Naming Rules (required by VB):

• Names are from 1 to 255 characters long
• Names must begin with a letter
• Names may consist of letters, digits and underscores
• Names must not contain blanks or periods
• Names cannot be Visual Basic reserved words (keywords to which VB has assigned some meaning. Examples are Print, Name, and Value)

Naming Conventions (not required by VB but good practice):

• Follow the now well-established Hungarian notation:
• After the appropriate prefix, capitalize each word of the name
• Used mixed case
• Naming conventions keep track of variable types
• Naming conventions help you remember which are constants and which are not
• They avoid data conversion
• You can detect misspellings by typing variables in lowercase

Examples:

VB’s Auto Fill feature helps you complete writing a Dim statement by presenting a list of available options.

Constants—Named & Intrinsic

Using named constants follows conventional programming practice.

Use named constants in lieu of "magic numbers"

• increases documentation,
• decreases likelihood of errors
• facilitates global value changes
• eases program maintenance

Examples of declaring Constants:

Const strRPTNAME As String = "October Report"

Const sngTAXRATE As Single = 0.075

• Named constants, ones you define yourself, can be changed at design time but not at run time.
• Examples in which you should use named constants are local, city, or state tax rates.

Rather than coding

curStateTaxDue = curGrossPay * 0.078

code instead

curStateTaxDue = curGrossPay * sngSTATE_TAX_RATE

where sngSTATE_TAX_RATE is a named constant

• By using sngSTATE_TAX_RATE throughout your program, you can change the state tax rate in source code in one place, recompile the program, and the change is automatically "rippled throughout your program.

Where do you place constant declarations?

• In the lowest "level" you can: within a subprocedure that is lengthy, in a form declaration outside each form’s subprocedures if used by the form only, or in a code module if the constant might be used throughout the entire application.

Examples of Constant Declarations:

Dim strCompanyName As String = "Proflowers.com"

Dim sngTaxRateSingle As Single = 0.125

Dim intNumberOfCourses As Integer = 4 

Intrinsic Constants

• Intrinsic constants are system-defined constants. They are already defined within Visual Basic: vbChecked, vbRed
• They use a two-character prefix to indicate the source

  vb means Visual Basic

  db means Data Access Objects

  xl means Excel

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

• Code module-level declarations in General Declarations section of a form.
  • Placing variable declarations in the General Declarations section of the form makes them available to all procedures in the form.
• Code Local and Static variables in subprocedures.
• As long as a variable is in scope, you can write code to change it, print it, and initialize it.

Global Variables

• Can be referenced from anywhere. They are declared in a module (.BAS) as Public.
• A common use of a global variable is to hold the name of the program's current user
• If there is a global and a local variable with the same name, then only the local variable can be referenced while in the procedure. That is, global variables are temporal in nature.

Always keep a variable’s scope as small as possible. This greatly reduces the chances of error.

Variable lifetime:

• A variable's lifetime is the period of time that the variable exists.
• each time you execute a procedure, the variable comes back to life and is reinitialized to zero (numeric) or an empty string (string variables).
• Lifetime of a module-level variable is entire time the form is loaded.
• To extend the lifetime of a variable across multiple form load operations, then you have to use a Static variable.

Module level variables frequently have the prefix m added to indicate their scope:

mcurTakeHomePay As Currency

 Calculations and Assignment Statements

Standard Operators: (), ^, *, /, +, -

intintSum = 3 + 7

intDifference = 3 - 7

txtTaxes.text = CurGrosspay * 0.35

intCount = intCount + 1

dblTotal = dblTotal * 100.00

Operator precedence: determines how an equation is evaluated

For expressions with more than 1 operator, which is calculated first?

