Introduction To The Visual Basic Programming Language

Introduction to the Visual Basic Programming Language Microsoft Visual Basic 2005 Express Edition is a fast and easy way to create programs for Microsoft Windows. Even if you are new to Windows programming, with Visual Basic you have a complete set of tools to simplify development. So what is Visual Basic? "Visual" refers to the method used to create what the user sees—the graphical user interface, or GUI. "Basic" refers to the BASIC (Beginners All-Purpose Symbolic Instruction Code) programming language, a language used by more programmers than any other language in the history of computing. You can create useful programs by learning just a few of its features. The following links will get you started with Visual Basic programming; each link includes examples, as well as access to additional information.

Programming Concepts What exactly is a programming language? The following links will give you some background on what a language is and how it stores different types of information.

Term The Basics: How Programming Works

Definition How a programming language works, along with basic terminology.

Representing Words, Numbers, and Values How variables store values and represent with Variables information, along with how to use variables. Words and Text: Using String Variables to How to use a String variable to represent words and Organize Words text. Arrays: Variables That Represent More Than One Value

How to use an Array variable to represent several values of the same type.

Arithmetic: Creating Expressions With Variables And Operators

How to write code that performs arithmetic.

Comparisons: Using Expressions to Compare Values

How to write code that compares numeric values.

Your First Program Ready for a bit of real-world programming? The following links will walk you through the creation of a simple program and will show you how to check your program for errors.

Term Making Your Computer Do Something: Writing Your First Procedure

Definition How to write code that tells your program to perform a particular action.

Making Your Program Repeat Actions: Looping With the For...Next Loop

How to write code that repeats actions in your program and counts how many times these actions have been performed.

Making Your Program Choose Between How to write code that does different things in Two Possibilities: The If...Then Statement response to different conditions. What To Do When Something Goes

How to write code that handles errors in your 1

Wrong: Handling Errors

programs. You will also learn about the different types of errors.

More About Visual Basic The following links will help you build on your knowledge of programming and of the Visual Basic 2005.

Term Closer Look: Understanding Properties, Methods, and Events Closer Look: Data Types

Definition How properties, methods, and events work. How data is stored using the different types of variables.

Closer Look: Converting from One Variable How to convert data from one type to another, Type to Another along with some common pitfalls of this process. How to use the Do...While and Do...Until Closer Look: Using Do...While and statements to repeat code based on certain Do...Until to Repeat Until a Condition is Met conditions. Closer Look: Using Select Case to Decide Between Multiple Choices

How to run code based on multiple conditions, where there are many choices.

Visual Basic Guided Tour

More of what you can do with the Visual Basic 2005 programming language

The Basics: How Programming Works Before you jump in and start learning the Visual Basic programming language, it may help you to understand what a programming language is and how it works, including some programming terminology. The best place to start is with the basics.

How Programming Works On its own, a computer isn't very smart. A computer is essentially just a big bunch of tiny electronic switches that are either on or off. By setting different combinations of these switches, you can make the computer do something, for example, display something on the screen or make a sound. That's what programming is at its most basic—telling a computer what to do. Of course, understanding which combination of switches will make the computer do what you want would be a daunting task—that's where programming languages come in.

What is A Programming Language? People express themselves using a language with many words. Computers use a simple language consisting of only 1s and 0s, with a 1 meaning "on" and a 0 meaning "off." Trying to talk to a computer in its own language would be like trying to talk to your friends using Morse code—it can be done, but why would you? A programming language acts as a translator between you and the computer. Rather than learning the computer's native language (known as machine language), you can use a programming language to instruct the computer in a way that is easier to learn and understand.

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A specialized program known as a compiler takes the instructions written in the programming language and converts them to machine language. This means that as a Visual Basic programmer, you don't need to understand what the computer is doing or how it does it, you just need to understand how the Visual Basic programming language works.

Inside the Visual Basic Language In many ways, Visual Basic is a lot like the language that you use every day. When you speak or write, you use different types of words, such as nouns or verbs, which define how they are used. Visual Basic also has different types of words known as programming elements that define how they are used to write programs. Programming elements in Visual Basic include statements, declarations, methods, operators, and keywords. As you complete the following lessons, you will learn more about these elements and how to use them. Written and spoken language also has rules, or syntax, that defines the order of words in a sentence. Visual Basic also has syntax—at first it may look strange, but it is actually very simple. For example, to state "The maximum speed of my car is 55", you would write:

Ca r.Speed .Max imum = 55 You will learn more about syntax later, and tools in Visual Basic such as IntelliSense provide you with guidance in using the correct syntax when writing programs. The language you write and speak also has structure: for example, a book has chapters with paragraphs that contain sentences. Programs written in Visual Basic also have a structure: modules are like chapters, procedures are like paragraphs, and lines of code are like sentences.

Next Steps In this lesson, you learned what a programming language is and how it works. In the next lesson, you will start learning how to use the Visual Basic programming language. Don't worry—you'll be speaking Visual Basic in no time at all!

Representing Words, Numbers, and Values with Variables Variables are an important concept in computer programming. A variable is a letter or name that can store a value. When you create computer programs, you can use variables to store numbers, such as the height of a building, or words, such as a person's name. Simply put, you can use variables to represent any kind of information your program needs. You might ask, "Why use a variable when I could just use the information instead?" As the name implies, variables can change the value that they represent as the program is running. For example, you might write a program to track the number of jelly beans you have in a jar on your desk. Because candy is meant to be eaten, the number of jelly beans in the jar is likely to change over time. Rather than rewriting your program every time you get a sugar craving, you can represent the number of jelly beans with a variable that can change over time. Storing Information in Variables There are three steps to using a variable:

1.

Declare the variable. Tell the program the name and kind of variable you want to use.

2.

