THEORY OF STRUCTURED PROGRAM DESIGN

. WANT TO SHOW HOW PROGRAMS ARE, IN FACT, DESIGNED.

. "PROGRAM DESIGN" ===> THE DEVELOPMENT OF A PROGRAM SO THAT ITS ELEMENTS FIT TOGETHER LOGICALLY AND IN AN INTEGRATED WAY.

. PROGRAMS PLANNED BEFORE CODING WILL BE BETTER DESIGNED!

... AND MINIMIZE ERRORS.

. PLANNING TOOLS:.. FLOWCHARTS.. PSEUDOCODE - STRUCTURED

PSEUDOCODESTANDARD PSEUDOCODE

.. HIERARCHY CHARTS - STRUCTURE CHARTS

A MODULE IS A SET OF INSTRUCTIONS THAT PERFORMS A SINGLE FUNCTION.

WE PERFORM MODULES - CONTROL IS TRANSFERRED MODULE IS EXECUTED CONTROL RETURNED TO NEXT SEQUENTIAL STATEMENT

1

THERE ARE OTHER "TRANSFERS OF CONTROL" STATEMENTS, SUCH AS THE GOTO, CALL.

. WILL MINIMIZE THE USE OF THE GOTO:

. BE AWARE HOW IT WORKS

. YOU WILL RUN INTO IT AND THE PROBLEMS IT CAUSES...

MODULES SHOULD BE CODED IN A HIERARCHICAL MANNER - . MAIN MODULES WRITTEN FIRST. SECONDARY MODULES PERFORMING SPECIFIC DETAILED

FUNCTIONS.

. CODING MODULES IN A HIERARCHICAL MANNER IS CALLED TOP DOWN PROGRAMMING

. THE TOP DOWN APPROACH IS SOMETIMES CALLED STEPWISE REFINEMENT OR FUNCTIONAL DECOMPOSITION.

2

THE NOTION OF MODULARITY AND ABSTRACTION

IF AM1-1 = AMT-2.... <---------DETAILS

....ELSE

.... <---------DETAILS

....END-IF

MODULAR APPROACH:IF AMT-1 = AMT-2

PERFORM 1000-EQUAL <== AN ‘ABSTRACTION’ELSE

PERFORM 2000-UNEQUAL <== AN ‘ABSTRACTION’END-IF

WE ARE ‘ABSTRACTING’ OUT THE DETAILS.MAIN FOCUS HERE IS TO NOTE THE CONTROL!!LEAVE THE DETAILS (AS PERFORMED IN CITED PARAGRAPHS)

UNTIL ‘THOSE’ PARAGRAPHS ARE DEVELOPED.

MAIN MODULE FOCUS ON PRIMARY ISSUE:ARE THE TWO FIELDS ARE EQUAL.

HERE, MAIN MODULE FOCUS ON CONTROL

THE DETAILS OF WHAT TO DO IF UNEQUAL OR UNEQUAL ARE ABSTRACT AND NOT INCLUDED HERE, WHERE THEY MIGHT OBSCURE THE REAL FUNCTIONS OF THE IF AND CONTROL.

PRINCIPLE OF ABSTRACTION IS VERY IMPORTANT IN ANY PROGRAMMING LANGUAGE.

BUT KNOWING THE LANGUAGE IS NECESSARY, BUT NOT ENOUGH.!SYNTAX IS NOT ENOUGH.

PROGRAM DESIGN TECHNIQUES TRANSCEND LANGUAGE CONSTRUCTS

LOGICAL CONTROL STRUCTURES:

THE SEQUENCEONE STATEMENT AFTER ANOTHER

THE SELECTIONEXECUTE THIS STATEMENT OR THAT STATEMENTDEPENDING ON THE EVALUATIN OF A ‘CONDITION.’

THE ITERATIONREPEATEDLY EXECUTE THIS STATEMENT (UNTIL SOME ‘CONDITION’ BECOMES TRUE OR FALSE)

(OTHERS: THE CASE....)

WELL-DESIGNED PROGRAMS CAN BE DESIGNED AND IMPLEMENTED IN CODE USING ONLY THESE THREE LOGICAL CONTROL STRUCTURES.

THESE THREE CONSTRUCTS: LANGUAGE INDEPENDENT

SO, HOW DO WE MODEL OUR ALGORITHMS USING LANGUAGE INDEPENDENT MEANS?

