Transcript Excel 5 Spreadsheet Databases

Programming in Visual Basic 6.0
Update Edition
Chapter 3
Variables, Constants, &
Calculations
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Programming in Visual Basic 6.0 Update Edition
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Variables, Constants, & Calculations
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Data—Variables, Constants, & Controls
Data Types
Naming Conventions
Variable Scope
Calculations
Val function
Arithmetic Operations
Formatting Data
Programming Hints—Counting/Summing
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Programming in Visual Basic 6.0 Update Edition
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Data—Variables & Constants
Data Types
– Boolean, Byte, Currency
– Date, Double, Integer, Long
– Single, String, Variant
intMaximumNumber = 100
intMaximumNumber
100
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Programming in Visual Basic 6.0 Update Edition
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Declaring Variables & Constants
• Variables:
Dim variable-name As data-type
• Constants:
Const constant-name As data-type = value
• Most important:
Force variables to be explicitly declared:
Select Tools, Options, Editor tab, and click the
“Require Variable Declaration” check box.
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Programming in Visual Basic 6.0 Update Edition
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Naming Variables & Constants
• Naming rules keep track of variable types
• Naming rules help you remember which are
constants and which are not
• They avoid data conversion
• Default data type is Variant
• You can detect misspellings by typing
variables in lowercase
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Programming in Visual Basic 6.0 Update Edition
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Naming Variables
• All lowercase variables are converted to
their “dimmed” spelling.
• Attach prefix to the variable name:
bln-Boolean, cur-currency, int-integer, sng-single
precision, str-string
• Examples: strSocialSecNum, intQuantity,
curHoursWorked, curPayRate
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Programming in Visual Basic 6.0 Update Edition
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Constants—Named & Intrinsic
• Using named constants follows
conventional programming practice
• Use named constants in lieu of “magic
numbers”
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increases documentation,
decreases likelihood of errors
facilitates global value changes
eases program maintenance
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Programming in Visual Basic 6.0 Update Edition
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More about Constants
• Examples of declaration:
Const strRptName As String = “October Report”
Const sngTaxRate As Single = 0.075
• Advantage of Constants
– Cannot be changed during run
– If global change needed, one adjustment will
change every use of the identifier
– For example when tax rate changes, just
change the decimal fraction in the declaration
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Programming in Visual Basic 6.0 Update Edition
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Intrinsic Constants
• Intrinsic constants are system-defined
constants
• They use a two-character prefix to indicate
the source
– vb means Visual Basic
– db means Data Access Objects
– xl means Excel
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Programming in Visual Basic 6.0 Update Edition
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Scope of Variables
• Scope is the availability of a variable
–Global: can always be used or seen across the entire
project
–Module: accessible by all procedures in the form
–Local: accessible only from single procedure in
which it is declared
–Static: local variables that are “remembered” across
invocations of a routine but not outside it
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Programming in Visual Basic 6.0 Update Edition
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Scope of Variables (continued)
• Code module-level declarations in General
Declarations section of a form.
• Code Local and Static variables in
subprocedures.
• Variable's lifetime is the period of time that
the variable exists.
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Programming in Visual Basic 6.0 Update Edition
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Calculations & the Val Function
• Standard Operators: (), ^, *, /, +, • Operator precedence determines how an
equation is evaluated
• Example: curTotal = curTotal + curLatest
• Val function converts Text to value; reduces
errors for blank entries in text box
– intQuantity = Val(txtQuantity.Text)
• Counting occurrences: intSum = intSum + 1
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Programming in Visual Basic 6.0 Update Edition
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Formatting Data
• Alter the displayed form of a number or string
by formatting it
• Special format functions exist to speed the
process including FormatPercent
• Form of a typical format function:
FormatCurrency(NumericExpressionToFormat)
• FormatCurrency(curTotalDue)
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Programming in Visual Basic 6.0 Update Edition
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Format Function Examples
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lblTot.Caption = FormatCurrency(curTotal)
lblDate.Caption = FormatDateTime(intBirth)
lblQuantity.Caption = FormatNumber(intDone)
lblPercentDone.Caption =
FormatPercent(txtDaysWork/txtTotalDays)
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Programming in Visual Basic 6.0 Update Edition
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Counting and Accumulating Sums
• Counting:
intCount = intCount + 1
• Accumulating a sum:
mintTotal = mintTotal + intNew
• Averaging:
msngAverage = mintTotalScore / intCount
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Programming in Visual Basic 6.0 Update Edition
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Calculation Program Example
summary
information
McGraw-Hill/Irwin
© 2002 The McGraw-Hill Companies, Inc. All rights reserved.