visualbasic lecture5.pdf

8/10/2019 visualbasic lecture5.pdf

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Copyright 2011 Pearson Addison-Wesley

Declaring Variables

A variable declaration is a statement that creates a variable in

memory

The syntax is:

Dim VariableName As DataType

Dim(short for Dimension) is a keyword

VariableNameis the programmer designated name

Asis a keyword

DataTypeis one of many possible keywords for the type

of value the variable will contain

!hapter "# $lide %


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Declaring &ultiple Variables

$everal variables may be declared in one

statement if they all hold the same type of

value

Dim intLength, intWidth, intHeight as Integer

'r this can be done in " separate statements

Dim intLength as Integer

Dim intWidth as Inte er!hapter "# $lide


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Variable aming *ules

The first character of a variable name must be

a letter or an underscore

$ubse+uent characters may be a letter,underscore, or digit

Thus variable names cannot contain spaces or

periods (or many other kinds of characters)

Visual -asic keywords cannot be used as

variable names

!hapter "# $lide "


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Variable aming !onventions

aming conventions are a guideline to help

improve readability but not re+uired syntax

A variable name should describe its use .ach data type has a recommended prefix, in

lower case, that begins the variable name

The %st letter of each subse+uent word in thevariable name should be capitali/ed

intours0orked # an integer variable

str1astame # a string (or text) variable !hapter "# $lide 2


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$etting the Value of a Variable

An assignment statement is used to set the

value of a variable, as in:

Assign the value %% to the variable length length = 112

Assign the string literal 34ood &orning 3 followed

by the contents of the text box txtame to thevariable greeting

greeting = "Gd !rning " t#tName$Te#t

An assignment changes only the left operand!hapter "# $lide 5


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Visual -asic Data Types

6nteger types

-yte

$hort

6nteger

1ong

7loating#8oint types $ingle

Double

Decimal !hapter "# $lide 9


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umber $ystem

0hat number system do we use

De%imal

6t;s called decimal because it has 1&symbols


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umber $ystem (cont@)

0hat is the value of this number %


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umber $ystem (cont@)

% -yte > bits

nsigned

%B=C%B9C%B5C%B2C%B"C%BC%B%C%B

C 92 C " C %9 C > C 2 C C % 55

$igned!hapter "# $lide ?

8osition: = 9 5 2 " % , range is

& t 2(( )2* +1=2((

$igned

#n#% to C(n#% #%), n number of bits

6f n >, range is

+ +!hapter "# $lide%",92> to

,%2=,2>",92=

1ong lng $igned integer from #?,","=,52,==5,>


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7loating#8oint Data Types

7or values that may have fractional parts

$ingle used most fre+uently

Double sometimes used in scientific calculations Decimal often used in financial calculations

!hapter "# $lide%

Data

Type

aming

8refixDescription

$ingle sng As large as %


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'ther !ommon Data Types

-oolean E variable naming prefix is bln

olds possible values, True or 7alse

!har E variable naming prefix is chr olds a single character

Allows for characters from other languages

$tring E variable naming prefix is str

olds a se+uence of up to billion characters

Date E variable naming prefix is dat or dtm!hapter "# $lide%"


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The $tring Data Type

A string literal is enclosed in +uotation

marks

The following code assigns the nameGose 4on/ales to the variable strame

Dim strName as string

strName = "/se Gn0ales"

An empty string literal can be coded as:

Two consecutive +uotation marks!hapter "# $lide%2


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The Date Data Type

Date data type variables can hold the date and time

or both

Hou can assign a date literal to a Date variable, as

shown here:

Dim dtm'irth As Date

dtm'irth = 3(4142&1&3

A date literal is enclosed within I symbols

All of the following Date literals are valid:

31241&42&1&3!hapter "# $lide%5


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Declaring Variables with

6ntelli$ense As you enter your program, V- often aids youby offering a list of choices that could be used

at that point

After typing JAsJ in a variable declaration, V-

will offer an alphabetical list of all possible

data types

Type the first few letters of the data type name

6ntelli$ense box will highlight the matching type

8ress the Tab key to select highlighted choice!hapter "# $lide%9


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Default Values and 6nitiali/ation

0hen a variable is first created in memory, it

is assigned a default value

numeric types are given a value of /ero

-oolean types are given a value of 7alse

strings are given a value of othing

dates default to %:


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6nitiali/ation of Variables

!an provide a starting or initiali/ation value for

any type of variable in a Dim statement

sually want to set an initial value unlessassigning a value prior to using the variable

Gust append = :al;eto the Dim statement

where value is the literal to be assigned to thevariable

Dim int!nths7er


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$cope and 1ocal Variables

$cope refers to the part of the program

where:

A variable is visible and

&ay be accessed by program code

Variables declared within a procedure are

called local variables and observe thesecharacteristics

$cope begins where variable is declared

.xtends to end of procedure where declared!hapter "# $lide%?

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