Introduction to Programming Structure Lesson 4 McManusCOP10061

Overview Pointers Modules & Functions Cohesion & Coupling Local & Global Variables Parameters Variable Names & Data Dictionaries Three Logic Structures McManusCOP10062

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 McManusCOP10063

Modules, Procedures & Functions The subparts to a Program McManusCOP10064

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 McManusCOP10065

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). McManusCOP10066

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 McManusCOP10067

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; } McManusCOP10068

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 McManusCOP10069

An Example of Control McManusCOP100610

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 McManusCOP100611

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 McManusCOP100612

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 McManusCOP100613

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 McManusCOP  Read and Data Validation  Reads and validates input data Usually separate modules Private Sub Verify(X) If X 10 Then lblMessage.Text = “Data Error” End If End Sub

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 McManusCOP100615

Global or Local Variables Scope! McManusCOP100616

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. McManusCOP100617

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!A nasty effect that should be avoided! McManusCOP100618

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

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 McManusCOP X, Y, Z A m C p B n 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

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 McManusCOP Scully & Mulder are what type of variables? What type of variable is Temp?

Another Example McManusCOP Scope of X, Y, Z, Procedure1 Scope of M, N, Me, X, You 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}

Cohesion & Coupling Making Modules Better! McManusCOP100623

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. McManusCOP100624

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. McManusCOP Term was coined by Larry Constantine in mid-1960’s

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. McManusCOP Type Measure Black Box Functional BestBlack Box Informational ** Best Sequential Communicational ProceduralGray Box Temporal Logical Coincidental WorstTransparent or White Box **Originally not part of Scale

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

Effects? Which pieces affect Cohesion and which affect Coupling? McManusCOP 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 is the code doing? What are the parameters? McManusCOP Private Sub Skinner(Scully As Long, Mulder As Long) Dim Temp As Long If Scully < Mulder Then Temp = Scully Scully = Mulder Mulder = Temp End If End Sub

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 McManusCOP100630

Goal of Cohesion & Coupling? High Cohesion –Functional or Information Low Coupling –Data, Stamp, Control McManusCOP100631

Parameters How we avoid side effects! McManusCOP100632

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 McManusCOP100633

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. McManusCOP100634

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. McManusCOP100635

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 McManusCOP100636

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 McManusCOP100637

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 McManusCOP100638

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 McManusCOP100639

Data Areas After a Call McManusCOP ?? Formal Parameters Local Variables Sum Average Num1 Num2 Actual Parameters Var1 Var2 Main program data area Procedure data area

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. McManusCOP100641

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 McManusCOP100642

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! McManusCOP100643

Parameter Relationships Call Parameters Actual Parameters Valued Variable McManusCOP Procedure Header Parameters Formal Parameters Valued Variable Parameter Interface using Global Variables

Another Look at Variables Names & the Data Dictionary McManusCOP100645

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 McManusCOP100646

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) McManusCOP100647

DD Example ItemName (no spaces) Data Type ModulesDomain (Range) Scope Hours worked HoursNumeric- real GetHours CalcGrossPay 0  n  168 Global Gross Pay Numeric- real CalcGrossPay CalcDeductions CalcNetPay PrintPayChecks 0  n  1 million Global Net Pay Numeric- real CalcNetPay PrintPayChecks 0  n  1 million Local Global McManusCOP100648

The Three Logic Structures SequentialSequential –One statement follows another SelectionSelection (Decision) –Allows choices based on the data –IfThenElse, Nested If’s, Case, Switch IterationIteration (Looping or Repetition) –Allows statements to be repeated a specified number of times –While, Do, For, Do Until, Repeat McManusCOP100649

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