Assign the variable. Give the variable a value to hold.

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3.

Use the variable. Retrieve the value held in the variable and use it in your program. Declaring Variables

When you declare a variable, you have to decide what to call it and what data type to assign to it. You declare a variable using the Dim and As keywords, as shown below.

Dim aNumber As I n teger This line of code tells the program that you want to use a variable named aNumber , and that you want it to be a variable that stores whole numbers (the Integer data type). Because aNumber

is an Integer, it can store only whole numbers. If you had wanted to store 42.5,

for example, you would have used the Double data type. And if you wanted to store a word, you'd use a data type called a String. One other data type worth mentioning at this point is Boolean, which can store a True or False value. Here are more examples of how to declare variables.

Dim aDouble As Doub le Dim aName As St r i ngDim YesOrNoAs Boo lean For more information about other variable types, see Closer Look: Data Types. Assigning Variables You assign a value to your variable with the = sign, which is sometimes called the assignment operator, as shown in the following example.

aNumber = 42 This line of code takes the value 42 and stores it in the previously declared variable named aNumber. Declaring and Assigning Variables with a Default Value As shown above, you can declare a variable on one line of code, and then later assign the value on another line. This can result in an error if you try to use the variable before assigning it a value. For that reason, it is a better idea to declare and assign variables on a single line. Even if you don't yet know what value the variable will hold, you can assign a default value. The code for declaring and assigning the same variables shown earlier would look like the following.

Dim aDouble As Doub le = 0 Dim aName As St r i ng = "default string" Dim YesOrNo As Boo lean = True By declaring variables and assigning default values on a single line, you can prevent possible errors. You can still use assignment to give the variable a different value later on. Try It! In this exercise, you will write a short program that creates four variables, assigns them values, and then displays each value in a window called a message box. Let's begin by creating the project where the code will be stored.

To create the project

1.

If it is not already open, open Visual Basic from the Windows Start menu.

2.

On the File menu, click New Project.

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3.

In the New Project dialog box on the Templates pane, click Windows Application.

4.

In the Name box, type Var i ab les and then click OK.

Visual Basic will create the files for your program and open the Form Designer. Next, you'll create the variables.

To create variables and display their values 1.

Double-click the form.

The Code Editor opens to a section of code called Fo rm1_Load . This section of code, which is called a procedure, contains instructions that will be carried out when the form is first loaded into memory.

2.

In the Fo rm1_Load procedure, type the following code.

Dim an In tegerAs I n tege r = 42 Dim aS ing leAs S ing le = 39 .345677653 Dim aSt r i ngAs St r i ng= "I like candy" Dim aBoo leanAs Boo lean = True This code declares four variables—an Integer, a Single, a String, and a Boolean—and assigns their default values.

Tip As you typed the code, you may have noticed that after you typed As, a list of words appeared below the cursor. This feature is called Intellisense. It allows you to just type the first few letters of a word until the word is selected in the list. Once selected, you can press the TAB key to finish the word. Note Whenever you represent actual text in a program, you must enclose it in quotation marks (""). This tells the program to interpret the text as actual text instead of as a variable name. When you assign a Boolean variable a value of True or False, you do not enclose the word in quotation marks, because True and False are Visual Basic keywords with special meanings of their own. 3.

Beneath the code you wrote in the previous step, type the following.

MsgBox(anInteger) MsgBox(aSingle) MsgBox(aString) MsgBox(aBoolean) End The first four lines of code tell the program to display each value that you assigned in the previous step in a new window, using the MsgBox function. The final line tells the program to end after executing this procedure—it uses the End statement. 4.

Press F5 to run your program.

Click OK for each window as it appears. Note that the value of each variable is displayed in turn, and then the program ends. After the program has finished, you can go back and change the values that are assigned in the code and run the application again—you'll see that the new values are displayed.

Closer Look: Data Types 5

Data types in Visual Basic determine what kind of values or data can be stored in a variable, as well as how that data is stored. Why are there different data types? Think of it this way: if you had three variables, two of which held numbers while the third held a name, you could perform arithmetic using the first two, but you can't perform arithmetic on the name. Assigning a data type to a variable makes it easier to determine how the variable can—or can't—be used.

Note Data types are also used in other programming elements such as constants, properties, and functions. You will learn more about the other uses of data types in a following lesson. Data Types for Numbers Most computer programs deal with numbers in some form or another. Since there are several different ways to express numbers, Visual Basic has several numeric data types to deal with numbers more efficiently. The numeric data type that you will use the most is the Integer, which is used to represent a whole number (a number without a fractional part). When choosing a data type to represent whole numbers, you will want to use the Long data type if your variable will be storing numbers larger than approximately two billion; otherwise an Integer is more efficient. Not all numbers are whole numbers; for example, when you divide two whole numbers, the result is often a whole number plus a fraction (9 divided by 2 equals 4.5). The Double data type is used to represent numbers that have a fractional part.

Note There are additional numeric data types such as Decimal, Short, SByte, and UInteger; these are typically used in very large programs where memory usage or speed is an issue. For now, the basic numeric data types are all you will need. Data Types for Text Most programs also deal with text, whether displaying information to the user or capturing text entered by the user. Text is usually stored in the String data type, which can contain a series of letters, numbers, spaces, and other characters. A String can be of any length, from a sentence or a paragraph to a single character to nothing at all (a null string). For a variable that will always represent just one character, there is also a Char data type. If you only need to hold one character in a single variable, you can use the Char data type instead of a String.

Other Data Types In addition to text and numbers, programs sometimes need to store other types of information, such as a true or false value, a date, or data that has a special meaning to the program. For values that can be represented as true/false, yes/no, or on/off, Visual Basic has the Boolean data type. A Boolean variable can hold one of two possible values: True or False. Although you can represent dates or times as numbers, the Date data type makes it easy to calculate dates or times, such as the number of days until your birthday or the number of minutes until lunch. When you need to store more than one type of data in a single variable, you can use a composite data type. Composite data types include arrays, structures, and classes. You will learn more about these in later lessons.

