. Example Dim statements with several variables in one line Dim a, b, c As Single, x, y As Double, i As Integer ' a, b, and c are all Single; x and y are both Double 28

29 Chapter 5 Variables, Input and Output Using variables to store values We can assign a value to a variable with the equal sign =. Once a new value is stored in the variable, the old value is forgotten. See the example below: Example Use of variables Dim x, y As Integer x = 5 ' x is set to 5 y = 3 ' y is set to 3 x = 2 ' x becomes, y is unchanged y = x + 2 ' y becomes (x+2)=(2+2)=4, x is unchanged x = y + 3 ' x becomes (y+3)=(4+3)=7, y is unchanged Class Work With reference to the programs below, write down the values of the variables after the execution of each of the statements. Program Value of a Value of b Value of t Dim t As Integer Dim a As Integer = 2 2 Dim b As Integer = t = a a = b b = t Program Value of p Value of q Dim p As Double = 3 Dim q As Double = 4.5 q = p p = 2 p = (p + q) / 2 q = (p - q) / 2 29

30 Introduction to Visual Basic - Book Scope of variables The variable can be declared inside the Class or inside a Sub procedure. The variable is only effective inside the Class or Sub structure. Once the given structure finishes execution, the variable is deleted and its value is forgotten. Here is an example: Variable inside Class Public Class Form1 Dim x As Integer = 20 Private Sub Button1_Click(...) Handles Button1.Click x = x + 10 'increase x by 10 Label1.Text = x End Sub End Class After execution of Sub Button1_Click, the new value of x is remembered. Therefore, x is increased by 10 every time we click the button. Variable inside Sub Public Class Form1 Private Sub Button1_Click(...) Handles Button1.Click Dim x As Integer = 20 x = x + 10 'increase x by 10 Label1.Text = x End Sub End Class After execution of Sub Button1_Click, the new value of x is forgotten because x is declared inside Sub Button1_Click. Therefore, x becomes = 30 every time we click the button. If a variable exists both inside and outside the sub, the variable inside the sub is used. You may also declare variable inside an If Then Else statement (see chapter 7) or other statement blocks. Like the case with Sub procedure, those variables will be forgotten after the statement block finish execution. 30

31 Chapter 5 Variables, Input and Output Class Work The following is a program with label Label1 and buttons Button1, Button2 and Button3. Public Class Form1 Dim x As Integer = 5 Private Sub Button1_Click(...) Handles Button1.Click Dim x As Integer x = 8 Label1.Text = x End Sub Private Sub Button2_Click(...) Handles Button2.Click End Sub Label1.Text = x Private Sub Button3_Click(...) Handles Button3.Click x = 12 Label1.Text = x End Sub End Class Now the program is run and the buttons is pressed step by step as follows. Determine the value of Label1.Text after each step: Step Action Value of Label1.Text 1 Run the program. 2 Click Button1. 3 Click Button2. 4 Click Button3. 5 Click Button1. 6 Click Button2. 31

32 Introduction to Visual Basic - Book Introduction to InputBox and MsgBox Besides using labels and textboxes, we can use InputBox and MsgBox to handle input and output. In this part, we only deal with the simplest way to handle input and output. The details will be discussed in the second term InputBox To use an InputBox to input a string, we use the following statement: variablename = InputBox("message") If you want to input a number, you should use the Val function to convert the string into a number, i.e. Without the Val function, a runtime error occurs if the user press Cancel in the InputBox. variablename = Val(InputBox("message")) Here is an example asking for your name using InputBox: * Example InputBox Dim s As String s = InputBox("What is your name?") * Starting from this chapter, there may be no Button1 and Button1.Click in the examples. You should create the button Button1 and the event Button1.Click yourself, and then input the code inside Button1.Click. 32

