cis 115 lecture05 vb-variables
TRANSCRIPT
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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?