CSI 101CSI 101Elements of ComputingElements of Computing

Spring 2009Spring 2009Lecture #6: Data Types and Lecture #6: Data Types and

Variables in Visual BasicVariables in Visual Basic

Wednesday, Februrary 11th, 2009Wednesday, Februrary 11th, 2009

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Importance of VariablesImportance of Variables

Everything in computers is data-relatedEverything in computers is data-related

Variables hold dataVariables hold data Serve as a repository for a data valueServe as a repository for a data value Save purpose as variables in mathSave purpose as variables in math

Variable is not permanent storageVariable is not permanent storage Lives only for particular life spanLives only for particular life span Called “scope”Called “scope” We'll discuss scope later in the semesterWe'll discuss scope later in the semester

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Importance of Data TypesImportance of Data TypesDefines the type of data contained by that Defines the type of data contained by that variablevariable Note this differs from variables in math, which Note this differs from variables in math, which

can hold any valuecan hold any value

Allows for easy formattingAllows for easy formatting

Data values of some data types cannot be Data values of some data types cannot be read by other data typesread by other data types Leads to data conversion functionsLeads to data conversion functions We'll discuss data conversion functions later We'll discuss data conversion functions later

in the semesterin the semester

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Categories of Data TypesCategories of Data Types

DiscreteDiscrete Holds an individual data valueHolds an individual data value

VariantVariant Could hold a value of different typesCould hold a value of different types

EnumeratedEnumerated Holds one of a specific list of valuesHolds one of a specific list of values Not discussed in this courseNot discussed in this course

ComplexComplex Holds multiple valuesHolds multiple values

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Discrete Data TypesDiscrete Data Types

BooleanBoolean Contains only two values Contains only two values Used for True and FalseUsed for True and False

Numeric data typesNumeric data types Integral or whole numbersIntegral or whole numbers Decimal numbersDecimal numbers

TextText

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Boolean data typeBoolean data typeHolds only True or FalseHolds only True or False Note that True and False are reserved words in Note that True and False are reserved words in

Visual Basic. They can't be used for ANYTHING Visual Basic. They can't be used for ANYTHING other the value of Boolean variablesother the value of Boolean variables

Used to hold results of comparisonsUsed to hold results of comparisons For example, A < B would result in True or FalseFor example, A < B would result in True or False

Can be set by program:Can be set by program: boolVar = TrueboolVar = True

Value can be tested in decision statementsValue can be tested in decision statements

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Integral Data TypesIntegral Data Types

ByteByte Holds values 0-255Holds values 0-255 Fastest calculationFastest calculation

ShortShort Holds -32,767 to 32,767Holds -32,767 to 32,767

IntegerInteger Holds positive or negative value up to over 2 Holds positive or negative value up to over 2

billionbillion

LongLong Holds extremely large positive or negative Holds extremely large positive or negative

valuesvalues

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Decimal Data TypesDecimal Data Types

SingleSingle Holds positive and negative values with 7 Holds positive and negative values with 7

digits and magnitude to 1.2 x 10digits and magnitude to 1.2 x 103838

DoubleDouble Holds huge values with 15 digitsHolds huge values with 15 digits

DecimalDecimal More decimal significance (29 digits)More decimal significance (29 digits) Magnitude to 7.9 x 10Magnitude to 7.9 x 102828

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String typeString type

Used for textUsed for text

Dynamic – adds space as text string growsDynamic – adds space as text string grows

Limit of over 250,000 charactersLimit of over 250,000 characters A character is any symbol on the keyboardA character is any symbol on the keyboard Includes punctuation marks and spaceIncludes punctuation marks and space

Can hold a single characterCan hold a single character Improvement over Char type, which holds only Improvement over Char type, which holds only

one characterone character

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Handy ReferenceHandy Reference

The FAQ file on the website lists theseThe FAQ file on the website lists these

There is also a list of Visual Basic data There is also a list of Visual Basic data types on the inside front cover of your text types on the inside front cover of your text bookbook Upper left cornerUpper left corner

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Numeric CalculationsNumeric Calculations

If two variables are contained in If two variables are contained in calculation, what happens?calculation, what happens? Any numeric types retain valueAny numeric types retain value Integral to decimal simply add decimal placesIntegral to decimal simply add decimal places Decimal to integral rounds off decimalDecimal to integral rounds off decimal Text to numeric (or vice versa) requires Text to numeric (or vice versa) requires

conversion functionconversion function

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Declaring a variableDeclaring a variable

All variables must be declared before usedAll variables must be declared before used

