variables programming statements

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1 1 VB PROGRAMMERS GUIDE LESSON 1 File: VbGuideL1.doc Date Started: May 24, 2002 Last Update: Dec 27, 2002 ISBN: Version: 0.0 INTRODUCTION TO VB PROGRAMMING VB stands for Visual Basic. Visual Basic is a programming language that lets you program window GUI programs very fast. GUI stands for Graphic User Interface. All windows programs are GUI's. These guides will teach you VB programming with step by step instructions. If you do all the exercises in these lessons you will become a very good VB programmer. In each lesson we use bold to introduce new programming concepts, underline for important programming concepts, violet for program definitions and blue for programming code examples. VB Data Types and Variables A program lists instructions as words to instruct a computer to do something. The words form a programming statement, just like we use words in a sentence. Which words are used defines the programming language. Each programming language as their own set of words. The words chosen for Visual Basic make it easy for some one to learn programming. All programs need data to represent values. Programming languages use numeric data like 10.5 or string data like "hello". These different types of data are known as data types. Here is a chart of all data types used in Visual Basic. Data Type Sample value Range of values String "hello there" Alphanumeric characters, digits and other characters Integer -5 Whole numbers ranging from -32,768 to 32,767 Long Large whole numbers having range: to Boolean True True or False Byte 10 small numbers range 0 to 255 Single Single-precision floating point numbers with six digits of accuracy. Double E30 Double-precision floating point numbers with 14 digits of accuracy Currency Decimal fractions, such as dollars and cents. Date #5/23/2002# month, day, year, various date formats available Object frm as Form Objects represent a group of many values Variant v as variant A variant can represent any data type. The variable's data type will depend on the value that was last assigned to it. Decimal Double-precision floating point number with 14 digits of accuracy. Data is stored in computer memory. The location and value where the data is stored is represented by a variable. A variable gets an identification name like: x. The identification name must start with a letter. Additional letters or digits may follow the first Letter, like: x1. VB Program variables programming statements

2 2 Declaring Variables Before you can use a variable it has to be declared. To declare a variable you use the Dim keyword and specify the data type. Dim x as Integer Code (blue) Dim identifier As datatype Definition (violet) In the above example x is declared as a variable of Integer data type. Keywords are used in programming languages as commands. Dim means declare variable, as means specify data type. Using Variables Think of a variable as a box that holds a value represented by an identifier. If you know the name of the identifier then you can get its value. The value is stored in computer memory represented by the variable. x 5 After you declare a variable, you can give it a value. When you give a value to a variable it is known as assigning or initializing. Here is an example declaring an Integer variable x and initializing to an integer value. Dim x As Integer x = 5 x 5 Here is an example of declaring a string variable and initializing it with a string value: Dim name As String name = "Ed" name "Ed" Later in your program you can assign different values to your variables: name = "Tom" name "Tom" In this situation the old value is replaced with the new value. The old value disappears, gone forever You can even assign a variable to another variable: Dim x As Integer Dim y As Integer x = 10 x 10 y = x y 10 In this situation the value of variable x is assigned to variable y. Remember, it is the value represented by the variable that is assigned not the name of the variable. The value is obtained from the name. As you use your variables the values will be always changing.

3 3 VB Sample Program We are now ready to write our first VB program. In this sample program we will initialize some variables and print their values to the screen using the debug.print statement. Programming languages use programming statements to do things. A statement includes a command word called a keyword and values. The debug.print statement allows you to print out messages on the computer screen. Messages are string constants enclosed by " double quotes ". Here we use the Debug.Print statement to print "Hi, my name is Tom" message on the computer screen. programming statement Debug.Print "Hi, my name is Tom" keywords string message You can also use a variable to print out values to the screen. Debug.Print name Here is our example program: Dim name as String Dim age as Integer name = "Tom" age = 20 Debug.Print "Hi, my name is " Debug.Print name Debug.Print "I am now " Debug.Print age Debug.Print "years old." Before you can write and run this program you first need to create a new Project. Make sure you have Visual Basic installed on your computer. We are using Visual Basic version 6.0. Open up Visual Basic, the following window pops up, select Standard EXE then click on the Open button.

