Lesson III.         Data Types, Arithmetic Operators & Precedence, and Common Functions

 

Objectives

ü       To determine what data types VB supports; how to declare variables, and how to assign data to variables

ü       To able to use Arithmetic Operators

ü       To use and understand the Val( ), Str( ), and isNumeric Functions

ü       To apply the rule of arithmetic operator precedence

ü       To able to write subroutines that solve mathematical problems

 

 

 

 


Programming involves data and ways of manipulating these data.  Thus, you should learn how to represent data in the computer. 

 

 

IN FOCUS: VARIABLES, TYPES, DECLARATION, AND SCOPE

 

You often need to store values temporarily when performing calculations with Visual Basic. In VB, you can store values in containers called variables.  Generally, variables have a name (a string you use to refer to the variable when you want to fetch or store value in the variable) and a data type (which determines the range of values the variable can store and the operations defined for that type).

 

Data types control the internal storage of data in Visual Basic. How do we associate a type to a variable?  VB does this automatically.  By default, Visual Basic uses the Variant data type.  But programmers can always associate variables to a type explicitly. 

 

The following table enumerates the common VB data types:

 

Data Type

Description and Range

Boolean

Data that is either True or False

Byte

Positive numeric values without decimals that range from 0 to 255

Double

Numeric values that range from

–1.79769313486232E+308 to 1.79769313486232E+308*

Integer

Numeric values with no decimal point or fraction that range from –32,768 to 32,767

Long

Integer values with a range beyond that of Integer data values.  It ranges from

–2,147,483,648 to –2,147,483,647

String

Data that consists of 0 to 65,400 characters of alphanumeric (letters and numbers) and special characters (punctuations, etc.). 

Variant

Data of any data type and used for control and other values for which the data type is unknown.

*The value of 1.2E+6 is computed by multiplying 1.2 by 6 raise to 10.  This is equal to 1.2 times 1,000,000 = 1,200,000.
 
 

 

 

Variable Declaration

Data have different sizes, thus a variable that would contain a String value should have enough space to accommodate strings.  Variable declaration specifies how much space VB should allocate for a variable. 

 

How do we declare variables?  You use the Dim statement through the following syntax:

 

Dim <var_name> As <data_type>

 

The following are examples:

 

Dim strName As String

Dim A As Boolean, B As Integer

 

 

Scoping Variables

A variable is scoped as either a procedure-level (local) or module-level variable.  This depends on how it is declared.

 

Scope

Private

Public

Procedure-level

Variables are private to the procedure in which they appear.

 

Declaration: Declare variables within a procedure.  For Example:

Private Sub cmdSum_Click( )

    Private x As Integer

    …

End Sub

 

Note: Variables declared within a procedure are by default private – the variables can only be accessed within the procedure’s scope.

 

Not applicable. You cannot declare public variables within a procedure.

Module-level

Variables are private to the module in which they appear.

 

Declaration: You create module-level private variables by declaring them with the Private keyword in the Declarations section at the top of the module.

Variables are available to all modules.

 

Declaration: You create module-level public variables by declaring them with the Public keyword in the Declarations section at the top of the module.

 

Note: You can declare 2 different variables with the same name but these variables must be declared in different scopes.  For example, you could have a public variable named Temp and then, within a procedure, declare a local variable named Temp.  If we refer to variable Temp within the procedure, we would access the local variable.  References to Temp outside the procedure would access the public variable.

 

 

IN FOCUS: ASSIGNMENT OPERATOR

 

We use the assignment operator = to assign data to a variable.  Assigning data to a variable is simply storing a value to the variable. The syntax is as follows:

 

<var_name> = <expression>

 

<var_name> is a variable name (which you might have declared using the Dim statement).  <expression> can be a literal, another variable, a mathematical expression, or a function call.  Take the following examples:

 

‘Assigning a literal to a variable

age = 18

stude_name = “Jay Fernandez”

 

‘Assigning the value of a variable to another variable

grade = score

 

‘Assigning the result of a mathematical expression

grade = score1 + score2

 

Always bear in mind that the left and right hand side values of the assignment operator should be of the same data type or at least of compatible type.  You cannot assign a String value to a Double variable.

 

 

IN FOCUS: MATH OPERATORS

 

You use math operators when you want to manipulate data.  If you want to get the average of 2 numbers, you will probably use the formula (x + y)/2.  The following table describes VB’s primary math operators.

 

Operator

Example

Description

+

Score1 + Score2

Adds two values

-

Price – Discount

Subtracts the second value from the first

*

Price * Discount

Multiplies two values

/

Total / 2

Divides the first value by the second value

\

Total \ 2

Divides the first value by the second value but returns only the whole number (e.g. 5\2 returns 2).  Expressions with this operator are called integer divisions.

^

2^2

Raises the first value to the second value

-

-2

Negates a value.  This operator is called unary minus.

