Visual Basic 6

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Visual Basic 6

• Variables, Constants and Calculations

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Variables and Constants

• Memory locations that hold data that can be
  changed during project execution are called
  variables. Ex. intX 17
• Constants are memory locations that do not change
  during project execution.
• All variables and constants used in a project
  should be declared in a declaration statement.

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Declaration Statements

• Establish your projects variables and constants,
  give them names and specify the type of data they
  will hold.
• Ex. Dim strName as String
• Here we are creating a variable called Name that
  will hold strings of characters or text.strName
  could be used for example to store the names of
  people entered into a text box.

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Constants

• You declare named constants using the keyword
  Const. You give the constant a name, data type
  and a value.The data type you declare and the
  data type of the value must match.
• Named constants allow for more readable code and
  make coding changes much easier.
• Ex. Const strNAME As String Mark
• Const curSALESTAX As Currency .15
• Note that Constant names are in uppercase.

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Scope of Variables

• A variable may exist and be visible for an entire
  project, for only one form, or for only one
  procedure.
• The scope or existence a variable is said to be
  global, module level or local.
• Global can be used anywhere in a project
• Module level are accessible anywhere in a form
• Local variable may be used only within the
  procedure in which it is declared.

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Data Types

• Data type indicates what type of information will
  be stored by a variable or constant. This is
  important because it uses memory more efficiently
  and allows programs to run faster.
• A table containing all VB data types can be found
  on page 79 of your guide.
• You should always specify data type

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Data Types

• We will use the following prefixes for data
  types bol for Boolean, byt for byte, cur for
  currency, dat for date, dbl for double, int for
  integer, lng for long, sng for single and var for
  variant.
• Always use the prefixes written in small case
  followed by the variable or constant name written
  with a capital letter.
• Ex. intNumber as Integer

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Option Explicit Programming

• This insures that all variables must be declared
  before they can be used in the program.
• This saves memory space and eliminates a number
  of common programming errors.
• To ensure all your programming is done in option
  explicit mode, go to the tools menu, select
  options, on the editor tab, make sure that
  Require Variable declaration is selected, click
  OK. Option explicit should now appear at the top
  of all forms that you create.

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Calculations

• In programming you can perform calculations on
  variables and with properties of certain objects.
• The properties you will use such as the text
  property and label caption are actually strings
  and must be converted into a numeric form before
  they can be used in calculations.

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Calculations

• What I mean by this is that if a user enters 15
  as there age in a text box on a form you have
  created, by default VB assumes the input, 15, to
  be a string value. By using the val function we
  convert 15 into a form that allows for
  calculations.

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Calculations

• When performing calculations in VB the following
  symbols are used for arithmetic operations
• for addition, - for subtraction, for
  multiplication, / for division, Sqr(x) for the
  square root of x, xy to raise x to the power of
  y.

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Using Calculations in Code

• Calculations are performed in assignment
  statements.
• In most cases you will assign calculation results
  to variables or to the caption properties of
  labels.
• Ex. curPaycheck curHours curPayrate or
• lblCommission.Caption curSales curCOMRATE

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Functions

• A function performs an action and returns a
  value. There are many built in functions provided
  in VB and you can create your own using the
  Function statement.
• The first function we will use is the val
  function which converts text data into a numeric
  value.
• Ex. intAge Val(txtAge.Text)
• The variable intAge can now be used in
  calculations

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Format Functions

• The format function allows you to control the way
  the output will look.
• The expression to be formatted may be numeric, a
  string, a constant, a variable or a property.
• In most cases you will be formatting numeric data
  to control the number of decimal positions.
• Ex. lblTotal.Caption Format(curTotalAmount,
  Currency)
• This will display Total Amount in the 3.00 form
  as opposed to 3.

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Counting and Accumulating Sums

• Programs often need to sum numbers. If you want
  to accumulate a total, you need a variable to
  hold the information.
• If you want a variable to retain its value for
  multiple calls, in order to accumulate totals,
  you must declarethe variable as module level,
  outside the subprocedure.

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Summing Numbers

• Create a module level variable to hold the total
• In the calculate procedure, add the current
  amount to the total
• Ex. curSum curSum curPrice
• This code adds the new price to the past total
  every time a new price is found.

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Counting

• If you want to count something, such as the
  number of items sold, you need a module level
  variable.
• Ex. Dim intCount as Integer
• Then in the calculate event add the following
  code intCount intCount 1
• Each time the calculate event is executed the
  counter will go up 1, this gives you a rumming
  count for example of how many items have been
  sold.

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Putting This Info to Use

• Three step process
• Convert input to numeric values
• Perform your calculations
• Display your output
• Do Calorie Counter example