4 4 The VB IDE Integreated Development Environment now appears as shown below. You write your VB program using the IDE. Just follow these step by step Instructions: [1] Right-click on Form1 in the Project Explorer window and select Remove Form1 from the pop up window. (this is an optional step) [2] From the Project menu click Add Module. [3] Click on the Open button to create Module1, then double-click the Module1 icon on the Project Explorer window to open it. [4] Enter the following program in the Module1 window as shown below. Notice, the program starts with Sub Main() and ends with End Sub Sub Main() Dim name as String Dim age as Integer name = "Tom" age = 20 Debug.Print "Hi, my name is " Debug.Print name Debug.Print "I am now " Debug.Print age Debug.Print "years old." End Sub

5 5 [5] Click on the View menu and select Immediate Window. [6] Click the Run menu and then click Start (or press F5 to run the program). The output of the program is shown in the Immediate window.

6 6 Understanding the example program Lets go through each line of the example program one by one for understanding. We will put a comment on each line. Comments are used to explain program code, the comments are never executed. Comments start with a ' single quotation. ' This is a Comment Our comments will be in color green to be easily recognized. Comments may start at the beginning of a line or at the end of a programming statement. age = 20 ' assign the number 20 to the variable age Programs are made up of programming statements. Programming statements are grouped together in a Sub. Subs are put into modules. When the VB program starts to run it executes the statements one by one contained in the Sub. When the last statement is executed the program ends. All Subs need a name for identification. The name of our Sub is Main. The Main is the first sub to be executed when the program runs. A VB program can only have 1 main sub. The round brackets () following Main means Main is a sub. Round brackets () distinguish a sub name from a variable name. Names of subs and variables are known as identifiers. Sub main() ' contains program Dim name as String ' declare a string variable to represent a persons name Dim age as Integer ' declare an integer variable to represent a persons age name = "Tom" ' assign the string "Tom" to the variable name age = 20 ' assign the number 20 to the variable age Debug.Print "Hi, my name is " ' print a message to the screen Debug.Print name ' print to the screen the persons name Debug.Print "I am now " ' print a message to the screen Debug.Print age ' print to the screen the persons age Debug.Print "years old." ' print a message to the screen End Sub ' ends program The program output:

7 7 RENAMING PROJECTS AND MODULES Every time you make a project it gets a default name Project 1. Every time you add a module, it gets a default name like Module 1. You should give projects and modules meaningful names. It's easy to do. Just go to the project window click on the module and in the Module properties box change the name. To change a project name right click on the project, select project properties from the pop up menu and the following dialog box appears. Dialog boxes allow you to enter information. In VB programming you will be using dialog boxes all the time. You will also be writing your own dialog boxes. Dialog boxes are also called forms. project type set startup to Main project name

8 8 Saving your projects and modules You need to save all the modules of your project separately from the file menu. From the file menu select Save Project As and then Save Module As. You only need to use "Save As" once. Next time just use Save Project and save Module. Be careful, save project only saves the projects but not the modules. You always need to save everything. VB LESSON1 EXERCISE 1 Write a program that has 3 variables to display a person's name, address and phone number. Declare and assign values to these variables. Print out to the screen using Debug.Print. Call your VB project Project1 and call your module Module1 VARIANTS You may need a variable to represent any kind of data type. In this situation you may declare the data type as Variant. This means that the data type of the variable will depend on the value assigned to it. Dim s As Variant s = "Hello" The String constant "Hello" is assigned to the variable s. Later in the program, you may assign a number to s. s = 10 Variants may be confusing to some but to others it may be more convenient. Variants are also the default data type. Dim s s is now a variant. In VB, you can declare variables on the same line separated by commas. Dim x, y As Integer You must be very careful here! x is a variant where y is an Integer. To make both Integer you need to write the statement as Dim x As Integer, y As Integer CONSTANTS There are situations in a program where you need to assign a value to variable but you never ever want that variable to change. These variables are called constants and are also known as "read only". This means you can read the value again but you can never write a new value to it. You use the keyword const to specify that a variable will be a constant and initialize it with a value. Const Max = 10

