Chapter 3 Part 1 1

Variables, Constants, and Calculations

Chapter 3 Section 3.3Chapter 3 Section 3.4

Chapter 3 Part 1 2

Section 3.3

Variables Constants

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So far…

Take user input, perform calculation, and display output.

Have not stored input or output for future processes within the application.

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Variable

What is it? Storage location in memory (RAM)

What does it store? Data during execution of the program

What are the uses? Copy and store user input Manipulate user input Store intermediate calculation results to be

used later Store results to display for output

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Why is it called a “variable”?

The data may change during execution of the program.

The data varies. Run the program several times to calculate

different employees’ gross pay.

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Variable Declaration

Statement that sets aside memory; creates the variable location. Specify variable name and data type

Syntax:Dim typeVariableName As DataType

Examples:Dim intAge As IntegerDim strName As String

Declaration before usage of variable

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Variable Name Rules (Required)

1st character must be a letter or underscore (not a numeric value)

Other characters may be letters, numeric digits, and underscores

No spaces allowed No use of keywords as variable names

Dim Integer Private

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Variable Names (Conventions)

Descriptive (not vague) names Camel Casing (Uppercase first letter each

word chained) dblSalesTax strFirstName intMaximumCapacity

Three-letter prefix abbreviation of data type Casing different to distinguish constant &

variables.

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Variable Name Exercise

Name Valid? Conventional?

Quantity

strCompany

decpayrate

Sub

Minimum Amt

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Data Type

Indicates the type of data that the variable can store.

Important: Specifies amount of memory used Dictates how the variable formats and stores

data in memory

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Data Types

Numeric Data Only numbers Available to use in mathematical calculations e.g., 34.56 amount of money e.g., 231 number of available hotel rooms

String Data Any symbol Not available for calculations e.g., ISYS 1200 (course number)

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“Numeric” Data

9.35 Weight in tons of crate

125000 Population of City

109.95 Dollar Amount

84097 Zip Code

629111111 Social Security Number

8018638843 Phone Number

If data will NEVER used in calculations (e.g., Phone Number), make it a string.

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Zip/Postal Code

84097 84097-0555 K1Z 8R7 (Ottawa, Ontario, Canada)

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Visual Basic .NET Data Types (p. 118)

Integer Decimal Double String Boolean

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Mini Quiz: Which data type…

Stores only whole numbers? Decimal, Double, or Long

Stores larger numbers? Decimal or Double

More appropriate for U.S. Zip Codes? Short, Long, String

Provides greater precision? Decimal or Double

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Default Values

Default value created when you declare a variable

Numeric (integer, decimal, double, etc.) Assign 0 as default value

String Special value known as Nothing

Boolean False

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Variable Initialization

Process of specifying an initial value upon declaration

Syntax:Dim variableName As DataType = initialValue

ExampleDim intUnitsSold As Integer = 12

Using a string variable without initialization halts the execution.Dim strName As String = “”

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Variable Assignment

Specifies contents for a variable name. Syntax:variablename = expression

Examples:decSalary = val(txtSalary.Text)decSalary = decSalary + decRAISE

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Constant or Variable

Permanent number or string throughout application that does not change or

Able to take on different numbers or strings during run time?

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Constants

Read-only data items whose values do not change while the program is running.

Declared & set at the beginning of application and maintain their values.

Examples: UVSC string literal constant 0.0625 numeric literal constant decSALESTAX symbolic constant

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String Literal Constants

Begins and ends with quotation marks. Occurs in pairs. Examples:

“314 North State” “Visual Basic” “Utah Valley State College”

lblData.Text = "ISYS 1200 class rocks!"

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Numeric Literal Constants

Numeric value that does not change during the operation of the program.

Example:lblResult.Text = Val(txtSubtotal.Text) *

0.0625

But…the above statement uses a magic number to display the constant 0.0625.

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Magic Numbers

When a literal constant number is used in an expression. (e.g., 0.0625 in previous slide)

Problems: Not immediately apparent to anyone but

original programmer what 0.0625 is Maintenance nightmare if use the magic

number throughout a complex program and the value changes

Sales tax rate changes from 6.25% to 6.6% Nightmare to ensure that you made all changes

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Magic Numbers, Continued

Solution: Create a named constant (a descriptive name) for

the literal numeric constant value Use the named constant in expressions

Example:Const decRAISE As Decimal = 1000decSalary = decSalary + decRAISE

Declaration: Set up memory and specify its permanent value Must initialize upon declaration (not optional)

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Magic Numbers, Continued

Exceptions:-1, 0, 1 when used as counters (such as in

loops)

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Constant Name Rules

Letters and digits First character must be letter No spaces or punctuation marks No reserved (keywords) used

Sub If End

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Constant Name Conventions

Use meaningful, descriptive names. Use all CAPITAL letters for symbolic constant

names. Include underscore between words. Include three-letter lowercase prefix for data

type identification. Examples:

strCOLLEGE_NAME decSALES_TAX_RATE

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Run Time

1) Creates empty table of symbolic constants.