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Finally, there are some cases in which the type of data that you need to store may be different at different times. The Object data type allows you to declare a variable and then define its data type later. You will also learn more about the Object data type in a later lesson.

Words and Text: Using String Variables to Organize Words In this lesson, you will learn how to use the String data type to represent words and text. In the previous lesson, you learned how to use variables to store data in your program, and that each variable must be of the appropriate type for the data that it will store. In this lesson, you will learn more about the String data type, which is used to store text. What Is a String? A string is any series of text characters, such as letters, numbers, special characters, and spaces. Strings can be human-readable phrases or sentences, such as "The quick brown fox jumps over the lazy dog," or an apparently unintelligible combination, such as "@#fTWRE^3 35Gert". String variables are created just as other variables: by first declaring the variable and assigning it a value, as shown below.

Dim aString As String = "This is a string" When assigning actual text (also called a string literal) to a String variable, the text must be enclosed in quotation marks (" "). You can also use the = character to assign one String variable to another String variable, as shown in this example.

Dim aSt r i ng As String = "This is a string" Dim bSt r i ng As String = "" The previous code sets the value of bSt r i ngto the same value as aSt r i ng(This is a string). You can use the ampersand (&) character to sequentially combine two or more strings into a new string, as shown below.

Dim aSt r i ng As String = "Across the Wide" Dim bSt r i ng As String = "Missouri" Dim cS t r i ng As String = "" cS t r i ng = aSt r i ng & bSt r i ng

The previous example declares three String variables and respectively assigns "Across the Wide" and "Missouri" to the first two, and then assigns the combined values of the first two to the third variable. What do you think the value of cString is? You might be surprised to learn that the value is Across

the WideMissouri because there is no space at the end of aString or at the beginning of bString. The two strings are simply joined together. If you want to add spaces or anything else between two strings, you must do so with a string literal, such as " ", as shown below.

Dim aSt r i ng As String = "Across the Wide" Dim bSt r i ng As String = "Missouri" Dim cS t r i ng As String = "" cS t r i ng = aSt r i ng & " " & bSt r i ng The text contained in cString now reads as Across the Wide Missouri. Try It!

To join strings

1.

On the File menu, click New Project.

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2.

In the New Project dialog box:

1.

In the Templates pane, click Windows Application.

2.

In the Name box, type Concatenation.

3.

Click OK.

A new Windows Forms project opens. 3.

Double-click the form to open the Code Editor.

4.

In the Form1.Load event procedure, declare four string variables and assign the string values, as shown below:

Dim aString As String = "Concatenating" Dim bString As String = "Without" Dim cString As String = "With" Dim dString As String = "Spaces" 5.

Add the following code to concatenate the strings and display the results:

' Displays "ConcatenatingWithoutSpaces". MsgBox(aString & bString & dString) ' Displays "Concatenating With Spaces". MsgBox(aString & " " &

The text displayed in the message box is the result of joining the string variables that were assigned in a previous step. In the first box, the strings are joined together without spaces. In the second, spaces are explicitly inserted between each string.

Arrays: Variables That Represent More Than One Value In this lesson, you will learn how to use arrays to store groups of values. As you learned in the previous lessons, variables are used to store different types of data for use by your program. There is another type of variable called an array that provides a convenient way to store several values of the same type. For example, suppose you were writing a program for a baseball team and you wanted to store the names of all of the players on the field. You could create nine separate string variables, one for each player, or you could declare an array variable that looks something like the code below.

Dim players() As String You declare an array variable by putting parentheses after the variable name. If you know how many values you need to store, you can also specify the size of the array in the declaration as follows.

Dim players(9) As String The size of the array is 9 because a baseball team has 9 players. An array is made up of a number of values, or elements, starting with element 0 and ending with one less than the number specified in the declaration. In this case, the array contains the elements 0 through 8, for a total of nine elements. When you want to refer to one of the players on the team, you just subtract 1. For example, to reference the first player, you reference element 0, to reference the ninth player, you reference element 8.

Assigning Values to Arrays

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As with other types of values, you need to assign values to arrays. To do so, you refer to the element number as part of the assignment, as shown below.

players(0) = "John" players(3) = "Bart" In the above code, the value J ohn is assigned to the first element of the array (element 0) and the value Bre t tis assigned to the fourth element (element 3). The elements of the array don't have to be assigned in order, and any unassigned element will have a default value—in this case, an empty string. As with other types of values, you can declare and assign values to an array on a single line as follows.

Dim p laye rs ( ) As Integer = {1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9} In this case, curly brackets indicate a list of values. The values are assigned to elements in the order listed. Notice that size of the array isn't specified—it is determined by the number of items that you list. Retrieving Values from Arrays Just as you use numbers to specify an item's position in an array, you use the element number to specify which value you want to retrieve.

Dim AtBat As String AtBat = p laye rs (3 ) The above code retrieves the fourth element of the array and assigns it to the string variable AtBat. Try It!

To store values in an array

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type MyFi r s tA r ray and then click OK.

A new Windows Forms project opens.

7.

4.

From the Toolbox, drag a Textbox control onto the form.

5.

From the Toolbox, drag a Button control onto the form.

6.

Double-click the Button to open the Code Editor.

In the Button1_Click event procedure, add the following code:

Dim p laye rs ( ) As String = {"Dan", "Fred", "Bart", "Carlos", _ "Ty", , , , "Pedro"} Dim i As Integer = C In t ( Tex tbox1 . Tex t ) "Juan" Notice that "Jay" the above"Sam" code uses the CInt function to convert the String value (Tex tBox1 . Tex) tto an Integer (i). You can learn more about conversions in Closer Look: Converting from One Variable Type to Another. 8.