33 Chapter 5 Variables, Input and Output MsgBox To use MsgBox to display a message, we use the following statement: MsgBox("message") Here is an example using MsgBox: Example MsgBox MsgBox("This is a wonderful message!") If you want to check the value of a variable, you can add a MsgBox statement in your source code.. If you want to display a message with more than one line, see the example below. The & character join strings together, and vbcrlf is similar to the Enter key in the text editor. (Note: the example below should be typed in a single line.) Example Multiline MsgBox MsgBox("You Won!!!" & vbcrlf & vbcrlf & "You rolled a 6" & vbcrlf & "and the computer rolled a 5!") 33

34 Introduction to Visual Basic - Book Miscellaneous examples Example Calculate the product of two numbers Dim x, y, product As Double x = Val(InputBox("Input the first number.")) y = Val(InputBox("Input the second number.")) product = x * y MsgBox("Their product is: " & product) Exercise 5 1. Rewrite Example into a program that does not use InputBox and MsgBox. Use text boxes and labels instead. 2. Write a program that uses InputBox to input two numbers when the form is loaded. The numbers are stored into two variables. Now the user can press one of the four buttons, that each of the buttons does a basic arithmetic operation (i.e. +,, or ) * on the two numbers, and then display the results using a MsgBox. Note: Do not use any text boxes or labels in this question. 3. Suggest a name and a data type of the variable for each of the following. Meaning of variable Name of variable Data type The name of a student. The class of a student. The date of the computer examination. * The symbol can be typed by pressing Alt It means holding the Alt key and typing 0215 in the numeric keypad while the Alt key is held. Similarly, can by typed by pressing Alt

35 Chapter 6 Operators and Expressions Chapter 6 Operators and Expressions You can enter expressions in Visual Basic to do calculations. The expressions in Visual Basic are similar to what you type in your calculator, Casio fx-50fh. Operators in Visual Basic can be classified into arithmetic operators, (string) concatenation operators, relational operators and logical operators. The first two types are discussed in this chapter, while the others are discussed in the next chapter. 6.1 Arithmetic Operators Here is a list of the arithmetic operators in Visual Basic You should get yourself familiar with the operators \ and Mod before examinations. Operator Meaning Example Result + Addition Subtraction (unary) Negation -2-2 * Multiplication 3*8 24 / Division (floating point) 14/4 3.5 \ Integer division Note: Avoid decimals. They are rounded off in a strange way. 14\ \ Mod Find the remainder of division (decimals are okay) 10 Mod Mod ^ Exponentiation 3^4 81 Class Work 6.1 Evaluate the following expressions: VB expression Result VB expression Result * ^ 5 15 / 7 5 ^ 2 15 \ 6 3 ^ (5-2) 15 Mod 6 5 ^ (-2) 35

36 Introduction to Visual Basic - Book Parentheses (singular: parenthesis) Parentheses (or brackets) are used to force some parts of an expression to be evaluated before others. Do not forget to add a pair of parentheses. E.g. should be a/(b+c) but not a/b+c. Class Work 6.2 Convert the following expressions into Visual Basic code: Mathematical expression a = b + c VB statement ( ) 6.3 Concatenation Operators Concatenation is the operation joining two strings together. The strings are simply joined together into a single string. There are two concatenation operators in Visual Basic: + and &. The & operators is preferred because it is defined for strings only. Example: "Con" & "caten" & "ation" evaluates to "concatenation" Note: No space is added to the strings. Adding a number and a string by the + sign, e.g., "4" + 2, will produce a runtime error! 36

