Variables & Function Calls

Overview u Variables  Programmer Defined & Intrinsic  Data Types  Calculation issues u Using Functions  The val() function  The msgbox() function

Types of Data u Numeric  Can contain only numbers  Real numbers, integers, currency, dates  Used for arithmetic calculations u String (text)  May contain any symbol  No arithmetic calculations  Append, insert, extract

Variables - The Concept u Variables are named containers of data  They are not displayed on a form  The contents can change during runtime u Variables vary in size and purpose  Some types of data require more memory than others  Each variable belongs to a type  The type of a variable determines its size, and what operations (such as add, substract, append) will work on the variable

VB Variables u Variables are not created using the tool box  Instead, they are declared within the program  Declaring a variable  sets aside memory for the variable  assigns a name to that space in memory  describes what type of data can be stored there u Variables don’t work with the properties window  Changes to a variable must be made in a program

Declaring a Variable u A declaration begins with the Dim statement  This alerts VB that a new variable is being defined u A declaration contains a variable name and type  Just as each control must have a (name), like lblInput, and type, like label u The form of a declaration: Dim as  For example: Dim intCount as Integer

Numeric Variable Types u There are several different numeric variable types, each with varying  Precision  Size  Representation scheme u Selecting the wrong type for variables can hurt the performance of a program

Counting Things u Integer types  BYTE  Small range of values, 0 to 255  INTEGER  -32,768 to +32,767  LONG  -2,147,483,648 to +2,147,483,647

Measuring Things u SINGLE  Positive or negative  As close to zero as E-45  As large as E38 u DOUBLE  Positive or negative  As close to zero as E-327  As large as E308

Highest Precision u Scaled integers u CURRENCY  +/-922,337,203,685, u DECIMAL  +/-79,228,162,514,262,337,593,543,950,335  +/  no rounding, slow but sure

Storage Space & Prefixes u BYTE 1 byte byt u INTEGER 2 bytesint u LONG 4 byteslng u SINGLE 4 bytessng u DOUBLE 8 bytesdbl u BOOLEAN 2 bytesbln u CURRENCY 8 bytescur u DECIMAL14 bytesdec

Establishing Values Dim intValue as integer intValue = 12 intValue = ”12” intValue = 12.5 intValue = Due to automatic type conversion, all result in the integer variable intValue being set to 12

Operators & Precedence 1.^  Exponentiation 2.-  Unary Negation 3.* /  Multiplication and Division 4.\  Integer Division 5. MOD  Remainder  Addition and Subtraction (otherwise left to right)

Precedence Examples sng A = ( ) / ( ) results: sng A = sng B = ( ) / results: sng B = 0.50 sng C = 3 * 4 / 2 * 3 results: sng C = sng D = ( 3 * 4 ) / ( 2 * 3 ) results: sng D = 2.00

Real & Integer Arithmetic sng A = 5 / 2 results: sngA = 2.50 sng B = 5 \ 2 results: sngB = 2.00 int C = 5 MOD 2 results: int C = 1 int D = 5 / 2 results: intD = 2

Text Strings u Used to hold an arbitrary list of characters u The size can be predetermined, or allowed to change as the program runs u The.caption property of a label is a string, as is the text property of a textbox

String Variables Variable and fixed length strings: Dim strNamer As String Dim strMiddle As String * 1 Dim strState As String * 2 strNamer = ”Jason Ogelthorpe” strMiddle = ”P” strState = ”WI”

String Variables Dim strSocSoc As String * 11 Dim strZipCode As String * 10 strSocSoc = ” ” strZipCode = ”53211” (strZipCode = ”53211 ”) (note | |)

Concatenation of Strings strSamp = ”Hi” & ” There” (strSamp = ”Hi There”) strSamp = ”Value” & 27 (strSamp = ”Value27”) strSamp = ”Value” & str(27) (strSamp = ”Value 27”)

Long Strings strSamp = ”We can build ” _ & ”a very, very, very, ” _ & ”long string.” (a concatenated string can be split over several lines, note underscore characters and note spaces contained within the quoted areas.)

Naming VB Variables u Programmer defined u Don’t use reserved words  Type names, control names, keywords (DIM or end) u Make the name meaningful  intNum vs intNumberOfGroceries u Capitalize first character after prefix  If multiple words, capitalize the 1 st character of each u use standard prefixes

Naming VB Variables u Are these valid names?  int Number  Sub  MinimumRate  Caption  CarCount  intNumberofcars

VB Constants u Types of Constants  Literals  sngTaxRate =.075  txtName = “Smith”  Symbolic  programmer defined with Const keyword const as = Const sngTaxRate as Single =.075  Some are defined by VB vbRed vbCenter u What are benefits of symbolic constants? u Why avoid literals in code?

Working with Variables u Declaring Variables  Dim statement, short for Dimension,  Sets aside memory  Labels the memory  Initializes variable based upon type u Dim intPhone as Integer  Creates variable that can store an integer number u Dim strLastName as String  Creates variable that can store a string (of variable length) u Dim strLastName as String * 5  Creates variable that can store a string of 5 characters.

Working with Variables u Assignment Statement  Each statement in Experiment 3 was an assignment statements u Basic format  =  Use an = (equals sign – read “is assigned”)  VB evaluates the expression to the right of the equal sign  If necessary, that value is converted so that it has the same type as the variable  The value is copied in to the variable – old data is overwritten u For Numeric Data Types  A = B + 8*C (Variable A is assigned the value of the resolved expression) u For String Data Types  strFull = strFirst & strLast (The right side is concatenated and assigned into strFull)

Working with Variables u Control Properties are Variables  lblName.Caption = “My label” u Type Mismatch intNumber = strLastName u Misspellings  Use OPTION EXPLICIT statement to make the computer find errors  If a variable used in an assignment is not in a DIM statement, then VB does not allow the program to compile

Using Functions u Some activities are common in programming tasks  Converting a string to a number  Pop up a warning message for the user  Extracting a substring from a string u Rather than forcing programmers to re-write the code which handles these activities each time they are needed, we can write one reusable function u Functions have names, like variables, and parameter lists

The form of a function call u The general form of a function call is  (,, … ) u Functions take input values, called parameters, and produce an output value which can be assigned to a variable  dblValue = squareRoot(intCount) u Visual Basic includes many functions we can use u We will learn how to create our own functions  Chapter 6

The VAL Function u This function allows conversion between string filled variables and numeric variables. u It works by “scanning” the characters of the string filled variable and builds a numeric value until the end-of-string or a non numeric is found.

A Simple Program Private Sub cmdComnd_Click() Label1.Caption = Val(Text1.Text) End Sub We can enter character strings into the box “Text1”, press the command button, and look at what appears in the box “Label1”.

Some Examples

The Message Box u VB allows the user to generate message boxes as a part of a user program. When invoked they look something like this:

Format of MsgBox u In its simplest form the MsgBox is called by this command: Dim intX As Integer intX = MsgBox("Read the Password")

The Details Dim intX As Integer intX = MsgBox("Do this now", _ vbCritical, "Prompting Issue")

Response Choices The value of “intZ” above depends on which response is chosen. “Yes” - intZ = 6 “No” -intZ = 7 (the numerical values are defined by Visual Basic.) Dim intZ As Integer intZ = MsgBox(“Should I Do This”, _ vbYesNo,”Prompting Issue”)

Icon Choices