Press F5 to run the program.

9.

Type a number between 0 and 8 in the text box and click the button. The name corresponding to that element is displayed in a message box.

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Tip You should write additional code to check that the data entered is valid. For example, you can check that the value entered is a numeric value between 0 and 8.

Arithmetic: Creating Expressions With Variables And Operators In this lesson, you will learn how to create expressions to perform arithmetic and then return values. An expression is a segment of code that performs arithmetic and then returns a value. For example, a simple addition expression is shown below. 5+4 The expression 5 + 4 returns the value 9 when evaluated and is made up of two parts: the operands (5 and 4), which are the values that the operation is performed on, and the operator (+), which specifies the operation to be performed. Using Values Returned by Expressions For an expression to be useful, you must do something with the value that is returned. The most common thing to do is to assign it to a variable, as shown below.

Dim an In tege r As Integer = 5 + 4 This example declares a new Integer variable called anInteger and assigns the value returned by 5 +

4 to it. Arithmetic Operators A common use for expressions is to perform arithmetic on variables: addition, subtraction, multiplication, or division. The following table describes the operators commonly used for arithmetic.

Operator + (addition)

Description Returns the sum of two operands

Example 5+4

- (subtraction)

Returns the difference of two operands

5-4

* (multiplication)

Returns the product of two operands

5*4

/ (division)

Returns the quotient of two operands

5/4

The kind of variable that you use when performing arithmetic can affect the result. Dividing two numbers often results in a return value that is not a whole number. For example, when you divide 3 by 2, the result is 1.5. If you assigned the return value of that expression to an Integer variable, it would be rounded to the nearest whole number, 2. When performing division, you should use a Double variable to store the return value.

Note You can also convert a variable from one data type to another by using the conversion functions of Visual Basic. For more information, To add numbers

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

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3.

In the Name box, type Ar i thmet i cand then click OK.

A new Windows Forms project opens.

4.

From the Toolbox, drag two Textbox controls onto the form.

5.

From the Toolbox, drag a Button control onto the form.

6.

Double-click the Button to open the Code Editor.

7.

In the Button1_Click event procedure, type the following code.

Dim A As Double = Tex tbox1 . Tex t Dim B As Double = Tex tbox2 . Tex t MsgBox(A + B) MsgBox(A - B) MsgBox(A * B) MsgBox(A / B) The first two lines declare the variables A and B, which will hold the numeric values used in this program, and assign the values of the two TextBox controls (their text) to the variables A and B. The final four lines create expressions with the two variables and each of the basic arithmetic operators, and display the results of those expressions in a message box. 8.

Press F5 to run the application.

9.

Type a number in each of the text boxes and click Button1.

Note If you type any other character into the text boxes, an error will occur. 10. Expressions are created using the two numbers that you enter and each of the four basic arithmetic operators (addition, subtraction, multiplication, and division). The result of each expression is displayed in a message box.

Closer Look: Converting from One Variable Type to Another As you have seen, variables come in different types. The type determines what kind of data a variable can hold. An Integer variable can hold only numeric data without decimal points. A String variable can hold only text. What happens when you want to display an Integer in a TextBox control that requires a String? The answer is that the data must be converted from one type to another. In this topic, you will explore how to convert data from one type to another, and you will learn some techniques used for data conversion, as well as some of its common pitfalls. Converting Variables to Text Every variable in Visual Basic can be converted to text by using a special function called CStr (which is shorthand for Convert to String). This function, as the name implies, returns the data represented by the variable as a String. The following procedure demonstrates a simple example of converting an Integer to text. Try It!

To convert a variable to text

1.

On the File menu, click New Project.

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2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type Convers i onand then click OK.

A new Windows Forms project opens. 4.

Double-click the form to open the Code Editor.

5.

In the Form1_Load event handler, type the following code.

Dim an In tege r As Integer = 54 MsgBox(CSt r (an In tege r ) ) This code declares an Integer variable called an In teger , assigns it a value of 54, and then converts that value to text and displays it in a message box by calling the CStr function.

6.

Press F5 to build and run your application. A message box that reads 54 is displayed.

Let's

try

something

just

for

fun.

In

the

Code

Editor,

change

the

line

that

reads

MsgBox(CSt r (an In tege r )to) read MsgBox(an In teger,) and press F5 to run. What happens? The program behaves exactly as it did before. Visual Basic is smart enough to know that what you really want is to convert the Integer to text to display in the message box. However, you cannot rely on this behavior for all cases—there are many variable types that cannot be automatically converted. Therefore, it is good practice to always use the CStr function, even if a variable would automatically be converted to text. In addition to converting Integer variables to text, the CStr function can be used on any numeric data type, such as Double or Long. It can also be used to convert the Date and Boolean data types to text. For more information on data types, see Closer Look: Data Types. Converting Between Numeric Data Types As you learned in the arithmetic lesson, sometimes the result of an arithmetic operation can't be expressed as an Integer. Just as Visual Basic has a function for converting numbers to text, it also has functions for converting variables from one numeric data type to another. For example, you can use the CDbl (Convert to Double) function in an arithmetic operation to return a fractional number when working with Integer variables. The following procedure shows how to use the CDbl function when dividing two integers. Try It!

To convert numeric data types 1.

In the Code Editor, delete the code that you entered in the previous procedure and type the following:

Dim A As Integer = 1 Dim B As Integer = 2 MsgBox(CDb l (A / B) ) This code declares two Integer variables (A and B), assigns them values of 1 and 2, and then converts the result of the division operation (A / B) using the CDbl function and displays it in a message box.

2.

Press F5 to build and run your application. A message box that reads 0.5 is displayed.

