Mastering Programming Structure: Pointers, Modules & Functions

1. Introduction to Programming Structure Lesson 4 COP1006

2. Overview • Pointers • Modules & Functions • Cohesion & Coupling • Local & Global Variables • Parameters • Variable Names & Data Dictionaries • Three Logic Structures COP1006

3. Pointers on Program Structure • Use Modules • Each part should have a particular function • Use the three logic structures • Sequential, Decision & Iteration • Don’t reinvent the wheel • Avoid rewriting identical processes • Use techniques to improve readability COP1006

4. Cohesion & Coupling Making Modules Better! COP1006

5. Cohesion & Coupling • Each module should • Be functionally independent • Perform one problem-related task • Calculating IRS Withholding is one problem-related task, but may have multiple operations within the module • When connected, use the smallest interface possible. COP1006

6. Cohesion • The degree of interaction withina module. • Each module should perform one functionally-related task…not necessarily one assignment statement. • Concentration is on what goes on within the module. Term was coined by Larry Constantine in mid-1960’s COP1006

7. Type MeasureBlack Box Functional Best Black Box Informational ** Best Sequential Communicational Procedural Gray Box Temporal Logical Coincidental Worst Transparent or White Box **Originally not part of Scale Scale of Cohesion • Stevens, Myers, Constantine, and Yourdon developed the Scale of Cohesion as a measure of the “black boxiness” of a module, and as a result, the maintainability of a module. COP1006

8. Best (Lowest Interaction) Normal Data Stamp Control Common Content Worst (Highest Interaction) Coupling • The degree of interaction between two modules. • Interaction is the interface, or lack thereof, between two modules. • The interface is the parameter list. COP1006

9. Effects? • Which pieces affect Cohesion and which affect Coupling? Private Sub Minimum(min As Long, y As Long, z As Long) If y < min Then min = y End If If z < min Then min = z End If lblSmallest.Caption = "Smallest value is " & min End Sub COP1006

10. What is the code doing? • What are the parameters? Private Sub Minimum(Scully As Long, Mulder As Long) Dim Temp As Long If Scully < Mulder Then temp = Scully Scully = Mulder Mulder = temp End If End Sub COP1006

11. Each Goal • Cohesion’s Goal • To create a procedure that performs one functionally-related task. • Coupling’s Goal • To protect global data and local data from being used within a procedure without declaring it on the procedure’s header COP1006

12. Goal of Cohesion & Coupling? • High Cohesion • Functional or Information • Low Coupling • Data, Stamp, Control COP1006

13. Modules, Procedures & Functions The subparts to a Program COP1006

14. Modules • “A module is a lexically contiguous sequence of program statements, bounded by boundary elements, having an aggregate identifier.” Yourdon & Constantine(1979) • A part of a larger system • Written and tested separately • Combined with other modules to form a complete system • Used in top-down programming • Procedures & Functions COP1006

15. Procedures • A smaller part of the main program. • 2 Advantages • Eliminates the need to program the same thing more than once. • Larger programs are easier to read when broken into procedures (and functions). COP1006

16. Functions • Functions • A subprogram that acts like a mathematical function: • given a particular set of argument values, the function returns a unique result. • Use Return values that are associated with the name of the function COP1006

17. Function Examples • Pascal FUNCTION doublenum(b : Integer) : Integer; BEGIN doublenum := 2 * b END; • Visual Basic Private Function Doublenum(b As Integer) As Integer Doublenum = 2 * b End Function • C++ Int doublenum ( int b) { return 2 * b; } COP1006

18. What do they have in common? • Each module • is an entity by itself • has a single purpose • should be easily read, modified and maintained • Length is governed by function and number of instructions contained within • Controls the order of processing COP1006

19. An Example of Control COP1006

20. Types of Modules/Procedures • Control • Demonstrates overall flow of data • Initialization & Wrap-Up • Processes instructions to be performed once (either at beginning or at the end of the program) • Used typically in batch processing COP1006 • Process Data • Calculation • Print • Read and Validation • Event • Used in OO and some event driven programming languages • More about these later

21. Control Modules • Most often called “Main” • All other modules, procedures and functions are subordinate to the control module Sub Main Call ProcedureA(X, Y) Call ProcedureB(A, B) End Main COP1006

22. Init Module • Used in some languages to initialize variables or processes • Examples • Opening files • Initializing variables • Printing report headings Procedure Begin Dim X, Y, Z As Integers Open Payroll file End Procedure ‘Begin COP1006

23. Process Data Modules • Read and Data Validation • Reads and validates input data • Usually separate modules • Calculation • Performs • Arithmetic operations • Accumulations • Sorting or Searching Private Sub Double (X, Y) Dim Temp as Integer Temp = X * Y End Sub • Private Sub Verify(X) • If X < 0 Or X > 10 Then • lblMessage.Text = “Data Error” • End If • End Sub COP1006

24. Wrap Up Module • Used to close out processes • Examples • Closing files • Printing reports • Returning updated data to databases Procedure End Close Employee file Close Payroll file End Procedure ‘End COP1006

25. Global or Local Variables Scope! COP1006

26. Scope • The area of a program where an identifier (variable) is visible • When an identifier has multiple declarations in different modules, the most local declaration is used each time that identifier is referenced. (overloading) • Global or “non-local” variables subject to side effects. COP1006