DESIGN TOOLS:

1. PROGRAM FLOWCHARTS

2. PSEUDOCODE

3. STRUCTURE CHARTS (HIERARCHY CHARTS) (AS STATED BEFORE)

BUT NOW, IN MORE DETAIL....

A PROGRAM FLOWCHART IS A DIAGRAM OR PICTORIAL REPRESENTATION OF THE INSTRUCTIONS AND LOGICAL CONTROL STRUCTURES THAT WILL BE USED IN THE PROGRAM.

GRAPHICS-BASED TOOL FOR ILLUSTRATING YOUR ALGORITHM

PSEUDOCODE IS A SET OF STATEMENTS THAT SPECIFIES THE INSTRUCTIONS AND LOGICAL CONTROL STRUCTURES THAT WILL BE USED IN A PROGRAM.

PSEUDOCODE IS A TEXT-BASED TOOL USED TO ILLUSTRATE THE ALGORITHM.

FLOWCHARTING CONVENTIONS:

1.. EACH SYMBOL DENOTES A TYPE OF OPERATION

2..A NOTE IS WRITTEN INSIDE EACH SYMBOL TO INDICATE THE SPECIFIC FUNCTION TO BE PERFORMED.

3..THE SYMBOLS ARE CONNECTED BY FLOWLINES

4..FLOWCHARTS ARE DRAWN AND READ FROM TOP TO BOTTOM UNLESS A SPECIFIC CONDITION IS MET THAT ALTERS THE PATH.

5. A SEQUENCE OF OPERATIONS IS PERFORMED UNTIL A TERMINAL SYMBOL DESIGNATES THE SEQUENCE'S END OR THE END OF THE PROGRAM.

6. SOMETIMES SEVERAL STEPS OR STATEMENTS ARE COMBINED IN A SINGLE PROCESSING SYMBOL FOR EASE OF READING.

REMEMBER: A FLOWCHART REPRESENTS AN ALGORITHM – AN APPROACH TO SOLVING A PROBLEM.

STATEMENTS IN YOUR ALGORITHM SHOULD NOT BE IN 1-1CORRESPONDENCE WITH YOUR ULTIMATE PROGRAMINSTRUCTIONS !!!!!!

EXPANSION OF THE PERFORM:

SEQUENCE OF STATEMENTS

FLOWCHARTS - OLDER TECHNIQUE

TRIED AND TRUE

VISUAL AND EASY TO WORK WITH

IN GENERAL, MORE DETAILED THAN PSEUDOCODE

GREAT FOR SHOWING COMPLICATED LOGIC...

STRUCTURED CODING IS MORE SIMILAR TO PSEUDOCODE.

PSEUDOCODE

. DESIGNED SPECIFICALLY FOR REPRESENTING LOGIC IN A STRUCTURED PROGRAM.

. HAS INDENTATION AND STRUCTURE DESIGNEDTO SUPPORT IMPLEMENTING THE PROGRAMLOGIC IN A STRUCTURED LANGUAGE.

. CAN ABBREVIATE AND NOT SHOW ALL PROCESSING DETAILS

. IS A LANGUAGE-INDEPENDENT TOOL

. LOGICAL CONTROL CONSTRUCTS ARE EASILY SPECIFIEDUSING PSEUDOCODE(SEQUENCE, SELECTION, ITERATION, THAT IS…)

CONSIDER: the PSEUDOCODE FOR IN-LINE PERFORMPERFORM

....

... <----STATEMENTS

....ENDPERFORM

. CALLED THE INLINE PERFORM SINCE ALL INSTRUCTIONS APPEAR DIRECTLY AFTER THE WORD PERFORM.

THEY ARE … ‘IN-LINE.’

I’D RATHER YOUR IN-LINE PSEUDOCODE BE:LOOP

….….

ENDLOOP

WHICH MAKES THIS QUITE LANGUAGE INDEPENDENT AND THUS CAN BE IMPLEMENTED IN ANY LANGUAGE READILY.

STANDARD PSEUDOCODE RESEMBLES COBOL-85 EXTREMELY CLOSELY

STANDARD PSEUDOCODE HAS THE SCOPE TERMINATORS IN IT.

