Variables / Scope

1. Variables / Scope

2. Variable • Memory location whose value can change as the program is running. • Used to hold temporary information • Used to control the type of data used in calculations • Val returns a Double-type, which is often larger than necessary • Can store only one piece of data at any time • Data is processed faster

3. Byte Boolean Currency Date Double Integer Long Object Single String Variant Data Types

4. Use the Appropriate Data Type • Integer or Long - Used to store whole numbers • Single, Double, Currency - Used to store numbers with a decimal fraction • String - Used to store strings • Boolean - Used to store Boolean values (True and False) • Date - Used to store date and time information • Object - Used to store a reference to an object • Byte - Used to store binary data • Variant - Flexible, but not efficient

5. Variable Names • Should be meaningful • First three characters should represent the data type • Remainder of name should represent the variable’s purpose

6. Byte byt Boolean bln Currency cur Date/Time dtm Double dbl Integer int Long lng Object obj Single sng String str Variant vnt Three-character Ids

7. Rules for Naming Variables • Name must begin with a letter. • Name can contain only letters, numbers, and the underscore. No punctuation characters or spaces are allowed. • Name cannot exceeds 255 characters. • Name cannot be a reserved word.

8. Creating (declaring) a Variable • Dim variablename [As datatype] • Public variablename [As datatype]

9. Assigning Values to Variables • Assignment statement • variablename = value • Examples: • sngHours = 38.5 • curBonus = curSales * .1 • strName = “Susan”

10. Literal constant an item of data whose value cannot change while the program is running Examples: 7 “Janet” Symbolic constant a memory location whose contents cannot be changed while the program is running Examples: conPi conRate Constants

11. Scope of a Variable • Indicates which procedures can use the variable • Determined by where the Dim or Public statement is entered. • Can be either global, form-level, or local.

12. Variable Scope Global Variable: Defined using a Public declaration in the general declarations section of a module Module/FormVariable: Defined using a Private or dim declaration in the general declarations section of a module or a form Local Variable:defined using dim or static within a subprocedure or function In general, when variables have the same name but different scope, the most local (closest) variable is used.

13. Local Variables • Created with the Dim statement. • The Dim statement is entered in an object’s event procedure (within a subprocedure or function) • Only the procedure in which it is declared can use the variable. • Same name can be used in different procedures • Removed from memory when the procedure ends.

14. Form-level Variables • Created with the Dim statement. • The Dim statement is entered in a form’s General declarations section. • Can be used by any of the procedures in the form. • initialized when module/form is first opened Removed from memory when the application ends.

15. Global Variables • Created with the Public statement. • The Public statement is entered in a code module’s General declarations section. • initialized when program starts • Used in multi-form projects and can be used by any of the procedures in any of the project’s forms. • Removed from memory when the application ends.

16. Static Variables • Static variables • defined using static instead of dim within a subprocedure or function • acts like a local but is initialized only on first use

17. Static Variable example • Goal: count the number of times a routine is entered private sub command1_click() dim counter as integer counter=counter+1 debug.print counter end sub • Does NOT work unless counter is declared using static instead of dim

18. Local vs Static Variables • A local variable is defined in an event procedure, and it is removed from memory when that event procedure ends. • A static variable also is defined in an event procedure, but it retains its value when that event procedure ends. • A static variable is a special type of local variable.

19. Swapping • To swap the contents of two variables: • assign the first variable’s value to a temporary variable • assign the second variable’s value to the first variable • assign the temporary variable’s value to the second variable

20. Option Explicit Statement • Doesn’t allow you to create variables “on the fly.” • Enter in every form’s, and every code module’s, General declarations section. • Use Tools, Options, Environment tab, Require Variable Declaration to have Visual Basic include Option Explicit in every new form and module.

21. Creating a Symbolic Constant • A memory location whose value cannot change during run time. • Syntax: [Public] Const constname [As datatype] = expression • Examples: • Const conPi As Single = 3.141593 • Public Const conMaxAge as Integer = 65

22. Scope of a Symbolic Constant • Indicates which procedures can use the symbolic constant. • Global: Public Const statement in a code module’s General declarations section. • Form-level: Const statement in the form’s General declarations section. • Local: Const statement in an event procedure.

23. Numeric Types • Integer - 2 bytes of memour • a 16-bit signed integer • whole number between -32768 and 32767 • long - 4 bytes of memory • a 32-bit signed integer (+/- 2^31) • whole number between 2,147,483,648 and 2,147,483,648 • single - 4 bytes of memory • a single-precision decimal floating point number • +/- 1.401298e-45 to 3.402823e38 • double - - 8 bytes of memory • a double-precision decimal floating point number • +/- 4.94065645841247e-324 to 1.79769313486232e208

24. Arithmetic Operations (1) • n=5 integer (long or short) or floating point • y=a+b y=a-b, y=a*b (add/subtract/multiply) • z=x / y divides two numbers and returns a floating point • z=x mod y divides two numbers and returns only the remainder. • z=x \ y divides two numbers and returns an integer result

25. Arithmetic Operations (2) • isnumeric(thing) returns TRUE if thing is numeric • randomize(number) initializes the random number generator • n=rnd(x) returns random number 0 <= x < 1 • s=str(number) converts a number to a string • n=val(string) converts a string to a number • n=rgb(red,green,blue) returns RGB color value • n=sgn(x) returns 1,0,-1 if x is pos., zero, neg.