They don’t need initial valuesThey don’t need initial values Most compilers initialize them when createdMost compilers initialize them when created Good practiceGood practice

Visual Basic command:Visual Basic command:DIM varname AS typeDIM varname AS type Example:Example:

DIM Count AS IntegerDIM Count AS Integer

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Declaring examplesDeclaring examplesDIM Count AS ByteDIM Count AS ByteDIM Sum AS IntegerDIM Sum AS IntegerDIM Sqr_root As SingleDIM Sqr_root As SingleDIM Salary AS DecimalDIM Salary AS DecimalDim Name As StringDim Name As String

Note I switched from all capital letters to Note I switched from all capital letters to mixed casemixed case Visual Basic compiler won't matterVisual Basic compiler won't matter In class, I'll usually use all capital letters to In class, I'll usually use all capital letters to

denote reserved keywords in Visual Basicdenote reserved keywords in Visual Basic

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Variable NamesVariable Names

Variable names can contain letters and Variable names can contain letters and numbers or underscores (_)numbers or underscores (_)

Must start with a letterMust start with a letter

Cannot contain spacesCannot contain spaces Spaces are used as keyword separators in Spaces are used as keyword separators in

Visual BasicVisual Basic That's why the underscore can be usedThat's why the underscore can be used

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Naming ConventionsNaming Conventions

Your text recommends starting your Your text recommends starting your variables with a three-letter designationvariables with a three-letter designation Designation states the data typeDesignation states the data type Not necessary, but it's a good practiceNot necessary, but it's a good practice

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Variable type prefixesVariable type prefixes

BooleanBoolean

ByteByte

ShortShort

IntegerInteger

LongLong

SingleSingle

DoubleDouble

DecimalDecimal

StringString

DateDate

BlnBln

BytByt

SrtSrt

IntInt

LngLng

SngSng

DblDbl

DecDec

StrStr

datdat

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VariantVariant

Sometimes you want to use a variable as Sometimes you want to use a variable as temporary placeholdertemporary placeholder Could hold different values at different timesCould hold different values at different times

Declare as Variant data typeDeclare as Variant data typeDIM temp As VariantDIM temp As Variant

Variants must be converted for any Variants must be converted for any calculation or processingcalculation or processing

Undeclared variables are automatically Undeclared variables are automatically made Variantmade Variant

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Complex Data TypesComplex Data Types

Date/TimeDate/Time

User-definedUser-defined Can contain different data typesCan contain different data types ““Wrapped” within a new data type nameWrapped” within a new data type name

ArrayArray Multiple values of same data typeMultiple values of same data type Have same variable name, but includes an Have same variable name, but includes an

index numberindex number

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Date/TimeDate/Time

Holds both date and timeHolds both date and time

Initialized to current date and timeInitialized to current date and time

Type is DATEType is DATE

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User-Defined Data TypesUser-Defined Data Types

Create custom data type for applicationCreate custom data type for application

Consolidates discrete items for easy Consolidates discrete items for easy referencereference

Allows user to declare variables of Allows user to declare variables of structurestructure

Individual items referenced using dot Individual items referenced using dot notationnotation

Uses STRUCTURE statementUses STRUCTURE statement

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User-Defined ExampleUser-Defined Example

Student record:Student record:Structure StudentStructure Student

Dim ID As LongDim ID As Long

Dim Name As StringDim Name As String

Dim ClassYear As IntegerDim ClassYear As Integer

Dim GPA As SingleDim GPA As Single

End StructureEnd Structure

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Using Student TypeUsing Student Type

Declare:Declare:DIM UAlb_Student As StudentDIM UAlb_Student As Student

Referencing:Referencing:UAlb_Student.ClassYear = 2010UAlb_Student.ClassYear = 2010

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ArraysArrays

Easily maintain set of same itemsEasily maintain set of same items Example: array of currency for expensesExample: array of currency for expenses Need to declare how many items are Need to declare how many items are

contained in arraycontained in array

Declaring:Declaring:DIM expenses(12) As SingleDIM expenses(12) As Single

Referencing:Referencing:Expenses(2) = 12.87Expenses(2) = 12.87

Number in parentheses is called indexNumber in parentheses is called index

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Array of studentsArray of students

Dim CSI101(200) as StudentDim CSI101(200) as Student

::

CSI101(1).ID = …CSI101(1).ID = …

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2-Dimensional Arrays2-Dimensional Arrays

Used for tablesUsed for tablesDeclare two limitsDeclare two limitsExample: 12x12 multiplication table:Example: 12x12 multiplication table:Dim MultTable(12,12) As ByteDim MultTable(12,12) As Byte