 

You can combine several mathematical expressions in one expression as in the example:

 

grade = (score1 + score2 + score3) / 3

 

 

IN FOCUS: OPERATOR PRECEDENCE

 

Another matter that you should take note of is operator precedence.  Precedence is the order of computation.  ^ is evaluated first, then * and /, then + and -.  If there are * and / in an expression, execution is done from left to right.  This holds true to + and -.  Let us demonstrate this in the following examples:

 

The expression 2 + 3 * 4 is equal to 14.  This was computed by doing the multiplication first before addition (since multiplication has higher precedence over addition).

 

The expression 4 * 3 / 2 is equal to 6.  We evaluate this expression from left to right, which is by doing the multiplication first before division.

 

You can override the operator precedence by using parentheses.  VB always evaluates expressions enclosed in parentheses before anything else in the expression.  Consider the following expression:

 

4 * (2 + 3)

 

This is equal to 20, computed by adding 2 and 3 and multiplying the sum by 4. Without the parentheses, the value of the entire expression is 11 (multiplying 4 with 2 and adding 3 to the product). 

 

 

IN FOCUS: VAL(), STR(), AND ISNUMERIC FUNCTIONS

 

The Val(), Str(), IsNumeric Functions are useful for data conversion: from numbers to Strings and from Strings to numbers.  Val() converts Strings to numbers.  Str() complements Val() – it converts numbers to Strings.  IsNumeric() returns True if a String argument can represent a number.

 

Val() is useful when you want to convert e.g. the Text  property of a TextBox to a number.  This is applicable in cases when you require the user to enter a number in the TextBox (e.g. the user’s age).  The Text property of TextBoxes and the Caption property of Labels are Strings in type, thus conversion must be done if you need their number value.  You use IsNumeric() in cases when you need to verify if whether a particular value can be represented as a number.  For example, if you require the user’s age, and the user enters letters, you might want to notify the user of wrong input.

 

The following are examples of how we use the 3 functions.

 

strNumber = “122”

intNumber = Val(strNumber) ‘intNumber is assigned the numeric value 122

strNum = Str(intNumber)                 ‘strNum is assigned the string “122”

intAge = Val(txtAge.Text)                 ‘intAge is assigned the numeric value of txtAge.Text

y = IsNumeric(intNumber)               ‘y receives True

 

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

The application asks for the 2 numbers and outputs “The average of the 2 numbers is <average>”.

 

1.       Create a new project.

2.       For the Form control, set the following properties:

 

3.       Drag 3 Labels to the Form.

Set the following Properties for the first Label:

§         Name                            lblNum1

§         BackStyle                       Transparent

§         Caption                         First Number

§         Font                              Arial, Size 14, Regular

§         Top                               360

§         Left                               240

§         Width                            2175

§         Height                           495

Set the following properties for the second Label:

§         Name                            lblLNum2

§         BackStyle                       Transparent

§         Caption                         Second Number

§         Font                              Arial, Size 14, Regular

§         Top                               960

§         Left                               240

§         Width                            2175

§         Height                           495

Set the following properties for the third Label:

§         Name                            lblResult

§         BackColor                      Light Blue

§         Caption                         None. Erase Default Value

§         Font                              Arial, Size 14, Italic

§         Width                            4815

§         Height                           975

§         Top                               1560

§         Left                               240

 

4.       Drag 2 TextBox controls to the Form.

Set the Following Properties for the first TextBox:

§         Name                            txtNum1

§         Font                              Arial, Size 14, Regular

§         Text                              None.  Erase Default Value

§         Width                            615

§         Height                           495

§         Top                               240

§         Left                               2520

Set the following properties for the Second TextBox:

§         Name                            txtNum2

§         Font                              Arial, Size 14, Regular

§         Text                              None.  Erase Default Value

§         Width                            615

§         Height                           495

§         Top                               840

§         Left                               2520

 

5.       Drag 2 CommandButton controls to the Form.

Set the following properties for the first CommandButton:

§         Name                            cmdAverage

§         Caption                         Average

§         Width                            1695

§         Height                           495

§         Top                               2760

§         Left                               1440

Set the following properties for the second CommandButton:

§         Name                            cmdQuit

§         Caption                         Quit

§         Width                            1695

§         Height                           495

§         Top                               2760

§         Left                               3360

 

6.       Write the following procedures:

 

Private Sub cmdAverage_Click()

    ave = (Val(txtNum1.Text) + Val(txtNum2.Text)) / 2

    lblResult.Caption = "Average of the 2 numbers is: " & Str(ave)

End Sub

 

Private Sub cmdQuit_Click()

    End

End Sub

 

7.       Run your application.

8.       Save your work as Lesson3.vbp.

 

 

 

 

 


1.   Write a code that declares these variables: your first name, your last name, your age, your course, and whether you are old or new student.

      ___________________________________________________________________________

      ___________________________________________________________________________

      ___________________________________________________________________________

      ___________________________________________________________________________

 

  1. Write an application that accepts your age in a TextBox and then displays, when you click a CommandButton, your age in 10 years time.

 

  1. Make an application that asks for an item price, the discount (if any), and the cash given by the customer.  Compute and output the selling price and the change (cash – selling price).  Refer to the Form below.