26. Boolean Operations (1) • Consume 2 bytes of memory • result = exp1 And exp2 • true only if both exp1 AND exp2 are true • result = expression1 or expression2 • true if either or both exp1 OR exp2 are true • result = NOT exp1 • true if exp1 is false, false if expression is true

27. Comparisons • x<y, x<=y, x=y, x<>y, x>y, x>=y where x, y can be strings or numbers • y = a is b returns true if a & b are the same object • y = string like pattern • y=StrComp(string1, string2[, compare]) compares two strings based on the compare type • typeofobject returns an object's type • Can only be used as part of an if statement if typeOf thing is label then thing.backcolor=vbGreen

28. String Operations

29. String Manipulation Functions • Left(string, length) • returns the leftmost length characters in the string • Right(string, length) • returns the rightmost length characters in the string • Mid(string, start[, length]) • returns length characters from string, beginning with start

30. Instr Function • Instr(start, string1, string2[,compare] • start is a numeric expression that sets the starting position for the search • string1 is the string expression being searched • string2 is the string expression being sought • compare is either 0 (default; case-sensitive) or 1 (case-insensitive) • returns the starting position of string2 within string1

31. Trim, LTrim, RTrim • Trim(string) - removes leading and trailing spaces from a string • LTrim(string) - removes leading spaces from a string • RTrim(string) - removes trailing spaces from a string

32. String Concatenation • Ampersand - & • Examples:(Assume strFirstName contains “Mary” and sngSales contains 1000) • “Hello “ & strFirstName • strFirstName & “ sold $“ & sngSales & “.” • Results: • Hello Mary • Mary sold $1000.

33. String Operations (1) • Strings consume 0-2 Billion bytes of memory, but on;y as much as needed - very efficient • s=“Strings are always defined in quotes” • & - String coercion and concatenation • s=str(number) converts the number to a string • n=val(string) tries to convert the string to a number • s=ucase(string) converts string to upper case • s=lcase(string) converts string to lower case • string1=lset(string2) left justifies string2 in string1with trailing spaces (there is also an rset)

34. String Operations (2) • n=asc(string) converts first character to a 0-255 ASCII character code • s=chr(character-code) returns a string containing the respective character • s=hex(num) returns the hex value of num as a string (oct returns octal value) • s=inputbox(prompt, title) Displays a prompt in a dialog box, waits for the user input and returns a String containing the contents of the text box

35. String Operations (3) • n=len(string) returns integer length of string • s=ltrim(string) removes leading spaces • s=rtrim(string) removes trailing spaces • s=trim(string) removes surrounding spaces • s=space(n) returns a string of n spaces • s=left (string,n) returns the left n characters of string • s=right(string,n) returns the right n characters of string

36. String Operations (4) • s=mid(string,start,n) Returns n characters beginning at character #start • Mid(stringvar, start[, n]) = str2 replaces n characters in stringvar with characters in str2 • n=InStr(string1, string2) searches for string2 inside string1 and returns the integer position of the start • s=string(n,char) returns an n-character string filled with char

37. Date / Time / Misc

38. Date/Time related Statements • Date Data Types consume 8 bytes of memory • Date comparison - if somedate < #12/31/1993# then …. • d = Now - returns system date and time as date type • d = date - returns system date as date type • d$ = date$ - returns system date as string • d = time - returnssystem time as a date type • time = valid-time - sets the system time !!!!!!!!!!!!! • d$ = time$ - returns system time as a string • i = timer - integer number of seconds since midnight • n=weekday|month|day|year (date) Returns whole number representing the weekday|month|day|year

39. DateAdd() – Add value to a date • DateDiff() – Returns difference between 2 dates • DatePart() – Return parts from a date • Dim FirstDate As Date • Dim K As String • Dim N As Integer • Dim Msg • k= "m" ‘we will be adding months to our date • FirstDate = InputBox("Enter a date") ‘like 9/1/2001 • N = InputBox("Enter number of months to add") • Msg = "New date: " & DateAdd(k, N, FirstDate) • MsgBox Msg • The above program will return 12/1/2002 when we add 15 (months) to 9/1/2001. If k =“d”, and it will add days.

40. Misc • beep - beeps the computer’s speaker (sort of) • n=vartype(variable) returns an integer corresponding to variable type • n=rgb(red,green,blue) sets n to a long integer representing a color made up of red, green, blue (0-255). 0,0,0 = black 255,255,255 = white • n=qbcolor(x) where x is one of the 16 Quick Basic colors 0-15