cis 115 lecture05 vb-variables

8/10/2019 CIS 115 Lecture05 VB-Variables

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CIS 115 Lecture 5


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A storage location in memory (RAM)

Holds data/information while the program is running

These storage locations can be referred to by their

names

Every variable has three properties:

Name- reference to the location - cannot be changed

Value- the information that is stored - can be changedduring program execution, hence the name variable

Data Type- the type of information that can be stored -

cannot be changed


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You the programmer make up a name for thevariable

Visual Basic associates that name with alocation in the computer's RAM

The value currently associated with thevariable is stored in that memory location

You simply use the name you chose when youneed to access the value


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Copy and store values entered by the user Perform arithmetic manipulation on values

Test values to see if they meet a criteria Temporarily hold and manipulate the value of

a control property Hold data/information so that it can be

recalled for use at a later point in the code


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Data type - Specifies type of data variable can store Integer variables: Long, Integer, Short, Byte Floating-point variables: Single, Double

Fixed decimal point variable: Decimal Boolean variables: True, False

Character variable: Char

Text variable: String

The Object variable Default data type assigned by Visual Basic

Can store many different types of data

Less efficient than other data types


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Data type Prefix Size ValuesByte byt 1 byte positive integer value from 0 to 255Short shr 2 byte integer from 32,768 to +32,767Integer int 4 byte integer from +/- 2,147,483,647

Long lng 8 byte integer from +/- 9,223,372,036,854,775,807Single sng 4 byte single-precision, floating-point numberDouble dbl 8 byte double-precision, floating-point numberDecimal dec 16 byte number with up to 28 significant digits

Char chr 2 byte Any single characterBoolean bln 2 byte True or False

String str (4 byte) Text - Any number/combination of charactersDate dtm 8 byte 8 character date: #dd/mm/yyyy#Object obj (4 byte) An address that refers to an object


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First character must be a letter or underscore Must contain only letters, numbers, and

underscores (no spaces, periods, etc.)

Can have up to 255 characters Cannot be a VB language keyword Naming Conventions Should be meaningful

Follow 3 char prefix style - 1st 3 letters in lowercaseto indicate the data type

After that, capitalize the first letter of each word

Example: intTestScore


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A variable declaration is a statement thatcreates a variable in memory

Syntax: Dim VariableNameAs DataType Dim (short for Dimension) - keyword

VariableName - name used to refer to variable

As - keyword DataType - one of many possible keywords to

indicate the type of value the variable will contain

Example: Dim intLength as Integer


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A starting or initialization value may bespecified with the Dim statement

Good practice to set an initial value unlessassigning a value prior to using the variable Syntax:

Dim VariableNameAs DataType= Value Just append " = value to the Dim statement

= 5assigning a beginning value to the variable

Example: Dim intLength as Integer = 5


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Variable MUST be declared prior to the codewhere they are used

Variable should be declared first in theprocedure (style convention)

Declaring an initial value of the variable in thedeclaration statement is optional

Refer to default values (next slide)


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Data type Default (Initial) value

All numeric types Zero (0)

Boolean FalseChar Binary 0

String or Object Empty

Date 12:00 a.m. on January 1, 0001


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Actual value/data/information Similar to a variable, but can NOT change

during the execution of a program. Examples of Literals:

Numeric: 5 ; 157 ; 195.38256

String: Paul ; Hello!!! ; Jackson, AL 36545

Char: a ; 1 ; ? ; @

Boolean: True ; False


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Programs often need to use given values

For example: decTotal *= 1.06

Adds 6% sales tax to an order total

Two problems with using literals for these typesof values

The reason for multiplying decTotal by 1.06 isnt

always obvious

If sales tax rate changes, must find and change every

occurrence of .06 or 1.06


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Use of named constantsresolves both these issues Can declare a variable whose value is set at

declaration and cannot be changed later: Syntax: Const CONST_NAMEAs DataType= Value

Looks like a normal declaration except:

Const used instead of Dim

An initialization value is required

By convention, entire name capitalized with underscore

characters to separate words


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The objective of our code is now clearer

Const sngSALES_TAX_RATE As Single = 1.06

decTotal *= sngSALES_TAX_RATE Can change all occurrences in the code simply

by changing the initial value set in thedeclaration

If tax rate changes from 6% to 7%

Const sngSALES_TAX_RATE As Single = 1.07


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What Indicates the part of the program where thevariable can be used

When From the variable declaration until the end ofthe code block (procedure, method, etc.) where it is

declared Variable cannot be used before it is declared

Variable declared within a code block is only visible tostatements within that code block

Called Local Variable Can be declared at the beginning of the class code window

(General Declarations section) and be available to all blocks Called Form Level Variable

Variables that share the same scope cannot have the samename (same name ok if different scope)


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What Indicates the part of the program where the

variable exists in memory When From the beginning of the code block

(procedure, method, etc.) where it is declared untilthe end of that code block

When the code block begins the space is created to holdthe local variables

Memory is allocated from the operating system When the code block ends the local variables are destroyed

Memory is given back to the operating system


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Syntax: variablename = expression Assigns the value of the expression to the

variable. (The variable must be on the left andthe expression on the right.)