TO MAKE THE STANDARD PSEUDOCODE LESS COBOL-LIKE, AVOID USING WORDS LIKE PERFORM…

USE LOOP OR DO OR EVEN CALL.

USING WORDS SUCH AS PERFORM IN YOUR PSEUDO-CODE IMPLIES A COBOL IMPLEMENTATION.

But PSEUDO-CODE SHOULD NOT CONSTRAIN A LANGUAGE IMPLEMENTATION OF THE DESIGN.

HENCE THE USE OF ‘LOOP’ OR ‘DO’, ETC.

STRUCTURED PSEUDOCODE

IN LIEU OF THE INLINE PERFORMS, WE CAN USE STRUCTURED PSEUDOCODE, WHERE THE INLINE PERFORMS ARE ABSTRACTED OUT TO A SEPARATE MODULE....RATHER THANHAVING THE PSEUDOCODE ‘IN-LINE.’

EXAMPLE:

STARTOPEN EMPLOYEE FILEREAD EMPLOYEE FILE RECORD....COMPUTE ....DO PRINT MODULE<MORE COBOL CODE....>

STOP

PRINT MODULE PERFORMED MODULES EXECUTED WRITE .... ELSEWHERE.WRITE ...WRITE...

RETURN

YOUR TEXT MAKES A BIG DEAL OF STANDARD PESUDO-CODE VERSUS STRUCTURED PSEUDO-CODE.

ONLY THE BASIC DIFFERENCES ARE NECESSARY.

STANDARD PSEUDOCODE AND COBOL 85 ARE VERY CLOSE.

STRUCTURED PSEUDOCODE CAN BE REALIZED IN EITHER COBOL 85, 2002, OR, OF COURSE, THE OLDER VERSIONS

CONTROL STRUCTURES.

WE KNOW WHAT SEQUENCE IS.

MERELY THE EXECUTION OF IMPERATIVE STATEMENTS OR STATEMENTS THAT ARE CONSIDERED SEQUENTIAL.

NO TRANSFERS OF CONTROL

SELECTION:

SELECTION IS A LOGICAL CONTROL CONSTRUCT THAT EXECUTES INSTRUCTIONS DEPENDING UPON THE TRUTH VALUE OF A CONDITION.

SOMETIMES CALLED: THE IFTHENELSE.

IN COBOL:

IF (CONDITION)...... <---- INDICATES ACTIONS IF CONDITION IS TRUE

ELSE...... <--- INDICATES ACTIONS IF CONDITION IS FALSEEND-IF

CONSIDER: COBOL 74 CODE:

IF AMT < ZEROADD 1 TO ERR-COUNTER

ELSEWRITE NEW-RECORD. (NOTE: PERIOD ENDS IF)

COBOL 85 AND NEWER CODE:

IF AMT < ZEROADD 1 TO ERR-COUNTER

ELSEWRITE NEW-RECORD

END-IF. (NOTE: SCOPE TERMINATOR)

PSEUDOCODE FORMAT:IF (CONDITION) EXAMPLE:THEN IF AMT IS LESS THAN ZERO

..... THEN

.... ADD 1 TO ERROR COUNTERELSE ELSE

.... WRITE A NEW RECORD

.... ENDIFENDIF

(NO HYPHENS; SPELL OPERATORS; ENGLISH-LIKE)

ITERATION

. NEXT LOGICAL CONTROL STRUCTURE...

. IN COBOL, IMPLEMENTED AS A PERFORM...UNTIL

. E.G., PERFORM 200-CALC-RTNUNTIL EOF = 1.

(STRUCTURED; TRADITIONAL)

OR, PERFORM UNTIL EOF=1…..END-PERFORM.

(STANDARD…)

IN EITHER EVENT, WE ARE ‘LOOPING’

WE ARE EXECUTING CODE OVER AND OVER UNTIL SOME CONDITIONBECOMES TRUE.

BOOK:

EXAMPLE OF STANDARD PSEUDOCODE FOR INLINE PERFORM:

PERFORM UNTIL condition......

ENDPERFORM

EXAMPLE OF STRUCTURED PSEUDOCDE FOR PERFORM:

100 MAIN MODULE ....PERFORM 200 CALCULATE SALARY

UNTIL END OF FILE REACHED.....