2) Adds row to the table showing the name and value of new constants.

3) Goes to table to look up value corresponding with symbolic name.

4) Substitutes literal value for the name.

5) Executes statement using literal value.

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Errors Will Result If:

You do not assign a value to a symbolic constant declaration.

The program contains event procedures that attempt to change a constant’s contents.

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Intrinsic Constants

System-defined constants Typically declared in system class libraries

and are available for use in VB programs Examples:

MessageBoxIcon.Question ContentAlignment.Left

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Scope & Lifetime

Scope Area of program (such as an event procedure)

where the variable is visible and may be accessed by programming statements.

Lifetime Time during which the variable exists in

memory.

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Scope Issues

Variables can not be used before being declared. See problem example on p. 121

Variables declared inside a procedure are visible only within that procedure. No other procedure can access that variable. See problem example on pp. 121-122

You cannot have multiple variables of the same name in the same procedure.

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Local and Module Declarations

Local Specific (event) procedure only No other procedure can access variables in

another procedure* Module (or Class-Level)

Available for any procedure on the form Declaration area at top of code window (below

programmer ID info); not Form Load though

*Can be passed through procedure calls and function calls. See Chapter 6.

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Lifetime

Variable set up and memory reserved upon declaration.

Variable exists in memory. Variable destroyed; ends it lifetime.

Local variables lifetime expires at the end of the event procedure.

Class-level variables lifetime expires when the form is removed from memory.

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Date Data Types

Read pp. 122-123 on your own.

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Implicit Type Conversion

VB attempts to convert data type into data type for specific variable

Example: Dim intCount As Integer intCount = txtData.Text ‘user enters 12.2 VB converts 12.2 to 12 Rounds to nearest whole number

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Conversion Concerns

Data entered in text boxes is really a string, but VB tries to convert to numeric data for numeric data types.

Type Mismatch or Type Conversion Error intCount = txtData.Text ‘type abc123 Run-time error because VB can’t convert “abc”

to integer value.

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Val Function

Converts strings to numeric values. Prevents Type Mismatch and

Type Conversion errors. Intrinsic function that carries out specialized

operation and returns a value to the program. Syntax

Val(argument)

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Val Function

intCount = Val(txtData.Text) Val function

receives data from text property of the txtData text box control.

converts argument from a string to a numeric value.

returns the numeric value to the statement that called the function

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Val Function

intCount = Val(txtData.Text)

Returns number 45 from the string 45

String “45”

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Val Function Examples (p. 126)

String Conversion

“34.90” 34.9

“86abc” 86

“$24.95” 0

“3,789” 3

“x29” 0

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ToString (p. 126)

Returns a string equivalent of data stored in a variable (sometimes used for output)

Dim intNumber As Integer = 123 lblNumber.Text = intNumber.ToString

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Option Strict On (pp. 126-127)

Prevents VB from performing most implicit type conversions.

Reduces errors that might result from implicit data type conversions.

Must appear at the top of the code window before any other statements (after programmer ID remarks)

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Section 3.4

Arithmetic Operators Calculations

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Arithmetic Operators

+ Addition

- Subtraction

* Multiplication

/ Division

^ Exponentiation

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Arithmetic Examples

decAmount = 4 + 8 (but has magic #s) dblTotal = dblPrice + dblTax decArea = decLength * decWidth dblAverage = dblTotal / intNumber dblResult = Val(txtInput.Text) * _

dblUNIT_PRICE

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Warning

When performing division, be sure that the operand to the right of the / operator is NOT zero.

An error results if you divide by zero.

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Special Division (p. 129)

Integer Division \ Result is always an integer intParts = 17 \ 3 intParts contains value 5

MOD Operator Modulus operator returns remainder of division intLeftOver = 17 MOD 3 intLeftOver contains 2

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Order of Precedence

1. Exponentiation 1st

2. Multiplication and Division

3. Integer Division

4. Modulus

5. Addition and Subtraction

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Examples

Outcome = 12 + 6 / 3 Outcome = 12 + 2 Outcome = 14

Outcome = (12 + 6) / 3 Outcome = 18 / 3 Outcome = 6

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More on Calculations

Algebra Class 6y means multiply 6 by y

Programming Class 6 * y is required Must use operator

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Combined Assignment Operators

Add current variable contents to 5 and save back to same variable intNumber = intNumber + 5 intNumber += 5

Multiply current variable contents by 10 and save back to same variable intOutcome = intOutcome * 10 intOutcome *= 10

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Type Conversion Functions

Read pp. 135-136 on your own.

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Checkpoints for Individual Review

page 127 page 137