37 Lowest Highest Chapter 6 Operators and Expressions 6.4 Operator Precedence Similar to Mathematics expressions, the evaluation of expressions in Visual Basic is ordered by its operator precedence. Here is the list * : (Note: operators that are introduced in the future are also listed here.) 11 Exponentiation ^ 10 Unary identity and negation + 9 Multiplication and floating-point division * / 8 Integer division \ 7 Modulus arithmetic Mod 6 Addition and subtraction + 5 String concatenation & 4 Relational/comparison operators = <> < <= > >= 3 Negation Not 2 Conjunction And 1 Inclusive disjunction Or For operators in the same order of precedence, the calculations are done left-to-right. Class Work 6.4 Evaluate the following expressions: VB expression "con" & "cent" & "rated" 4 * 3 & 5 * / 2 * \ 2 * 3-35 Mod 3 ^ 3 Result * The operator precedence table above is only valid of Visual Basic. It is different for Excel. A unary operator has only one expression next to it. e.g. the sign in. 37

38 Introduction to Visual Basic - Book Increment and Decrement One of the most common operations in a program is to increase and decrease the value of a variable. See the following example for an idea. ' Assigns the value 10 to the variable. applessold = 10 ' The following statement increments the variable. applessold = applessold + 1 ' The variable now holds the value 11. In the previous example, the statement applessold = applessold + 1 increase the value of the variable applessold by 1. You may also write applessold += 1 instead. In the example, += is an assignment operator, which add a certain value to a variable. -=, *=, /=, \=, ^= and &= are having similar meanings. Class Work 6.5 Express the following actions into Visual Basic code: Action Increase x by 100 Decrease y by 5 Multiple product by -3 Add "sugar" to the end of the str1 Move the form downwards by 3 pixels Move the form to the left by 20 pixels VB statement 6.6 Val function In Chapter 5, you have learnt to use the Val function to convert strings into numbers. We use the Val function in either of the following ways: variablename = Val(InputBox("message")) variablename = Val(TextBox1.Text) We call Val a function because it returns a value that can be assigned to a variable. Similarly, InputBox is also a function. 38

39 Chapter 6 Operators and Expressions 6.7 Mathematical Constants and Functions Here is a list of mathematical constants and functions you will use in Visual Basic: Function Meaning Example Result Math.Sqrt Find the square root of a number. Math.Sqrt(4) 2 Math.Abs Math.Sign Absolute value (remove the negative sign). The sign of the number. 1 = positive, 0 = zero, -1 = negative Math.Abs(-4) 4 Math.Sign(-5) -1 Math.PI The value of. Math.PI Int (or Math.Floor) Fix (or Math.Truncate) Round down to the nearest integer. Find the integral part of a number. Int(2.3) Int(-2.3) Fix(2.3) Fix(-2.3) Math.Ceiling Round up to the nearest integer. Math.Ceiling(2.3) Math.Ceiling(-2.3) Math.Round Round off a number to an integer or a Math.Round(12.56) 13 specified number of decimal places. Math.Round(12.56,1) 12.6 (Note: The behavior of this function is a bit different from what you learn in Mathematics. See MSDN for details.) Math.Max Returns the larger of two numbers. Math.Max(2, 5) 5 Math.Min Returns the smaller of two numbers. Math.Min(2, 5) 2 Class Work 6.7 Convert the following expressions into Visual Basic code: Mathematical expression VB statement ( ) 39

40 Introduction to Visual Basic - Book Miscellaneous Examples Example Find the value of a polynomial Public Class Form1 Private Sub Button1_Click(...) Handles Button1.Click Dim x As Double = Val(TextBox1.Text) LabelResult.Text = -x ^ 2 + x + 3 TextBox1 End Sub End Class Button1 LabelResult Example Cycling through 0 to 9 Public Class Form1 Private Sub Button1_Click(...) Handles Button1.Click Dim n As Integer = Val(Label1.Text) Label1.Text = (n + 1) Mod 10 End Sub End Class Label1 Button1 Example Public Class Form1 Marks Counter Private Sub ButtonPlus20_Click(...) Handles ButtonPlus20.Click Dim marks As Integer = Val(LabelMark.Text) LabelMarks marks = marks + 20 LabelMark.Text = marks ButtonPlus20 End Sub Private Sub ButtonMinus10_Click(...) Handles ButtonMinus10.Click Dim marks As Integer = Val(LabelMark.Text) marks = marks - 10 LabelMark.Text = marks End Sub End Class ButtonMinus10 40