9 9 Once you initialize the constant variable with a value it can never change. The constant is only available for use in your module. To let other modules use your constant you proceed the constant declaration with the Public keyword. Public Const Max = 10 Once you declare and initialize your constant you use it just like any variable. Dim x As Integer x = Max Now x has the value 10. Remember you can never assign new values to your constant variable. Max = 20 EXPLICIT OPTION It is not always necessary to use the Dim statement to declare variables. You can use variables right away. x = 10 It is advised that you declare variables before you use them. To make sure you declare every variable before you use it, you put the Option Explicit directive at the top of your program. Here's the last program example using Option Explicit Option Explicit Sub main() Dim name as String Dim age as Integer name = "Tom" age = 20 Debug.Print "Hi, my name is " Debug.Print name Debug.Print "I am now " Debug.Print age Debug.Print "years old." End Sub VB LESSON1 EXERCISE 2 Change your Exercise 1 to use Variants, Constants and the Explicit Option. Call your module Module2. You can put a second module in your project and in the Project Properties you can set the startup to this module. You will have to rename Main to Main1 in Module 1. If you run your program many times you need to clear the Immediate window. It retains all previous results. Just highlight all the entries and press the delete button, or right click on it using the mouse and select cut from the pop up menu.

10 10 INPUT BOX Your program will need to get data values from the keyboard. A good way to get data from the keyboard is to use an Input Box. These look very professionals and impressive. Each Input Box gets a prompt message to tell the user what to do. Once they enter the data they can click on the OK button or Cancel button. InputBox is a built in VB function. Built in functions are pre-defined code that you can use right away. Functions receive values and return values. Using built in functions let you program very fast. You need to know how to use built in functions. Every built in function has a syntax that specifies how to use it. The syntax for the Input Box function is a little overwhelming. InputBox (prompt[, title] [, default] [, xpos] [, ypos] [, helpfile, context]) Every function gets arguments. arguments are used to pass values to the function. The InputBox function needs the prompt message. The other arguments enclosed in square brackets [ ] are optional meaning you do not need to supply values for them. Here are the arguments and their descriptions. Don't be too concerned right now about all the arguments right now. Argument prompt title Description Required. String expression displayed as the message in the dialog box. The maximum length of prompt is approximately 1024 characters, depending on the width of the characters used. If prompt consists of more than one line, you can separate the lines using a carriage return character (Chr(13)), a linefeed character (Chr(10)), or carriage return linefeed character combination (Chr(13) & Chr(10)) between each line. Optional. String expression displayed in the title bar of the dialog box. If you omit title, the application name is placed in the title bar.

11 11 default xpos ypos helpfile context Optional. String expression displayed in the text box as the default response if no other input is provided. If you omit default, the text box is displayed empty. Optional. Numeric expression that specifies, in twips, the horizontal distance of the left edge of the dialog box from the left edge of the screen. If xpos is omitted, the dialog box is horizontally centered. Optional. Numeric expression that specifies, in twips, the vertical distance of the upper edge of the dialog box from the top of the screen. If ypos is omitted, the dialog box is vertically positioned approximately one-third of the way down the screen. Optional. String expression that identifies the Help file to use to provide contextsensitive Help for the dialog box. If helpfile is provided, context must also be provided. Optional. Numeric expression that is the Help context number assigned to the appropriate Help topic by the Help author. If context is provided, helpfile must also be provided. The following string constants can be used anywhere in your code in place of actual values: Constant Value Description vbcr Chr(13) Carriage return vbcrlf Chr(13) & Chr(10) Carriage return linefeed combination vbformfeed Chr(12) Form feed; not useful in Microsoft Windows vblf Chr(10) Line feed vbnewline Chr(13) & Chr(10) or Chr(10) vbnullchar Chr(0) Character having the value 0 vbnullstring String having value 0 Platform-specific newline character; whatever is appropriate for the platform Not the same as a zero-length string (""); used for calling external procedures vbtab Chr(9) Horizontal tab vbverticaltab Chr(11) Vertical tab; not useful in Microsoft Windows Functions receive values do a calculation and return the calculated value. It's easy to use the InputBox function, all you have to do is assign the function to a variable. The variable gets the value that the user has entered into the input box Dim name As String name = InputBox("What is your name?") Here's a sample program that asks someone for their name and then prints their name to the screen. Sub main() Dim name As String name = InputBox("What is your name?") Debug.Print "Your name is " & name End Sub