Visual Basic has functions for other types of numeric variables as well. For example, if you add two variables of the type Double and want to round the result to the nearest whole number, use the CInt

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function. Other numeric conversion functions include CByte, CDec, CLng, and CShort. For a list of all the Visual Basic conversion functions, see Type Conversion Functions.

Comparisons: Using Expressions to Compare Values In this lesson, you will learn how to use comparison operators to create expressions that compare values. In the last lesson, you learned how to use arithmetic operators to create numeric expressions and return numeric values. Another kind of operator, the comparison operators, can be used to compare numeric values and return Boolean (True or False) values. Comparison operators are most frequently used to compare values and make decisions based on that comparison. Making decisions in your program will be covered in-depth in Making Your Program Choose Between Two Possibilities: The If...Then Statement. The following table summarizes the comparison operators:

Operator

Description

Examples 5 = 4 (false) Returns True if the number on the left-hand side is = (equals) 4 = 5 (false) equal to the number on the right-hand side. 4 = 4 (true) 5 <> 4 (true) Returns True if the number on the left is not equal to <> (not equal to) 4 <> 5 (true) the number on the right. 4 <> 4 (false) 5 > 4 (true) Returns True if the number on the left is greater than > (greater than) 4 > 5 (false) the number on the right. 4 > 4 (false) 5 < 4 (false) Returns True if the number on the left is less than the < (less than) 4 < 5 (true) number on the right. 4 < 4 (false) 5 >= 4 (true) >= (greater than or Returns True if the number on the left is greater than or 4 >= 5 (false) equal to) equal to the number on the right. 4 >= 4 (true) 5 <= 4 (false) <= (less than or equal Returns True if the number on the left is less than or 4 <= 5 (true) to) equal to the number on the right. 4 <= 4 (true) Try It!

To compare expressions

1.

On the File menu, click New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type Compar i son and then click OK.

A new Windows Forms project opens.

4.

From the Toolbox, drag two Textbox controls onto the form.

5.

From the Toolbox, drag a Button control onto the form.

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6.

Double-click the Button to open the Code Editor.

7.

In the Button1_Click event handler, type the following code:

Dim A As Double = CDbl(Textbox1.Text) Dim B As Double = CDbl(Textbox2.Text) > A B)and MsgBox(A MsgBox(A = B) The first two lines declare MsgBox(A the variables the numeric values used in this B, which <willB)hold program; they use the CDbl statement to convert the text from Textbox1 and Textbox2 into numeric values. Finally, the last three lines create expressions to compare the two variables using three basic comparison operators, and display the results of those expressions in three message boxes. 8.

Press F5 to run the application.

9.

Type a number in each of the text boxes and click Button1.

The first message box will display True if A (the number that you entered in the first text box) is greater than B (the number that you entered in the second text box); otherwise it will display False. The second message box will display True if A is less than B, and the third message box will display True if both numbers are the same. Try typing different numbers into the text boxes to see how the results change.

Making Your Computer Do Something: Writing Your First Procedure In this lesson, you will learn how to create a procedure, a self-contained block of code that can be run from other blocks of code, and then how to create parameters for your procedures. A procedure is simply a piece of code that tells the program to perform an action. Although you may not have realized it, you have already used procedures in the previous lessons. For example, the MsgBox function is a built-in procedure that performs the action of displaying a dialog box. While Visual Basic has many built-in procedures for performing common actions, there will always be cases when you want your program to perform an action that a built-in procedure can't handle. For example, the MsgBox function cannot display a dialog box with a picture. You need to write your own procedure to accomplish this task. What is a Procedure? A procedure is a self-contained block of code that can be run from other blocks of code. Generally speaking, procedures each contain the code necessary to accomplish one task. For example, you might have a procedure called PlaySound, which contains the code required to play a wave file. While you could write the code to play a sound every time your program needed to make a noise, it makes more sense to create a single procedure that you can call from anywhere in your program. A procedure is run, or executed, by calling it in code. For example, to run the PlaySound procedure, you simply add a line of code to your program with the name of your procedure, as shown below.

PlaySound That's all there is to it! When the program reaches that line, it will jump to the PlaySound procedure and execute the code contained there. The program will then jump back to the next line that follows the call to PlaySound.

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You can call as many procedures as you like. Procedures are run in the order that they are called. For example, you might also have a procedure called DisplayResults; to execute it after executing the PlaySounds procedure, call the procedures as shown below.

P laySounds Disp layResu l t s Functions and Subs There are two kinds of procedures: functions and subroutines (sometimes called subs). A function returns a value to the procedure that called it, whereas a sub simply executes code. Subs are called when a line of code that contains the name of the sub is added to the program, as in the following example.

DisplayResults Functions are different, because functions not only execute code, they also return a value. For example, imagine a function called GetDayOfWeek that returns an Integer indicating the day of the week. You call this function by first declaring a variable in which to store the return value, and then assigning the returned value to the variable for later use, as shown below.

Dim Today As Integer Today = GetDayOfWeek In this example, the value returned by the function is copied to the variable named Today and stored for later use. Writing Procedures You write procedures by first writing a procedure declaration. A procedure declaration does several things: states whether the procedure is a function or a sub, names the procedure, and details any parameters the procedure might have (parameters will be discussed in detail later in this lesson). The following is an example of a simple procedure declaration.

Sub MyFi r s tSub( ) End Sub The Sub keyword tells the program that this procedure is a sub and will not return a value. The name of the sub (MyFi r s tSub ) comes next, and the empty parentheses indicate that there are no parameters for this procedure. Finally, the End Sub keyword indicates the end of the subroutine. All code that is to be executed by this sub goes between these two lines. Declaring functions is similar, but with the added step that the return type (such as Integer, String, etc.) must be specified. For example, a function that returned an Integer might look like this.