200 CALCULATE SALARY ........ <----------- INSTRUCTIONS TO BE PERFORMED

RETURN

I PREFER:

LOOP CALCULATE SALARY REPEATEDLYUNTIL END OF FILE IS REACHED.

LOOP UNTIL END OF FILE IS REACHED ….….END LOOP

REMEMBER: PSEUDO-CODE SHOULD BE LANGUAGEINDEPENDENT AND SHOULD THUS NOT CONSTRAIN A LANGUAGE IMPLEMENTATION (SOLUTION) OF YOUR ALGORITHM (DESIGN).

THE INFINITE LOOP:

AN ERROR TO BE AVOIDED.

1.. HOW TO GET THEM:NO TERMINATING CONDITION FOR LOOP

GET PRIMING READ OKAY,FORGET READ AS LAST STATEMENT OF PERFORMED

PARAGRAPH

2. SAY, PERFORM UNTIL COUNT > 10.INITIALIZE COUNT BEFORE LOOP, BUT DON'T INCREMENT COUNT WITHIN LOOP.

3. REINITIALIZE CONTROL VARIABLE WITHIN BODY OF LOOP EACH ITERATION.

CAN YOU PROVIDE AN EXAMPLE OF EACH????

THE CASE STRUCTURE:

THE CASE STRUCTURE IS A SPECIAL LOGICAL CONTROL STRUCTURE USED WHEN THERE ARE SEVERAL PATHS TO BE FOLLOWED DEPENDING ON THE CONTENTS OF A GIVEN FIELD.

EXAMPLE:AN INPUT CODE CAN RESULT IN ANY OF SEVERAL ACTIONS TO BE TAKEN:

FOR EXAMPLE, IF CODE = 1, PERFORM 2000-UPDATE,IF CODE = 2, PERFORM 3000-NEW-HIRE;

IF CODE = 3, PERFORM 4000-TERMINATE; IF CODE IS NOT 2, 3, OR 4, PERFORM 5000-ERROR

BETTER: USE THE COBOL EVALUATE VERB: (COBOL-85)

EVALUATE UPDATE-CODE (AN 88 LEVEL???)WHEN 1

PERFORM 2000-UPDATEWHEN 2

PERFORM 3000-NEW-HIREWHEN 3

PERFORM 4000-TERMINATEWHEN OTHER

PERFORM 5000-ERROREND-EVALUATE

. NOTE THE ABSTRACTION (AS IMPLEMENTED)

. GREAT FOR DESIGNING AND PROCESSING MENUS

. GREAT FOR VALIDATION OF ACCEPTABLE VALUES FOR VARIABLES READ INTO THE PROGRAM

SO WHAT WOULD THE PSEUDO-CODE LOOK LIKE?

STRUCTURED PSEUDOCODE:CASE UPDATE CODE

WHEN 1DO UPDATE

WHEN 2DO NEW-HIRE

WHEN 3DO TERMINATE ROUTINE

WHEN OTHERDO ERROR ROUTINE

ENDCASE

UPDATE...

NEW-HIRE...

TERMINATE...

ERROR...

STANDARD PSEUDOCODECASE UPDATE-CODE

WHEN 1DO

SALARY UPDATE RTNEENDDO

WHEN 2DO

NEW HIRE PROCEDUREENDDO

WHEN 3DO

TERMINATE PROCEDUREENDDO

ENDCASE

NOTE: . NO HYPHENS. ALL LOGIC IN LINE. MODULAR

FLOWCHARTS AND PSEUDOCODE - RULES REVIEW:

FLOWCHART RULES:

1. A FLOWCHART IS DRAWN AND READ FROM TOP TO BOTTOM UNLESS A SPECIFIC CONDITION ALTERS THE PATH.

2. DIFFERENT SYMBOLS ARE USED TO DENOTE DIFFERENT FUNCTIONS

3. ALL SYMBOLS HAVE EXPLANATORY NOTES INDICATING THE SPECIFIC OPERATIONS TO BE PERFORMED.

THE SYMBOL ITSELF DENOTES THE TYPE OF OPERATION SUCH AS INPUT/OUTPUT OR PROCESSING, AND A NOTE WITHIN THE SYMBOL DESCRIBES SPECIFIC OPERATION TO BE PERFORMED SUCH AS READ A RECORD OR ADD AMOUNT TO TOTAL.