27. Side Effects • Caused when the value of a global variable is changed within a procedure or function • Any effect of one module on another module that is not a part of the explicitly defined interface between them • Also caused when a variable name is used in globally and locally (causes overloading) • A nasty effect that should be avoided! COP1006

28. X, Y, Z A C B Global Scope • Declared within the main program • Can be referenced anywhere in the program • Is visible and accessible everywhere X, Y & Z are Global to modules A, B & C COP1006

29. X, Y, Z A m C p B n Local Scope • Declared within a module • Has no effect outside the procedure or function in which it is declared • Can be referenced only within a procedure or a function Within A, Variable m is defined, but can also see X, Y, & Z Within B, Variable n is defined , but can also see X, Y, & Z Within C, Variable p is defined , but can also see X, Y, & Z COP1006

30. Global or Local? Private Sub Minimum(Scully As Long, Mulder As Long) Dim Temp As Long If Scully < Mulder Then temp = Scully Scully = Mulder Mulder = temp End If End Sub Scully & Mulder are what type of variables? What type of variable is Temp? VB Example COP1006

31. Another Example program ShowScope; var X, Y, Z : Real; procedure Procedure1 (var M, N, Me : Real); var X, You : Real; begin {Procedure1} ....... end; {Procedure 1} begin {ShowScope} Procedure1(X, Y, Z) end. {ShowScope} Scope of X, Y, Z, Procedure1 Scope of M, N, Me, X, You Pascal Example COP1006

32. Parameters How we avoid side effects! COP1006

33. Parameters • Are the variables that are passed into and out of modules • Use global parameters • (to the procedure or function) • Pass values through the use of variables • Actual and Formal parameters • Call-by-reference & Call-by-value COP1006

34. Parameter Communication • A measure of the quantity of data passing through a module’s interface. • Is also a measure of the module’s coupling. • The goal is to strive for a minimal amount of information being passed. COP1006

35. How Parameters are Used • Input Parameter • Information passed into a procedure, but not returned or passed out of the procedure. • Output Parameter • Information returned to the calling program from a procedure. • Input/Output Parameter • Information passed into a procedure, perhaps modified, and a new value returned. COP1006

36. Parameters • Provide the communication links between the main program and its modules. • Make procedures and functions more versatile. • Different data can be manipulated each time the module is called. • Come in two types: • Actual • Formal COP1006

37. Actual & Formal Parameters • Actual Parameters • Are substituted for the formal parameter at the time the procedure is called. • Parameters used in the call statement • Statements that transfer control to a procedure. • Data types must be assignment compatible with its corresponding formal parameter • Can be a variable, constant or an expression • Can be call-by-value or call-by-reference COP1006

38. Actual & Formal Parameters • Formal Parameters • Parameters declared in the procedure header • Is a list of “place marker” names used in the procedure’s declaration. • Can include the data type of the valued parameters. • Must be a variable • Can be call-by-value or call-by-reference COP1006

39. Parameter Correspondence Rules • Determined by position in respective parameter lists • Lists must be the same size, although the names may differ • Data Types of corresponding actual and formal parameters must be identical COP1006

40. Formal Parameters Actual Parameters Var1 Var2 Num1 Num2 8.0 8.0 10.0 10.0 Local Variables Sum Average ? ? Data Areas After a Call Main program data area Procedure data area COP1006

41. Valued & Variable Parameters • By Reference (Call-by-Reference) • Passing a variable to a procedure is called passing an argument by reference, because a variable can be modified by a procedure and returned to the calling module. • By Value (Call-by-Value) • Passing a literal value (such as a string in quotation marks) to a procedure is called passing an argument by value, because a value cannot be modified by a procedure. COP1006

42. Call-by-Reference • The default for parameter passing • Gives access to the contents of the storage area where values are stored • Giving the called procedure the ability to directly access the caller’s data • Allowing changes in the data COP1006

43. Call-by-Value • Protects the data being passed • Accomplished by creating a copy of the value • without affecting the original value of the variable • Thus… • Called procedure is unable to change the values stored in the variable’s storage area • Helps avoid Side Effects! COP1006

44. Parameter Relationships Parameter Interface using Global Variables Call Parameters Actual Parameters • Valued • Variable COP1006 Procedure Header Parameters Formal Parameters Valued Variable

45. Another Look at Variables Names & the Data Dictionary COP1006

46. Variable Names • Use mnemonic terms • Use a variable name that relates the name of the variable to its usage • Contributes to self-documenting code • Which reduces the amount of commenting required • Z = X * Y What is it doing (besides multiplication?) • SalesTax = SalesTaxRate * Cost (this you know) • Examples • SalesTax, SalesRate, PayRate, Temp COP1006

47. The Data Dictionary • Defines all of the variables used within a program • Lists: • Names • Data type • Location defined & accessed • Test Data (or error checking) • Domain (range of possible values) COP1006

48. DD Example COP1006

49. The Three Logic Structures • Sequential • One statement follows another • Selection (Decision) • Allows choices based on the data • IfThenElse, Nested If’s, Case, Switch • Iteration (Looping or Repetition) • Allows statements to be repeated a specified number of times • While, Do, For, Do Until, Repeat COP1006

50. Next? Problem Solving with Sequential Logic COP1006