Function MyFi r s t Func t i on ( ) As Integer End Function The As I n tegerkeywords indicate that this function will return an Integer value. To return a value from a function, use the Return keyword, as shown in the following example.

Function GetTheNumberOne( ) As Integer Return 1 End Function This procedure will return the number 1. Try It!

To create procedures

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1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type MyFi r s tP rocedure and then click OK.

A new Windows Forms project opens. 4.

Double-click the form to open the Code Editor.

5.

In the Code Editor, locate the line that reads End C lass . This is the end of the code section that makes up your form. Immediately before this line, add the following procedure:

Function GetT ime( ) As String Return CSt r (Now) End Function This function uses the built-in Now procedure to get the current time, then uses the CStr function to convert the value returned by Now into a human-readable String. Finally, that String value is returned as the result of the function.

6.

Above the function you added in the previous step, add the following Sub.

Sub Disp layT ime( ) MsgBox(GetT ime) End Sub This sub calls the function GetT ime and displays the result returned by it in a message box.

7.

Finally, add a line to the Form1_Load event handler that calls the DisplayTime sub, as shown below.

Disp layT ime( ) 8.

Press F5 to run the program.

When the program starts, the Fo rm1_Load event procedure is executed. This procedure calls the DisplayTime sub, so program execution jumps to the DisplayTime sub procedure. That sub in turn calls the GetTime function, so program execution then jumps to the GetTime function. This function returns a String representing the time to the DisplayTime sub procedure, which then displays that string in a message box. After the sub is finished executing, the program continues normally and displays the form. Parameters in Functions and Subs At times you will need to provide additional information to your procedures. For example, in the PlaySound procedure, you may want to play one of several different sounds. You can provide the information on which sound to play by using parameters. Parameters are a lot like variables. They have a type and a name, and they store information just like variables. They can be used just like variables in a procedure. The two main differences between parameters and variables are:

•

Parameters are declared in the procedure declaration, not in individual lines of code.

•

Parameters are usable only in the procedure they are declared in.

Parameters are declared in the procedure declaration in the parentheses that follow the procedure name. The As keyword is used to declare the type, and each parameter is generally preceded by the ByVal keyword. This keyword will be added automatically by Visual Basic if you do not add it, and it has a fairly advanced function that is beyond the scope of this lesson to explain.

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An example of a sub with parameters is shown below.

Sub PlaySound(ByVal SoundFile As String, ByVal Volume As Integer) My.Computer.Audio.Play(SoundFile, Volume) End as Sub You would then call the sub with the values for the parameters shown below. PlaySound("Startup.wav", 1) You can also declare parameters for functions in exactly the same way you would for subs. Try It!

To create a function with parameters

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type paramete rs and then click OK.

A new Windows Forms project opens.

4.

From the Toolbox, drag two Textbox controls onto the form.

5.

From the Toolbox, drag a Button control onto the form.

6.

Double-click the Button to open the Code Editor.

7.

Immediately after the End Sub line of the But ton1_C l i ckevent handler, add the following procedure:

Function AddTwoNumbers ( ByVal N1 As Integer, ByVal N2 As Integer) _ As N1 + N2 End Function Integer Return procedure, add the following code: ton1_C l i ck 8. In the But Dim aNumber As Integer = C In t ( Tex tbox1 . Tex t ) Dim bNumber As Integer = C In t ( Tex tbox2 . Tex t ) MsgBox(AddTwoNumbers (aNumber , bNumber ) )

This code declares two integers and converts the text in the two text boxes to integer values. It then passes those values to the AddTwoNumbers function and displays the value returned in a message box. 9.

Press F5 to run the program.

10. Type a number value in each text box and click the button. The two numbers are added, and the result is displayed in a message box.

Making Your Program Repeat Actions: Looping With the For...Next Loop In this lesson, you will learn how to use the For...Next statement to repeat actions in your program, and to count how many times these actions have been performed.

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When you write a program, you frequently need to repeat actions. For example, suppose you are writing a method that displays a series of numbers on screen. You will want to repeat the line of code that displays the numbers as many times as necessary. The For...Next loop allows you to specify a number, and then repeat code contained within that loop for the specified number of times. The following example shows how a For...Next loop appears in code.

Dim i As Integer = 0 For i = 1 To 10 DisplayNumber(i) Next The For...Next loop begins with a counter variable, i. This is a variable that the loop uses to count the number of times it has executed. The next line ( For i

tells the program how many times to = 1 to ) 10

repeat the loop and the values i is going to have. When the code enters the For...Next loop, it starts with i containing the first value (in this case 1). The program then executes the lines of code between the For line and the Next line, in this case calling the DisplayNumber method with a parameter of i (in this case also 1). When the line Next is reached, 1 is added to i and program execution jumps back to the For line again. This repeats until the value of i is larger than the second number in the For line, in this case 10. When this occurs, the program continues with any code after the Next line. Try It!

To use the For...Next statement

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type ForNext and then click OK. A new Windows Forms project opens.

4.

From the Toolbox, drag one TextBox control and one Button control onto the form.

5.

Double-click the Button to open the Code Editor

6.

In the Button1_Click event handler, type the following code:

Dim i As Integer = 0 Dim NumberOfRepet i t i ons As Integer = C In t ( Tex tbox1 . Tex t ) For i = 1 To NumberOfRepet i t i ons 7.

Press F5 to run the program.

8.

In the text box, type a number and click the button.

MsgBox( "This line has been repeated " & i & " times") Next A message box appears as many times as you indicated in the text box.

Closer Look: Using Do...While and Do...Until to Repeat Until a Condition is Met In this lesson, you will learn how to use the Do...While and Do...Until statements to repeat code based on certain conditions.

