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VB 2008

VB 2008. Creating Variables and Constants Intro to Datatypes Assignment Statement. Variables. A variable is a “friendly” way to refer to a piece of memory in the computer This memory location stores a value

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VB 2008

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  1. VB 2008 Creating Variables and Constants Intro to Datatypes Assignment Statement

  2. Variables • 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”

  3. Variable Names • 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

  4. Variable Names • Hungarian Notation • DIM intPasswordRetention as Integer • Pascal Notation • DIM PasswordRetention as Integer • Camel Notation • DIM passwordRetention as Integer

  5. Variables • Creating a variable • Use the “DIM” statement DIM intX as Integer intX = 22 //Assignment statement • Same as DIM intX as Integer = 22

  6. Expressions • What is an expression? • Something to be evaluated • Not an instruction per se • Examples: 3 + 5 dblRate * intNumberYearsMember intAge > 21

  7. Assignment Statement • 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

  8. Constants • 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

  9. Datatypes • 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)

  10. Datatypes • Numeric • Integral (whole number) types • Signed • SByte 8 bit • -128 through 127 • Short 16 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 8 bit • UShort 16 bit • UInteger 32 bit • ULong 64 bit

  11. Datatypes • 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

  12. Datatypes • Integer data math is faster (performance) than using non-integer types • Reference • http://msdn.microsoft.com/en-us/library/ae55hdtk.aspx

  13. Datatypes • 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

  14. Datatypes

  15. Datatypes • 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”

  16. Datatypes • 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

  17. Inputting • 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!!!

  18. Inputting • 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 * taxrateand that amount is200 • 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”

  19. Order of Operations • Parenthesis ( ) • Exponentiation ^ • Positive / negative + - • Multiplication, division, integer division and modulus * / \ MOD • Addition and subtraction + -

  20. Casting • 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 ‘Rounded to 94 Dim grade As Integer = txtSubtotal.text‘may throw an exception Dim average As Integer = (a + b + c) / 3 ‘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

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