Introduction to Programming Structure

1. Introduction to Programming Structure Lesson 4 COP1000

2. Overview • Guidelines • Modules & Functions • Cohesion & Coupling • Local & Global Variables • Parameters • Variable Names & Data Dictionaries • Three Logic Structures COP1000

3. Guidelines 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 COP1000

4. Cohesion & Coupling Making Modules Better! COP1000

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. COP1000

6. Cohesion • The degree of interaction within a 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 COP1000

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. COP1000

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. • Create it, use it, lose it COP1000

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 What does this code do? COP1000

10. What is the code doing? • What are the parameters? Variables? 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 Global &Parameters Local Variable COP1000

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 • Create as close to where you’re going to use it as possible, use it, then lose it as soon as possible COP1000

12. Goal of Cohesion & Coupling? • High Cohesion • Functional or Information • Low Coupling • Data, Stamp, Control COP1000

13. Modules, Procedures & Functions The subparts to a Program COP1000

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 COP1000

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). COP1000

16. Functions • Functions • A subprogram that acts like a mathematical function: • given a particular set of argument values, the function returns a unique result. • Big Difference Between Procedures & Functions: • Use Return values that are associated with the name of the function COP1000

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++ Intdoublenum ( int b) { return 2 * b; } All three do the same thing, just with different syntax COP1000

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 COP1000

19. An Example of Control VB Example COP1000

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 • Process Data • Calculation • Print • Read and Validation • Event • Used in OO and some event driven programming languages • More about these later COP1000

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 COP1000

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 Does this violate Cohesion or Coupling? COP1000

23. Process Data Modules • Calculation • Performs • Arithmetic operations • Accumulations • Sorting or Searching Private Sub Double (X, Y) Dim Temp as Integer Temp = X * Y End Sub • Read and Data Validation • Reads and validates input data • Usually separate modules • Private Sub Verify(X) • If X < 0 Or X > 10 Then • lblMessage.Text = “Data Error” • End If • End Sub COP1000

24. Wrap Up Module • Used to close out processes • Examples • Closing files • Printing reports • Returning updated data to database Procedure LastProcedure Close Employee file Close Payroll file End Procedure ‘LastProcedure Does this violate Cohesion or Coupling? COP1000

25. Global or Local Variables Scope! COP1000

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. COP1000

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! COP1000

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 COP1000

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 COP1000

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 & Mulderare what type of variables? What type of variable is Temp? VB Example COP1000

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 COP1000

32. Parameters How we avoid side effects! COP1000

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 COP1000

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. COP1000

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. Programmers now determine how their parameters are used. COP1000

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 COP1000

37. 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 • If they don’t, your program will blow up! • Can be a variable, constant or an expression • Can be call-by-value or call-by-reference COP1000

38. Formal Parameters • Is a list of “place marker” names used in the procedure’s declaration. • Parameters declared in the procedure header • Can include the data type of the valued parameters. • Must be a variable • Can be call-by-value or call-by-reference COP1000

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 COP1000

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 COP1000

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. • You can use it AND you CAN change it! • 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. • You can use it BUT you CAN’T change it! COP1000

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 COP1000

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! COP1000

44. Parameter Relationships Call Statements Actual Parameters • Valued • Variable Parameter Interface using Global Variables Procedure Header Parameters Formal Parameters Valued Variable COP1000

45. Another Look at Variables Names & the Data Dictionary COP1000

46. Variable Names-Reminder • 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, Sales_Rate, PayRate, Temp COP1000

47. The Data Dictionary • Defines all of the variables used within a program • Lists: • Item Name • Variable Names • Domain(range of possible values) • Data type • Location defined & accessed • Test Data (or error checking) Required! COP1000

48. DD Example COP1000

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 COP1000

50. Next? Problem Solving with Sequential Logic COP1000