41 Chapter 6 Operators and Expressions Exercise 6 1. Write a program to find the value of the n-th triangle number, i.e.. 2. Write a program to calculate the value of in. 3. Rewrite the programs in the previous questions, using InputBox for input and MsgBox for output. 4. Write a program similar to Example However, the number in the box should cycle backwards, i.e. 0, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 9, 5. Write a program that calculates percentage changes. The program should be able to find the original value, the new value and the percentage change. (Hint: What are the formulas?) 6. Write a program with four buttons: up, down, left and right. When you press the button, the form moves itself by 10 pixels, to the direction indicated by the button. 41

42 Introduction to Visual Basic - Book 1 Chapter 7 Flow Control (1) 7.1 Decision Making A logical expression can give Boolean results, i.e. True or False. A computer can use these results to make decisions. 7.2 Relational Operators Relational operators (or comparison operators) are the equality signs and inequality signs in Mathematics. If the equality or the inequality is satisfied, then the result is True, otherwise the result is False. Here is the list of the relational operators: Operator Meaning Example Result Opposite = Equal to 9 = 11 > Greater than 11 > 9 < Less than 11 < 9 >= Greater than or equal to 15 >= 15 <= Less than or equal to 9 <= 15 <> Not equal to 9 <> 9 Class Work 7.2 Convert the following comparison into Visual Basic code: Comparison 1. Is Peter (PHeight) taller than Mary (MHeight?) 2. Is Linda s age (LAge) not equal to May s age (MAge)? 3. The passing mark is 50. Has Gigi (GMark) failed the test? 4. It is free to travel by MTR for a person with height 100 cm or below. Is it free for Kitty (KHeight) to travel by MTR? 5. Is X an even number? VB code 42

43 Chapter 7 Flow Control (1) 7.3 If...Then...Else Statement The If...Then...Else statement conditionally executes a group of statements, depending on the value of an expression. (See the flowchart below.) The basic If Then Else syntax is listed here. If condition Then Statement1 Statement2... Else Statement3 Statement4... End If Sometimes we do not want to execute anything if the condition is false. In this case, we can skip the keyword Else, i.e. If condition Then Statement1 Statement2... End If 43

44 Introduction to Visual Basic - Book 1 The following example shows the use of If...Then...Else statements: Example Pass or failure Dim marks As Integer Dim result As String marks = Val(InputBox("Enter the marks:")) If marks >= 50 Then result = "Pass" Else result = "Fail" End If MsgBox(result) start input marks marks>=50? No result="fail" Yes result="pass" In the above example, if the marks is more than or equal to 50, then the result is Pass. Otherwise (i.e. marks less than 50), the result is Fail. output result end 7.4 Multiple conditions with keyword ElseIf If you want to have more than two outcomes, you can add additional conditions using the keyword ElseIf (else and if together in one word). If condition1 Then Statement1... ElseIf condition2 Then Statement2... (more ElseIf conditionals if applicable) Else Statement3... End If The spelling of the word ElseIf is E-L-S-E-I-F, not E-L-S-E-L-F! 44

45 Chapter 7 Flow Control (1) To see how the keyword ElseIf works, see the example below: Example Grading the results Dim marks As Integer start Dim grade As String marks = Val(InputBox("Enter the marks:")) input marks If marks >= 80 Then grade = "A" ElseIf marks >= 70 Then marks>=80? Yes grade = "A" grade = "B" No ElseIf marks >= 50 Then grade = "C" marks>=70? Yes grade = "B" Else grade = "D" End If MsgBox("Your grade is: " & grade) No marks>=50? No Yes grade = "C" In this example, the marks are used to determine the grade. When we check for the second condition marks >= 70, we already know that marks >= 80 is false. Therefore, only people with marks from 70 to 79 get a grade of B. grade = "D" output grade (Note: In this example, the marks are integers. This is NOT true in Tang Hin, where the marks are correct to 2 decimal places.) end Class Work With reference to Example 7.4.1, please fill in the table below: Marks 80 Grade A B C D 45