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In the previous lesson, you learned how to use the For...Next statement to loop through a block of code a specified number of times—but what if the number of times that the code needs to be repeated is different for certain conditions? The Do...While and Do...Until statements allow you to repeat a block of code while a certain condition is True, or until a certain condition is True. For example, suppose you had a program to add a series of numbers, but you never wanted the sum of the numbers to be more than 100. You could use the Do...While statement to perform the addition as follows:

Dim sum As Integer = 0 Do While sum < 100 sum = sum + 10 Loop In the above code, the Do Whi le line evaluates the variable sum to see if it is less than 100: If it is, the next line of code is run; if not, then it moves to the next line of code following Loop. The Loop keyword tells the code to go back to the DoWhile line and evaluate the new value of sum.

To use a Do...While statement

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type DoWhile and then click OK.

A new Windows Forms project opens.

4.

From the Toolbox, drag one TextBox control and one Button control onto the form.

5.

Double-click the Button to open the Code Editor.

6.

In the Button1_Click event handler, type the following code:

Dim sum As Integer = 0 Dim counte r As Integer = 0 Do While sum < 100 sum = sum + C In t ( Tex tbox1 . Tex t ) counte r = counte r + 1 Loop MsgBox( "The loop

7.

Press F5 to run the program.

8.

In the text box, type a number and click the button.

has run " & CSt r ( counte r ) & " times!")

A message box appears displaying how many times the number was added to itself before reaching 100.

9.

On the Debug menu, choose Stop Debugging to end the program. Keep this project open. We are about to add to it. Do...Until Statement

The Do...While statement repeats a loop while a condition remains True, but sometimes you might want your code to repeat itself until a condition becomes True. You can use the Do...Until statement as follows:

Dim sum As Integer = 0 Do Until sum >= 100 sum = sum + 10 Loop This code is similar to the code for the Do...While statement, except that this time the code is evaluating the sum variable to see if it is equal to or greater than 100.

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To use a Do...Until statement

1.

Add the following code beneath the MsgBox line.

Dim sum2 As Integer = 0 Dim counte r2 As Integer = 0 Do Until sum2 >= 100 sum2 = sum2 + C In t ( Tex tbox1 . Tex t ) counte r2 = + 1theLoop MsgBox( "The loop has run " & CSt r ( counte r2 ) & Press F5 r2 to run program. 2. counte 3.

In the text box, type a number and click the button. A second message box appears displaying how many times the number was added to itself before equaling 100 or greater.

Making Your Program Choose Between Two Possibilities: The If...Then Statement In this lesson, you will learn to use the If...Then statement to run code based on conditions. Programs need to do different things in response to different conditions. For example, you might want your program to check what day of the week it is, and then do something different depending on the day. The If...Then statement allows you to evaluate a condition and to then run different sections of code based on the results of that condition. The following example demonstrates how the If...Then statement works.

If My.Compute r .C lock . Loca lT ime .DayOfWeek = DayOfWeek .Monday Then MsgBox( "Today is Monday!") End If When this code is run, the condition (the part between If and Then) is evaluated. If the condition is true, the next line of code is run and a message box is displayed; if it is false, the code skips to the End If line. In other words, the code states "If today is Monday, then display the message."

To use the If...Then statement

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type I f Thenand then click OK.

A new Windows Forms project opens. 4.

Double-click the form to open the Code Editor.

5.

In the Form1_Load event handler, type the following code.

If My.Compute r .C lock . Loca lT ime .DayOfWeek = DayOfWeek .Sa tu rday Or _ My.Compute r .C lock . Loca lT ime .DayOfWeek = DayOfWeek .Sunday Then 6.

Press F5 to run your program.

MsgBox( "Happy Weekend!") End If

If today is Saturday or Sunday, a message box appears telling you Happy Weekend! Otherwise, no message box appears.

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7.

On the Debug menu, choose Stop Debugging to end the program. Keep this project open. You will add to it in the next procedure, "To use the Else clause".

You may have noticed in the above example that the If...Then statement used the Or operator to evaluate multiple conditions ("if it is Saturday Or if it is Sunday"). You can use the Or and And operators to evaluate as many conditions as you want in a single If...Then statement. The Else Clause You have seen how to use the If...Then statement to run code if a condition is true—but what if you want to run one set of code if a condition is true, and another if it is false? In this case, you can use the Else clause. The Else clause allows you to specify a block of code that will be run if the condition is false. The following example demonstrates how the Else clause works.

If My.Computer.Clock.LocalTime.DayOfWeek = DayOfWeek.Friday Then MsgBox("Today is Friday!") Elseif itMsgBox("It isn't yet!") If first In this example, the expression is evaluated; is true, then the next Friday line of code is run, End and the message box is displayed. If it is false, then the code skips to the Else clause, and the line following Else is run, displaying the second message box. Try It!

To use the Else clause

1.

Change the code in the If...Then statement to the following.

If My.Computer.Clock.LocalTime.DayOfWeek = DayOfWeek.Saturday Or _ My.Computer.Clock.LocalTime.DayOfWeek = DayOfWeek.Sunday Then 2.

MsgBox("Happy Weekend!") Else MsgBox("Happy Weekday! Don't work

Press F5 to run your program. Your program will now display a message box stating whether it is a weekend or a weekday, with appropriate content.

Note You can change the day of the week by double-clicking the time on the Windows taskbar, if you want to test the execution of both code blocks. (The taskbar is the bar that contains the Windows Start button; by default, it is at the bottom of the desktop, and the time appears in the right corner.)

