Writing Code with Visual Basic .NET

This chapter is from the book

Sams Teach Yourself Visual Basic .NET in 21 Days

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This chapter is from the book

This chapter is from the book 

Sams Teach Yourself Visual Basic .NET in 21 Days

Learn More Buy

Introduction

Now that you've become more familiar with the Visual Basic .NET development environment, it's time to begin writing code. Although Visual Basic .NET makes it easy to write a simple program without using much code, any program simpler than a demo will need to keep track of information, and do simple calculations and similar tasks. To write code that performs these tasks, you'll need a good understanding of variables. By understanding the use and types of variables, you're creating the foundation for your understanding of Visual Basic .NET. Similarly, just as when you began learning simple arithmetic, you will need to learn some of the simple operators that can be used to work with number and string variables in your programs. Today, you will learn

• The types of variables you can create with Visual Basic .NET

• Some simple operators and functions available in Visual Basic .NET

• The fundamentals of writing code in Visual Basic .NET, including how to write procedures

Variables and Assignment

Variables and assignment are at the core of every programming language. Variables enable you to store information for later use, and assignment is the way you get information into variables.

What Is a Variable?

A variable is a bucket. It's a place to hold information until you need it. You will use variables throughout your programs to hold temporary values during calculations, to store user input, and to prepare information that will be later displayed to users.

Available Variable Types

Just as with pants, one size does not fit all variables. Although it is possible to create and use a variable that is capable of holding anything, it's not always the best solution. It's easy to imagine that a variable holding strings must do something different from a variable designed to hold numbers. In addition, even different types of numbers require different variable types. Some numbers, such as 1 or 5280, don't have a decimal place, whereas 3.14159265358979 and 16.50 do. A variable made to hold a number with a decimal must do something specifically to keep track of the values after the decimal. Of course, that means that decimal numbers probably take up more memory. Any time a computer or program does more work, it usually needs more memory. So, it is important to keep in mind not only the type of information you need to store, but also the amount of memory the computer will have to manipulate to keep track of the variable.

There are three broad types of variables you can create with Visual Basic .NET. The first set encompasses variables that hold simple values, such as numbers or strings. There are many of these, each designed to hold values of various sizes. The second category is complex variables, which hold some combination of simple variables, and include arrays and user-defined types. Arrays are variables that hold a series of other variables, and user-defined types enable a user to create new types of variables. The third category of variables is object variables.

User-defined types (also known as structures) and object variables will be covered later, on Day 7, "Working with Objects." Today's discussion of variables will focus on simple variables and arrays.

Simple Variables

As described previously, the simple variable types "hold" values such as numbers and words. So, you might think that there only needs to be two types of variables: numbers and words. However, there are actually a number of different simple variable types—each made to hold different sizes or kinds of numbers or strings.

Try to use the best variable type for the situation. Sometimes, you only need to keep track of a small number—for example, if you're keeping track of the months of the year. Other times, you need to work with big numbers with many decimal places—for example, if you're writing a program that performs engineering or scientific calculations.

The simple variables can be divided into four subgroups. The first, and largest, group is for integers, numbers that have no decimal places. The second group is used for numbers that have decimal places. Strings and characters make up the third group, and the fourth group is best described as "other." Let's look at each of these groups and see when it is appropriate to use each.

Integer Variables

The integer variables hold the familiar whole numbers (that is, numbers without decimal places). Integers are one of the most common variables you will use in programs, and the easiest for computers to deal with. Because of this ease, they should be your first choice as a variable type when you need to work with numbers. Table 3.1 shows a number of different integer variables, each designed to hold numbers of different sizes, and use different amounts of memory. The amount of memory used is measured in bytes. (One byte contains eight bits, which is just a fancy way of saying that each byte has eight 1's or 0's, or a combination of 1's and 0's.) Although there is no harm in using a variable designed to hold larger values than needed, it wastes memory. In addition, it could cause your program to run slower because it would have to keep track of larger sections of memory, even when that memory is never used.