12 12 Here's the input box Here's the program output: VB LESSON1 EXERCISE 3 Change your exercise 2 to use an Input Box. Ask someone for their name, address and age. Call your module Module3. Rename the Main sub in Exercise 2 to Main2 MSG BOX A message box lets you display information in a window. The message box is displayed until the OK button is pressed. A Message Box function has the following syntax: MsgBox(prompt[, buttons] [, title] [, helpfile, context]) Again the optional supplied argument values are in square brackets[ ]. We just use prompt and title. Here are the descriptions of the named arguments: Part prompt buttons title helpfile context Description Required. String expression displayed as the message in the dialog box. The maximum length of prompt is approximately 1024 characters, depending on the width of the characters used. If prompt consists of more than one line, you can separate the lines using a carriage return character (Chr(13)), a linefeed character (Chr(10)), or carriage return linefeed character combination (Chr(13) & Chr(10)) between each line. Optional. Numeric expression that is the sum of values specifying the number and type of buttons to display, the icon style to use, the identity of the default button, and the modality of the message box. If omitted, the default value for buttons is 0. Optional. String expression displayed in the title bar of the dialog box. If you omit title, the application name is placed in the title bar. Optional. String expression that identifies the Help file to use to provide context-sensitive Help for the dialog box. If helpfile is provided, context must also be provided. Optional. Numeric expression that is the Help context number assigned to the appropriate Help topic by the Help author. If context is provided, helpfile must also be provided.

13 13 Settings The buttons argument settings are: Constant Value Description vbokonly 0 Display OK button only. vbokcancel 1 Display OK and Cancel buttons. vbabortretryignore 2 Display Abort, Retry, and Ignore buttons. vbyesnocancel 3 Display Yes, No, and Cancel buttons. vbyesno 4 Display Yes and No buttons. vbretrycancel 5 Display Retry and Cancel buttons. vbcritical 16 Display Critical Message icon. vbquestion 32 Display Warning Query icon. vbexclamation 48 Display Warning Message icon. vbinformation 64 Display Information Message icon. vbdefaultbutton1 0 First button is default. vbdefaultbutton2 256 Second button is default. vbdefaultbutton3 512 Third button is default. vbdefaultbutton4 768 Fourth button is default. vbapplicationmodal 0 Application modal; the user must respond to the message box before continuing work in the current application. vbsystemmodal 4096 System modal; all applications are suspended until the user responds to the message box. vbmsgboxhelpbutton Adds Help button to the message box VbMsgBoxSetForegro und Specifies the message box window as the foreground window vbmsgboxright Text is right aligned Specifies text should appear as right-to-left vbmsgboxrtlreading reading on Hebrew and Arabic systems The first group of values (0 5) describes the number and type of buttons displayed in the dialog box; the second group (16, 32, 48, 64) describes the icon style; the third group (0, 256, 512) determines which button is the default; and the fourth group (0, 4096) determines the modality of the message box. When adding numbers to create a final value for the buttons argument, use only one number from each group. Note these constants are specified by Visual Basic for Applications. As a result, the names can be used anywhere in your code in place of the actual values. Return Values Constant Value Description vbok 1 OK vbcancel 2 Cancel vbabort 3 Abort vbretry 4 Retry vbignore 5 Ignore vbyes 6 Yes vbno 7 No