Total = Hours * Rate + Overtime * Rate * 1.5

The order evaluation of operators, called precedence, is as follows (from first/highest to last/lowest)

• Parentheses ()
• Exponentiation ^
• Multiplication *, Division /, or Integer Division \
• Addition or Subtraction + -

Thus, there is a particular order in which every expression is evaluated:

dblAnswer = 1 + 17 * 24 - 15 * (201 - 6) / 8

Val function

• converts Text to value; reduces errors for blank entries in text box
• Values of text boxes and labels are actually strings of text characters.
• They should not be used directly in calculations, just in case they are not numbers.
• The Val function converts value of a property of a control to a numeric form.

intTotalScore = intTotalScore + Val(txtScore.Text)

If the text box called txtScore contains a character string that is not numeric, then the value zero is returned--a convenient behavior.

If a control contains a combination of numbers and non-numbers and begins with a series of digits (and decimal points or signs), then the leading digits right up to the first non-digit are returned as a value:

Val("1234BeThere") is 1234

1. intSum = intNum1
2. intSum = intNum1 * intNum2 + intNum1
3. intSum = intNum1 + intNum2 * intNum1
4. intSum = intNum2 \ intNum1
5. intSum = intNum1 ^ intNum1
6. intSum = Val(txtNum1.text)

Answers.

Formatting Data

This means altering the displayed form of a number or string by formatting it. It does NOT affect the stored value in memory.

Special format functions exist to speed the process, such as FormatPercent

The form of a typical format function:

FormatCurrency(NumericExpressionToFormat)

FormatCurrency(curTotalDue)

When you want to format data for display or the printer, you use one of the four Format functions.

FormatCurrency

FormatNumber

FormatPercent

FormatDateTime

• You can choose to display a dollar sign, percent sign, and commas; and you can specify the number of decimal places to display

Examples:

FormatCurrency(12.345) $12.34

FormatNumber(1278.569,1) 1,278.6

FormatPercent(0.8326) 83%

FormatDateTime("3/15/2000",vbShortDate) 3/15/00

Format Function Examples

lblTot.Caption = FormatCurrency(curTotal)

lblDate.Caption = FormatDateTime(intBirth)

lblQuantity.Caption = FormatNumber(intDone)

lblPercentDone.Caption =

FormatPercent(txtDaysWork/txtTotalDays)

Note that format rounds fractional values to requested number of decimal places. This does not affect the stored value.

Counting and Accumulating intSums

Counting:

intCount = intCount + 1

Accumulating a sum:

intTotal = intTotal + intNew

Averaging:

sngAverage = intTotalScore / intCount

To sum sales, the trick is to define a module-level variable to hold the running total. Each time a new sale is made, add the new sale amount to the module-level variable. Doing so accumulates the sales over time.

Debugging Tip: You cannot declare a total inside a subprocedure that handles sales. That variable would be reinitialized each time the subprocedure is invoked.

totalSalesTax = totalSalesTax + taxThisSale

Counting things is the same as accumulating sums, except that you add one to the running count rather than a random sale amount. The basic form of counting across events of any kind is to define a module-level count accumulator and then to add one to it each time the countable event occurs:

totalWebPageHitCount = totalWebPageHitCount + 1

Calculation Program Example

• The hands on example illustrates variables, the Val function, and the FormatCurrency function.
• Three text boxes, seven labels, and four command buttons comprise the solution

Note: Be aware that there is a problem with the format command in general. The problem is that the discount amount is visually rounded by the FormatCurrency function to display in the "15% Discount" label. However, the calculation of the discount can create an answer that contains three or four decimal places. When the discount is subtracted from the Extended price, the answer can appear to be incorrect.

Convince yourself. Try this: 10 units at $39.95. The answer displayed for Extended price is $399.50, which is correct. The answer displayed for the discount is $59.93 (rounded up from the actual answer of 59.925). The amount displayed for the Discounted Price is $339.58, which is the rounded result of 399.50 – 59.925. The solution—without using the Round function because it has not been introduced yet—is to add 5/10ths of a penny (.005) to the discount. Then both the discount and the resulting discounted price will both appear to be correct and will be correct!