Closer Look: Using Select Case to Decide Between Multiple Choices In this lesson, you will learn to use the Select Case statement to run code based on multiple conditions. In the previous lesson, you learned how to use If...Then statements to run different blocks of code for different conditions. While it's possible to evaluate more than two conditions in an If...Then statement by using the ElseIf keyword, the Select Case statement provides a much better way to evaluate multiple conditions. The Select Case statement allows you to use as many conditions (or cases) as you need, making it convenient to write code for situations in which there are many choices. For example, suppose your program used a String variable to store a color choice and you needed to get the color value. The code for the Select Case statement might look like the following:

21

Select Case Color Case "red" MsgBox("You selected red") Case "blue" MsgBox("You selected blue") Case "green" selected green") When this code is run, the Select Case line determines theMsgBox("You value (Co lo r) of the expression. Assume that Co lo r is a String variable and that this variable is a parameter for a method that contains the Select Case statement. The value of Color is then compared to the value for first Case statement. If the value matches, the next line of code is run, and then the code skips to the End Select line; if the value doesn't match, then the next Case line is evaluated. The Case statement can take many different forms—in the example above, it is a String. However, it can be any data type or expression. You can evaluate a range of numbers by using the To keyword, as follows:

Case 1 To 10 In this example, any number between 1 and 10 would result in a match. You can also evaluate multiple values in a single Case statement by separating them with commas, as follows:

Case "red", "white", "blue" In this example, any of the three values would result in a match. You can also use comparison operators and the Is keyword to evaluate values, as follows.

Case Is > 9 In this example, any number greater than 9 would result in a match. Case Else The above example works when you know all possible conditions, but what happens if there is a condition you didn't account for? For example, if the value of Color was yellow, the code would simply evaluate the three cases without finding a match, and no message box would display. The Case Else statement can be used to run code when no match is found, as in the following example.

Select Case Co lo r Case "red" MsgBox( "You selected red") Case "blue" MsgBox( "You selected blue") Case "green" MsgBox( "You selected green") In the above code, if the value of Color is yellow, the code compares it with the first three Case lines

Case Else MsgBox( "Please choose red, blue, or green") End Select

without finding a match. When the Case Else line is reached, the next line of code is run before moving to End Select.

To use the Select Case statement

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type SelectCase and then click OK.

A new Windows Forms project opens.

22

4.

From the Toolbox, drag one TextBox control and one Button control onto the form.

5.

Double-click the button to open the Code Editor.

6.

In the Button1_Click event handler, type the following code.

Dim Number As Integer = CInt(Textbox1.Text) Select Case Number Case 1 MsgBox("Less than 2") Case 2 To 5 MsgBox("Between 2 and 5") Case 6, 7, 8 MsgBox("Between 6 and 8") Case 9 To 10 MsgBox("Greater than 8") Case Else

7.

Press F5 to run the program.

8.

In the text box, type a number and click the button.

A message box appears displaying the message for the Case statement matching the number that you entered.

Closer Look: Using Select Case to Decide Between Multiple Choices In this lesson, you will learn to use the Select Case statement to run code based on multiple conditions. In the previous lesson, you learned how to use If...Then statements to run different blocks of code for different conditions. While it's possible to evaluate more than two conditions in an If...Then statement by using the ElseIf keyword, the Select Case statement provides a much better way to evaluate multiple conditions. The Select Case statement allows you to use as many conditions (or cases) as you need, making it convenient to write code for situations in which there are many choices. For example, suppose your program used a String variable to store a color choice and you needed to get the color value. The code for the Select Case statement might look like the following:

Select Case Color Case "red" MsgBox("You selected red") Case "blue" MsgBox("You selected blue") Case "green" MsgBox("You green")Assume End When this code is run, the Select Case line determines the value (Co loselected r) of the expression. that Co lo r is a String variable and that this variable is a parameter for a method that contains the Select Case statement. The value of Color is then compared to the value for first Case statement. If the value matches, the next line of code is run, and then the code skips to the End Select line; if the value doesn't match, then the next Case line is evaluated. The Case statement can take many different forms—in the example above, it is a String. However, it can be any data type or expression. You can evaluate a range of numbers by using the To keyword, as follows:

Case 1 To 10 In this example, any number between 1 and 10 would result in a match. You can also evaluate multiple values in a single Case statement by separating them with commas, as follows:

Case "red", "white", "blue" In this example, any of the three values would result in a match.

23

You can also use comparison operators and the Is keyword to evaluate values, as follows.

Case Is > 9 In this example, any number greater than 9 would result in a match. Case Else The above example works when you know all possible conditions, but what happens if there is a condition you didn't account for? For example, if the value of Co lo r was ye l l ow , the code would simply evaluate the three cases without finding a match, and no message box would display. The Case Else statement can be used to run code when no match is found, as in the following example.

Select Case Co lo r Case "red" MsgBox( "You selected red") Case "blue" MsgBox( "You selected blue") Case "green" MsgBox( "You selected green") In the above code, if the value of Co lo r is ye l l ow , the code compares it with the first three Case lines

Case Else MsgBox( "Please choose red, blue, or green") End Select

without finding a match. When the Case Else line is reached, the next line of code is run before moving to End Select.

To use the Select Case statement

1.

On the File menu, choose New Project.

2.

In the New Project dialog box, in the Templates pane, click Windows Application.

3.

In the Name box, type Se lec tCaseand then click OK. A new Windows Forms project opens.

4.

From the Toolbox, drag one TextBox control and one Button control onto the form.

5.

Double-click the button to open the Code Editor.

6.

In the Button1_Click event handler, type the following code.

Dim Number As Integer = C In t ( Tex tbox1 . Tex t ) Select Case Number Case 1 MsgBox( "Less than 2") Case 2 To 5 MsgBox( "Between 2 and

7. 8.

5") Case 6 , 7 , 8 MsgBox( "Between 6 and 8") Case 9 To 10 Press F5 to run the program.

In the text box, type a number and click the button. A message box appears displaying the message for the Case statement matching the number that you entered.

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