PSEUDOCODE RULES:

1. PSEUDOCODE IS WRITTEN AND READ FROM TOP TO BOTTOM

2. THE LOGICAL CONTROL STRUCTURE OF PSEUDOCODE IS DEFINED WITH THE USE OF KEY TERMS SUCH AS (BOOK PERFORM...ENDPERFORM, PLEASE USE DO…ENDDO OR LOOP…END LOOP OR CALL….RETURN, IFTHENELSE,...ENDIF, AND CASE ...ENDCASE.

ALL HAVE SCOPE TERMINATORSNO HYPHENS OR LANGUAGE-DEPENDENT FEATURESHAS A ‘THEN’

3. THE OPERATIONS TO BE EXECUTED WITHIN A LOOP, IFTHENELSE OR CASE CAN BE CODED IN SEQUENCE OR IN A EPARATE MODULE.

HEURISTICS:

STRUCTURED PSEUDOCODE (SEPARATE MODULES)

BETTER FOR DETAILED OR COMPLEX SETS OF INSTRUCTIONS.

BETTER TO COMMUNICATE (THEREFORE) ABSTRACTIONOFTEN CLEANER

TRADITIONAL WAY TO CODE (OLDER; MAINTENANCE!)

CORRESPONDS TO STRUCTURE CHARTS MUCH MORE READILY THAN STANDARD PSEUDOCODE.

STANDARD PSEUDOCODE

STANDARD PSEUDOCODE (WITH INLINE PERFORMS) IS PREFERRED.

BETTER FOR LESS COMPLICATED LOGIC

BETTER FOR FEWER NUMBERS OF STATEMENTS

NO TRANSFERS OF CONTROL

ALL LOGIC IS CLEARLY VISIBLE

HIERARCHY CHARTS FOR TOP DOWN PROGRAMMING

FLOWCHARTS AND PSEUDOCODE USED TO MODEL OUR DESIGN USING THE STRUCTURED DESIGN PARADIGM.

USED FOR DETAIL DESIGN ONCE MODULES ESTABLISHED....

ARCHITECTURAL DESIGN (PRELIMINARY DESIGN)

. BUILDING BLOCKS

. A HIERARCHY OR STRUCTURE CHART PROVIDES A GRAPHICAL METHOD FOR SEGMENTING A PROGRAMINTO MODULES.

. ITS MAIN PURPOSE IS TO PROVIDE A VISUAL/GRAPHICALOVERVIEW OF THE RELATIONSHIPS AMONG MODULES IN A PROGRAM.

. A MODULE IS A WELL-DEFINED SEGMENT THAT PERFORMSA SPECIFIC FUNCTION.

MODULES MAY BE:

HEADING ROUTINESCOMPUTATIONAL ROUTINESROUTINES THAT DO NOTHING BUT ACONTROL@PRINT ROUTINESCOMMUNICATION ROUTINES, ETC.

COMMON MODULES?? SHOW.

WELL-WRITTEN PROGRAMS MAY WELL HAVE A MIXTURE OF BOTH STRUCTURED CODE AND STANDARD.

ARCHITECTURAL DESIGN IS FOLLOWED BY DETAIL DESIGN.

DRIVERS AND STUBSGIVEN A STRUCTURE CHART (INFERRED FROM CODE BELOW…)CAN SAY: (DISCUSS!)

0000-MAIN....PERFORM 1000-FUNCTPERFORM 2000-FUNCTPERFORM 3000-FUNCTPERFORM 4000-FUNCT.

1000-FUNCT.DISPLAY “GOT TO 1000-FUNCTION”.

2000-FUNCT.PERFORM 2100-FUNCTPERFORM 2300-FUNCTPERFORM 2500-FUNCT.

3000-FUNCT.DISPLAY “GOT TO 3000-FUNCTION”.

4000-FUNCT.DISPLAY “GOT TO 4000-FUNCTION”.

2100-FUNCT.DISPLAY “GOT TO 2100 FUNCTION”.

2300-FUNCT.DISPLAY “GOT TO 2300 FUNCTION”.

2500-FUNCT.... ETC.

DISCUSS DRIVERS AND STUBS.

ITERATIVE DEVLEOPMENT; STEPWISE VERIFICATION; ISOLATION OFPROBLEMS… DIVIDE AND CONQUER!!

NES...