Example: intNumber1 = 4

intNumber2 = 3 * (2 + 2) intNumber3 = intNumber1

IntNumber1 = intNumber1 + 6


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A value of one data type can be assigned to a variable

of a different type

An implicit type conversion is an attempt to automatically

convert to the receiving variables data type A widening conversion suffers no loss of data

Converting an integer to a single

Dim sngNumber as Single = 5

A narrowing conversion may lose data Converting a decimal to an integer

Dim intCount = 12.2 intCount becomes 12


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VB provides a set of functions that perform data type

conversions These functions will accept a literal, variable name, or

arithmetic expression The following narrowing conversions require an

explicit type conversion

Double to Single

Single to Integer Long to Integer

Boolean, Date, Object, String, and numeric types

represent different sorts of values and require

conversion functions as well


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The Val functionis a more forgiving means ofperforming string to numeric conversions

Uses the form Val(string) If the initial characters form a numeric value,

the Val function will return that Otherwise, it will return a value of zero


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Val Function Value Returned

Val("34.90) 34.9

Val("86abc) 86

Val("$24.95) 0

Val("3,789) 3

Val(") 0

Val("x29) 0 Val("47%) 47

Val("Geraldine) 0


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Returns a string representation of the value inthe variable calling the method

Every VB data type has a ToStringmethod Uses the form VariableName.ToString For example

Dim number as Integer = 123

lblNumber.text = number.ToString

Assigns the string 123 to the text property of thelblNumber control


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Arithmetic Operators

^ Exponential

* Multiplication/ Floating Point Division

\ Integer Division

MOD Modulus (remainder from division)

+ Addition

Subtraction

& String Concatenation (putting them together)


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Examples of use: decTotal = decPrice + decTax

decNetPrice = decPrice - decDiscount

dblArea = dblLength * dblWidth

sngAverage = sngTotal / intItems

dblCube = dblSide ^ 3


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The backslash (\) is used as an integer divisionoperator

The result is always an integer, created bydiscarding any remainder from the division

Example intResult = 7 \ 2 result is 3

shrHundreds = 157 \ 100 result is 1 shrTens = (157 - 157 \ 100 * 100) \ 10

result is ?


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This operator can be used in place of thebackslash operator to give the remainder of a

division operationintRemainder = 17 MOD 3 result is 2

dblRemainder = 17.5 MOD 3 result is 2.5

Any attempt to use of the \ or MOD operatorto perform integer division by zero causes aDivideByZeroException runtime error


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Concatenate: connect strings together Concatenation operator: the ampersand (&) Include a space before and after the & operator

Numbers after & operator are converted to strings

How to concatenate character strings strFName = "Bob"

strLName = "Smith"

strName = strFName & " Bob

strName = strName & strLName Bob Smith

intX = 1 intY = 2

intResult = intX + intY

strOutput = intX & + & intY & = & intResult 1 + 2 = 3


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Often need to change the value in a variable andassign the result back to that variable

For example: var = var 5 Subtracts 5 from the value stored in var

Operator Usage Equivalent to Effect+= x += 2 x = x + 2 Add to

-= x -= 5 x = x

5 Subtract from*= x *= 10 x = x * 10 Multiply by/= x /= y x = x / y Divide by\= x \= y x = x \ y Int Divide by&= x &= . x = x & . Concatenate


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Operatorprecedencetells us the order in whichoperations are performed

From highest to lowest precedence:

Exponentiation (^) Multiplicative (* and /)

Integer Division (\)

Modulus (MOD)

Additive (+ and -) Parentheses override the order of precedence Where precedence is the same, operations

occur from left to right


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Parenthesis Exponential

Multiplication / Division Integer Division MOD Addition / Subtraction String Concatenation Relational Operators (< , > , >= ,


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6 * 2 ^ 3 + 4 / 2 = 50

7 * 4 / 2 6 = 8

5 * (4 + 3) 15 Mod 2 = 34

intX = 10

intY = 5

intResultA = intX + intY * 5 'iResultA is 35

iResultB = (intX + intY) * 5 'iResultB is 75

dResultA = intX - intY * 5 'dResultA is -15

dResultB = (intX - intY) * 5 'dResultB is 25


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Redo the Calculate Gross Pay example fromLecture 4 using variables.

Redo the Calculator from HW2 usingvariables.


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Homework 3

Visual Basic - Variables

See handout for details and due date Questions?

cis 115 lecture05 vb-variables

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