Table 3.1 Integer Variable Types

Numbers with Decimal Places

A lot of number crunching is done without decimal places. However, more calculations, especially in engineering, finance, and science, require that you also keep a decimal value. Table 3.2 describes the two main decimal variable types. Deciding which to use depends on the degree of accuracy you need to track, rather than the size of values, because they all can hold rather large numbers. In case you don't remember scientific notation from school, the superscript number above the 10 is the number of times you need to multiply by 10 (if positive) or divide by 10 (if negative). Therefore 10⁶ is 10 six times, or 1,000,000, and 10^-6 is 0.000001.

Table 3.2 Decimal Variable Types

Strings and Characters

Numbers are fine if you need to track quantities or time, but you also need to deal with words frequently in programming. Visual Basic .NET gives you variables for storing strings: the Char and the String. The Char is good for storing only a single character (thus the name), whereas the String can hold strings of great length. Table 3.3 describes the two data types in more detail.

Table 3.3 String Variable Types

Why does each Char and each character in a String take up two bytes? After all, there are only 26 characters used in English, plus numbers and symbols—surely, you don't need two bytes (65,536 possible values). You wouldn't if every language used the same character set.

That scenario is what the formerly popular ASCII (or ANSI) character set defined. In ASCII, one byte equals one character, and every computer that used ASCII always positioned the same characters at the same position in the list. So the ASCII value 65 was always the letter "A," and the symbol "@" was had the value 64. If you wanted to do the same for all the other symbols people use in writing, however, you needed more characters. To accommodate this, a new system was developed, called Unicode.

With Unicode, each character is represented as two bytes. This allows for the storage of all the characters in the ASCII chart, as well as characters for Russian, Greek, Japanese and Thai languages, for mathematics, and so on. In Unicode, 65 still represents the letter "A", but 8800 is the character for . The Japanese Hiragana character "No," is represented as character 12398. Visual Basic .NET uses the Unicode values for all characters, so the Char uses two bytes, and each character in a String adds two bytes of additional storage space.

Other Simple Variable Types

It never fails when you try to categorize things. Something always defies being categorized (just imagine the first zoologist to come upon the platypus). Likewise, there are some variables that don't fit neatly into the previously described categories. In Visual Basic .NET, there are two such "other" variable types, the Boolean, and the Date. They are described in more detail in Table 3.4.

Table 3.4 Other Simple Variable Types

Declaring Variables

Now that you know the types of variables available, how do you create them in your programs? The simplest way is with the Dim (short for "Dimension") keyword, followed by the name of the variable, the keyword As, and finally the type of the variable. It looks like

Dim iSomeNumber As Integer

This would create the new variable iSomeNumber, which takes up four bytes and can hold a number as large as +/– 2 billion. Here are some more possible variable declarations:

Dim sFirstName As String
Dim dblGrossDomesticProduct As Double
Dim bLearned As Boolean

One new feature in Visual Basic .NET is the capability to give a value to a variable as you are creating it. You simply assign a value to the new variable on the same line as the Dim statement:

Dim dtDateOfMagnaCartaSigning As Date = #June 15, 1215#
Dim lPeopleOnEarth As Long = 6000000000

We'll look at some other ways of declaring variables later, when we discuss scope.

Arrays

A variable's capability to hold one of anything is handy, even essential, in programming. However, you might need to store a number of related items. For example, if you were writing a program to play chess, the squares on the board would need to be represented as a collection of related items. You use arrays to create variables that will store all the related items together. For the chess program, you would likely store the chessboard as an array of positions, each position holding the type of chess piece (or none) at that position. If you did not use an array variable, you would have to use 64 separate variables. You might also need to track a list of strings, for example, a list of the names of students in a class. Whenever you need to store a list of items, you use an array.

As with declaring simple variables, you declare an array with the Dim keyword. However, there are some differences between simple variable declarations and array declarations because arrays are collections of variables. Listing 3.1 shows three possible ways of declaring arrays.