46 Introduction to Visual Basic - Book Official Syntax of If...Then...Else Statement Here is the official syntax of the If Then Else statement. The parts inside [] are optional. If condition [ Then ] [ statements ] [ ElseIf elseifcondition [ Then ] [ elseifstatements ] ] [ Else [ elsestatements ] ] End If -or- If condition Then [ statements ] [ Else [ elsestatements ] ] When you enter your code into the Visual Basic IDE, it will do the following changes automatically: Add the missing keyword Then Change Else If into ElseIf Change EndIf into End If And here is an example of the single line syntax. The symbol : is used to separate multiple statements. (Note: you may use : outside the If Then Else statement.) If A > 10 Then A = A + 1 : B = B + A : C = C + B 46

47 Chapter 7 Flow Control (1) Exercise 7 1. Write a program that the user will enter the number. If, output A big number! in a label, otherwise output A small number!. 2. Write a program that the user will enter the amount of pocket money he/she spends per week. Then the program will output a message according to the table below: Pocket Money ($) Message 0 You didn't spend any money. Are you lying? >0 and <$100 Below average. What a good student! >=$100 and <$200 Average. >$200 Above average. Consider to spend less! 3. BMI Calculator: The Body Mass Index (BMI) is a method to see if you are overweight or underweight. Your program should receive two inputs: mass (in kg) and height (in cm). Calculate the BMI by to the formula. (Be careful with the unit!!!) Finally, your program should determine the result according to the following table: BMI Classification <18.5 Underweight Average Overweight >= 28 Obese 4. Write a program that the user will enter his/her marks in Chinese, English and Mathematics subjects. Then output the number of subjects that he/she has failed (i.e. marks less than 50). 47

48 Introduction to Visual Basic - Book 1 Alphabetical Index A AppendText (method) As (keyword) B BackColor (property) Boolean (data type) Button (control) C Class (keyword) Click (event) Close (method) code comment compiler... 5 concatenation control... 12, 15 D data type... 18, 27 design time Dim (statement) Double (data type) DoubleClick (event) E Else (keyword) ElseIf (keyword) Enabled (property) event event procedure F False (keyword) Font (property) ForeColor (property) form form designer G graphical user interface GUI... See graphical user interface H Hide (method) I IDE... See Integrated Development Environment If (keyword) If...Then...Else (statement) InputBox Integer (data type) Integrated Development Environment K keyword L Label (control) Left (property) Load (event) M machine code... 5 mathematical functions Me (keyword)... 17, 22 method Microsoft Developer Network.. 14 Mod (operator) MouseEnter (event) MouseLeave (event) MSDN... See Microsoft Developer Network MsgBox MyBase (keyword) N Name (property) New Project... 9 O object program... 5 Open Project operator precedence operators arithmetic operators assignment operators concatenation operators relational operators P parentheses Paste (method) procedure program... 4 programming... 4 programming language... 5 BASIC... 6 C implementation... 5 JavaScript... 7 Logo... 6 Pascal... 6 PHP... 7 specification... 5 Visual Basic.net... 7 Properties Window property R reserved keywords runtime S Save All scope of variables Show (method) ShowDialog (method) Single (data type) Solution Explorer source code... 5, 12 String (data type) Sub (keyword) T Text (property) TextAlign (property) TextChanged (event)... 22, 23 Then (keyword) Toolbox Top (property) translator... 5 True (keyword) U user interface V Val (function)... 32, 38 variable vbcrlf (constant) Visible (property) Visual Basic 2010 Express... 8 W Windows Form Application.. 9, 15 48