Programming Concepts and Logic for Beginners

1. Extreme Xbasic Dr. Peter Wayne

2. variables assignment flow control Elements of a programming language

3. character date logical numeric Variables - Simple Types

4. Explicit: dim myname as c myname="peter" Variable Declaration and Assignment Implicit: hisname="selwyn"

5. dim fname as c dim lname as c dim myname as c fname="peter" lname="wayne" myname=fname + " " + lname Character Variables ? myname = "peter wayne"

6. dim today as d today=date() ? today = {02/15/2002} Date Variables

7. dim istrue as l istrue="alpha">"microsoft" ? istrue = .F. Logical Variables

8. dim x as n x=12 y=2*x ' * is the multiplication operator ? y = 24.000000 Variables - numeric

9. x=12 x=x*2 Assignment is Not Algebra

10. “any” or variant pointer blob time function collection Variables - Complex

11. “Any” variable type dim anyvar as A anyvar=12 ? anyvar = 12.000000 anyvar=date() ? anyvar = {02/15/2002} anyvar="today is "+anyvar ? anyvar = "today is 02/15/2002"

12. tables, indexes, queries fields layout objects (forms, buttons, etc.) chunks of memory Pointer variables

13. Pointer Example: Table Pointer

14. Properties and Methods of Table Pointers

15. Pointer to Form

19. Here all properties are restricted except for changing the record:

20. Navigation and querying is prevented by the property settings.

21. if (something) then…(else)...end if for…next while…end while select…case…end while end on error goto…resume *for_each() (advanced) Flow Control

22. x=1 for i=1 to 5 x=x*2 if x>10 then exit for end if next i ui_msg_box("x is",str(x)) if…end if

23. x=1 for i=1 to 5 x=x*2 next ? x = 32.000000 For…Next

24. User Defined Functions FUNCTION reverse AS C (input AS C ) dim i as n dim length as n length=len(input) output="" for i=length to 1 step -1 output=output + substr(input,i,1) next i reverse=output end function

26. Creating a New Script

27. Structure of MyCheckbook.dbf

28. Creating a script

29. Calculate current checkbook balance Can you follow this script? t=table.open("mycheckbook") balance=0 t.index_primary_put("date") t.fetch_first() while .not. t.fetch_eof() amt=if(t.type$"DIE",t.amount,-t.amount) balance=balance+amt t.fetch_next() end while t.close() ui_msg_box("balance is",str(balance,10,2))

30. Applying a Query to a Table t=table.open("mycheckbook") credits=0 query.filter="type$'DIE'" query.order="" query.options="" qry=t.query_create() t.fetch_first() while .not. t.fetch_eof() credits=credits+t.amount t.fetch_next() end while qry.drop() t.close() ui_msg_box("total credits is",str(credits,10,2))

31. Events Form, Browse, and Object events Record and Field Events

32. Form, Browse, Object Events Simplest Example: OnPush event for button The OnPush event for a button is triggered when the button is pressed. You can attach Xbasic code to any event for any object. Of course, the code may not make sense if you attach it to the wrong event!

33. Sample OnPush event script This next form starts with multiple restrictions. The "Allow Edits" button removes the restrictions. An if..then..else..end if construct is used to toggle the form in and out of editing mode.

35. Right-click on the object, then choose Events to see a list of the events defined for the type of object. Different objects have different events. Buttons and hotspots have OnPush events:

36. OnPush event script for "Allow edits" button. "this" refers to the object containing the script, and "topparent" refers to the form containing the button. if this.text="Allow edits" then topparent.Restrict_change = .f. topparent.Restrict_delete = .f. topparent.Restrict_enter = .f. topparent:Browse1.Browse.Readonly = .f. this.text="Prevent edits" this.fill.forecolor="Red" else topparent.Restrict_change = .t. topparent.Restrict_delete = .t. topparent.Restrict_enter = .t. topparent:Browse1.Browse.Readonly = .t. this.text="Allow edits" this.fill.forecolor="Bright Green" end if

37. Events With few exceptions, there are 2 basic event types: CanXXX events, whose scripts can be used to deny permission for an event, using cancel() OnXXX events, whose scripts fire after the named event occurs Examples: CanSave and OnSave form level events

38. Example of CanXXX event: CanExit

40. Pressing the Close button produces this:

41. By contrast, OnExit event:

42. Sometimes events behave in ways that are not immediately obvious. Button OnPush code: parent.close() This bypasses the CanExit event, but the OnExit event still fires.

43. Xbasic can work on tables or objects, with similar results. Example: Program a "mark" button for a set-based form that marks either the active child record or the parent and all its children, depending on what has focus before the delete button is pressed. We can use the form's .active_prev() method to determine what had focus before "mark" is pressed.

44. First, verify that the user wants to perform the marking operation. name_of_active=parent.active_prev() if name_of_active="Browse1" then do_it=ui_msg_box("Confirm","Mark single detail item?",\ UI_YES_NO+UI_SECOND_BUTTON_DEFAULT) else do_it=ui_msg_box("Confirm","Mark entire transaction and all detail items?",\ UI_YES_NO+UI_SECOND_BUTTON_DEFAULT) end if if do_it=UI_NO_SELECTED then end end if

45. And then perform the mark, in this case with form-level methods: if name_of_active="Browse1" then browse1.mark_record() else lastrec=0 browse1.fetch_first() rec=table.current(2).recno() ' record number of the first browse record while rec<>lastrec lastrec=rec browse1.mark_record() browse1.fetch_next() rec=table.current(2).recno() end while parent.mark_record() end if

46. t2=table.current(2) t1=table.current(1) if name_of_active="Browse1" then t2.change_begin() t2.mark() t2.change_end(.t.) else t2.fetch_first() while .not. t2.fetch_eof() t2.change_begin() t2.mark() t2.change_end(.t.) t2.fetch_next() end while t1.change_begin() t1.mark() t1.change_end(.t.) end if Alternatively, one can mark the record at the table level using Table-Level Methods

47. Form-level vs Table-Level Form-level methods: Imitate a live user Field rules are respected Table-level methods Field rules are overridden

48. Example of field rule CanChangeRecord event for table:

49. CanChangeRecord event for mycheckdetail.dbf t=table.get("mycheckbook") status=t.status if status="R" then cancel() ui_msg_box("Error",\ "Changes not allowed to reconciled transactions!") else dim shared old_date as d old_date=t.date end if

50. Here is a reconciled transaction (status="R"):