Listing 3.1 Declaring Arrays

 1 'Simple declaration
 2 Dim iValues(3) As Integer
 3 Dim dtDates() As Date
 4 Dim I As Integer
 5 For I = 1 To 3
 6  iValues(I-1) = I
 7 Next

 8 'Changing the size of an existing array
 9 ReDim dtDates(4)
10 'fill the list of dates
11 dtDates(0)="6/15/1215" 'Signing of Magna Carta
12 dtDates(1)="8/28/1962" 'Martin Luther King Jr. delivers "I have a dream"
13 dtDates(2)="7/20/1969" 'Apollo 11 lands on Moon
14 dtDates(3)="2/14/1946" 'ENIAC unveiled to public
15 'Declaration with Initialization
16 Dim sMonths() As String = {"Jan","Feb","Mar","Apr","May","Jun", _
17   "Jul","Aug","Sep","Oct","Nov","Dec"}

18 'Using arrays
19 Console.WriteLine("")
20 Console.WriteLine("Second value in iValues = {0}", iValues(1))
21 Console.WriteLine("Third date in dtDates = {0}", dtDates(2))
22 Console.WriteLine("Eleventh month of the year = {0}",
sMonths(10))

In the first example, iValues is declared as a three-member array. Each item in the array is an Integer. That much should make sense. The potentially confusing part is in the way you refer to each member of the array. This is shown in the For...Next loop beginning at lines 5 through 7 (Note that we will be covering the For...Next loop tomorrow). Notice that the three members of the array are actually numbered beginning with 0 and going up to 2. Therefore, the second member of the array is at position 1, not 2.

The second array created, the months of the year, is declared and initialized. Just as you can initialize simple variables as you declare them, you can do the same with arrays. In the case of arrays, however, you put each of the elements of the array in a comma-delimited list, wrapped in braces (curly brackets), as shown on line 16. This creates a 12-member array, with each element holding a string. Remember, however, that if you wanted to refer to each element, they are numbered from 0 to 11—so sMonths(10) would be "Nov", not "Oct".

The final declaration is of a dynamically sized array. This array can be resized later to the correct size with the ReDim keyword as shown on line 10. Just as with the other array types, the items of the dynamic array are numbered from 0 to I minus 1. After the array is sized, you can then use it as any other array. This form of declaration is useful if the size of the array depends on the value of another variable, so that it wouldn't be known until runtime.

Dim sngValues(15 To 51) As Single

Assignment

Assignment has been simplified in Visual Basic .NET. Earlier Visual Basic versions (Visual Basic 4.0 to 6.0) had two different ways of assigning a value to a variable—one for simple variables (including structures and arrays) and one for object variables. Fortunately, the developers of Visual Basic .NET decided to remove the assignment method used for object variables, and rely only on the method used for simple variables. You assign values to variables (either simple or object) by putting the variable left of an equal sign, as shown in the following code:

iSomeVar = 1234
oObjectVar = New Something()

iSomeVar = 1234
Set oObjectVar = New Something

Constants

Constants are another class of values that you can use in your Visual Basic .NET programs. Constants are values that do not change, either for the lifetime of your program, or ever. For example, the months in a year, the value of pi, and the database server from which your program retrieves data are all constant values. You can define a value as being constant when you declare it. Any attempts to change the value of a constant will be marked as an error while you are still in the IDE (Integrated Development Environment), and before you attempt to run the application. Constants are declared using one of the two forms shown in the following code:

Const PI = 3.1415 As Double
Const DSN As String = "Random"

If the type of the constant is not described in a declaration, the compiler must use the value type that best fits. However, it does not always select the best possible type. Generally, when declaring constants, if you do not include the type of the value, Visual Basic .NET will create the following variable types

• Long For any undeclared whole numbers.

• Double For any undeclared decimal numbers. Note: If the value is actually too large for a Double, it will be truncated.

• String For any character values.