Transcript VB 2008 - Long Island University

Creating Variables and Constants
Intro to Datatypes
Assignment Statement
 A variable is a “friendly” way to refer to a piece of
memory in the computer
 This memory location stores a value
 The “DIM” command creates and associates this piece
of memory with a name in your program
 Programmer specifies the “type”
 Rules
 Must start with an “alpha” character
 Up to 256 alphanumeric and special characters
 $ _ are special characters
 Cannot be a reserved word
 Reserved = for VB’s usage, such as “DIM”
 NO SPACES!!!
 A space is a delimiter
 VB interprets the space as the end of something and beginning of
something else
 Hungarian Notation
 DIM intPasswordRetention as Integer
 Pascal Notation
 DIM PasswordRetention as Integer
 Camel Notation
 DIM passwordRetention as Integer
 Creating a variable
 Use the “DIM” statement
DIM intX as Integer
intX = 22
 Same as
DIM intX as Integer = 22
//Assignment statement
 What is an expression?
 Something to be evaluated
 Not an instruction per se
 Examples:
3+5
dblRate * intNumberYearsMember
intAge > 21
 The instruction which assigns a value to something
intAge = 10
boolAdult = intAge > 21
dinnerTip = subTotal * tipRate
 The expression on the right of the equal sign is
evaluated, and then assigned to the variable on the left
of the equal sign
A=8
A=A+1
System.Console.WriteLine(A)
//displays 9
 Declaration and assignment of a value to a name
 Once assigned, the value cannot change
 Same name rules as variable
 Perhaps you might wish to put a “c” before the name to
indicate that it’s a constant
Const cintPasswordLimit as Integer = 3
 Datatypes use a certain number of bytes in memory
 Defines the possible range of values a specific type can
represent
 Some types are “signed” while others are “unsigned”
 One of the bits is used to indicate the positive or
negative value
 This bit is not used for value, and halves the possible value
that can be held
 Signed Short Integer uses 15 bits for value and 1 for sign
 (-32768 thru +32767)
 Unsigned Short Integer
 (0 thru 65535)
 Numeric
 Integral (whole number) types
 Signed
 SByte
8 bit
 -128 through 127
 Short16 bit
 -32,768 through 32,767
 Integer
32 bit
 Long
64 bit
 -9,223,372,036,854,775,808 through 9,223,372,036,854,775,807
 Unsigned




Byte
UShort
UInteger
ULong
8 bit
16 bit
32 bit
64 bit
 Numeric
 Non-integral (non whole numbers)
 Decimal
 79,228,162,514,264,337,593,543,950,335 through
79,228,162,514,264,337,593,543,950,335 (if no decimal places used)
 7.9228162514264337593543950335E+28
 Floating Point (larger ranges than decimal, but might have
rounding errors)
 Single
 Smallest values
 -1.401298E-45 for negative values and 1.401298E-45
 Largest values
 3.4028235E+38
 Double
 Smallest values
 4.94065645841246544E-324 for negative values and 4.94065645841246544E-324 for
positive values
 Largest values
 1.79769313486231570E+308
 Integer data math is faster (performance) than using
non-integer types
 Reference
 http://msdn.microsoft.com/en-us/library/ae55hdtk.aspx
 String
 Group or “string” of characters
 Can hold up to about 2 billion Unicode characters
 Char
 Holds a single Unicode character
 Boolean
 Value is either True or False
 Used in decisions
 In many cases a “non-zero” value equates to a True
 Sometimes the data you get is not the datatype that
you need
 You cannot manipulate or use it directly
 Convert it to the type you DO need and can use
 If you ask a user for taxrate they will typically use the
keyboard to type in the response
 This is a bunch of letters (keystrokes) that merely
resemble numbers
 If you can type it on the keyboard, or display it on the
monitor / printer, then it is character data
 A long continuous group of characters is a character “string”
 Accepting string input which should be numeric
 Convert string data to numeric data
 Input comes from:
 Keyboard (command line)
 Form’s text box
 Text file
 Presenting (outputting) to the user a number on the
screen
 Convert the number to string / text data
 Output goes to:
 Screen (command line)
 Form’s text box
 Text file
 Getting a numeric value
 Valid characters which can be typed
 All keyboard characters
 Valid characters you can convert to numeric from string
 0-9
 Minus sign
 Decimal point
 Invalid characters which will not convert
 Any other characters, including $ and comma
 If you allow users to type in non-numeric characters, you must
strip away those characters before converting the remaining
characters to a numeric value usable by your program
 Do not assume that the user will type in anything correctly or in
the way you expect
 Validate the data!!!
 Using the tax and taxrate example
 When prompted for the taxrate we’ll assume the user wants 10%
 We will also assume the expression tax = amount * taxrate and that
amount is 200
 If the user types 10%
 The “%” sign cannot be converted, we must strip it out
 If the user types 10
 Then tax would be 2000

The correct answer should be 20!
 The user could’ve typed in .1 which is 10% or 10/100 (10 per 100)
 How many users know this?
 If you didn’t tell the user exactly how to enter the value, you’re guessing (and
hoping) as to what is entered
 Your program should do the conversion, “behind the scenes”, for the user
tax = amount * taxrate / 100
 Now if the user types “10” for taxrate the answer for tax would be “20”
 Parenthesis
( )
 Exponentiation
^
 Positive / negative
+ Multiplication, division, integer division and modulus
* / \ MOD
 Addition and subtraction
+-
 Process of converting data from one datatype to another
 In many cases, VB will do this “automatically” (Implicit Casts)
 Widening
 The receiving variable is “wider” and can hold the value being received
Dim a As Double = 6.5
Dim b As Integer = 6
Dim c As Integer = 10
Dim average As Double = (a + b + c) / 3
 b and c are implicitly converted to Double values
 Narrowing
 The receiving variable is not “wide enough” to hold the value
 A casting exception is thrown if receiving variable cannot hold the value
 Shouldn’t use this unless you KNOW the receiving variable can hold the value
Dim grade As Integer = 93.75
Dim grade As Integer = txtSubtotal.text
Dim average As Integer = (a + b + c) / 3
‘Rounded to 94
‘may throw an exception
‘average is 8
 Explicit Casts using code
 CInt(x) or CDec(y) will convert the value to the type for usage
 Explicit casts are performed before other operations in arithmetic
expressions