14 14 Remarks When both helpfile and context are provided, the user can press F1 to view the Help topic corresponding to the context. Some host applications, for example, Microsoft Excel, also automatically add a Help button to the dialog box. If the dialog box displays a Cancel button, pressing the ESC key has the same effect as clicking Cancel. If the dialog box contains a Help button, context-sensitive Help is provided for the dialog box. However, no value is returned until one of the other buttons is clicked. Note To specify more than the first named argument, you must use MsgBox in an expression. To omit some positional arguments, you must include the corresponding comma delimiter Here's our example program using a MsgBox Sub Main() Dim name As String name = InputBox("What is your name?") MsgBox "Your name is " & name End Sub Notice we use the '&' operator to join a message and a variable value. Operators are used to do operations with variables. The '&' can also be used to join strings together but not two numeric values. VB LESSON1 EXERCISE 4 Change your exercise 1 to use include a MsgBox. Ask the user to enter their name, address and age using the Input Box and then display them in a MsgBox. Use a line feed character Chr(10) or Vbcrlf to separate lines in the MsgBox. Call your module Module4 ARRAYS Arrays represent many variables of the same data type all accessed using a common name. Each variable of the array is known as a cell or element. x x0 x1 x2 x3 x4 You may want to use an array to record temperature of every hour for a certain day. temps It's easy to make an array you just declare a variable stating a lower bound and an upper bound. Dim a(0 to 4) as Integer Dim ( lowerbound to upperbound ) The lowerbound starts at 0 and can be omitted. Dim a(4)

15 15 Once you declare your array you can store values in it, this is known as accessing. You access the array cell elements by an index. Each cell element in the array is located using an index. a(2) = 5 Our declared array has 5 elements with indexes from 0 to 4. The value 5 will be located at index You can get the value from an array by specifying the array element index and assign the value at he specified index to another variable, Dim x as Integer x = a(2) Now variable x has the value 5. Arrays of 1 row are known as 1 dimensional arrays. VB LESSON1 EXERCISE 5 array name index value Declare a 1 dimensional array of 5 elements to store string values. Get 5 different names using an Input Box and store in the array. Using another Input Box ask the user to type in a number between 1 to 5. Display the name corresponding to the number in a MsgBox. Call your module Module5. Warning to not declare an array as name(4). Some people like their arrays to start at 1 rather than 0. Use the following statement at the top of your program to make all arrays to start at lower bound 1. Option base 1 2 dimensional arrays An array may have many rows and columns these arrays are known as 2 dimensional arrays, there are now two dimension rows and columns. To declare a two dimensional array you must declare the number of rows and columns. columns dim b ( 2, 3) As Integer rows rows columns

16 16 You assign values to the array cells by specifying row and column indexes row b(1, 2) = 5 column columns rows VB LESSON1 EXERCISE 6 Declare a 2 dimensional array of 4 rows and two columns. Using an Input Box ask the user for 4 names. Store each name in the first column of each row. For each name ask the user their favorite hobby, using an InputBox. Store the hobby in the second column of each row. Ask the user to type in a number between 1 and 4. using an Input Box. Display the person's name and hobby in a Message Box. Call your module Module6

17 17 IMPORTANT You should use all the material in all the lessons to do the questions and exercises. If you do not know how to do something or have to use additional books or references to do the questions or exercises, please let us know immediately. We want to have all the required information in our lessons. By letting us know we can add the required information to the lessons. The lessons are updated on a daily bases. We call our lessons the "living lessons". Please let us keep our lessons alive. all typos, unclear test, and additional information required to: courses@cstutoring.com all attached files of your completed exercises to: Order your next lesson from: students@cstutoring.com This lesson is copyright (C) 2002 by The Computer Science Tutoring Center " This document is not to be copied or reproduced in any form. For use of student only