An Introduction to Structured Program Design in COBOL

1. An Introduction to Structured Program Design in COBOL Chapter 1

2. Chapter Objectives To familiarize you with • Why COBOL is a popular business-oriented language. • Programming practices and techniques • History of COBOL • Use of the current ANSI standard versions of COBOL • Four divisions of a COBOL program

3. Chapter Contents • Computer Programming: An Overview • Applications Program Development Process • Nature of COBOL • History of COBOL and the ANS Versions

4. Chapter Contents • Techniques for Improving Program Design • Sample Programs • Entering and Running a COBOL Program on Your Computer • This is particular to our system • We’ll be using the “Alpha” which is a VAX • Telnet into it, you’ll love the editor.

5. Program Languages Machine language • Only language computer understands • All programs executed on computer must be in machine language • Machine language programs difficult to write

6. Program Languages Symbolic language (like COBOL) • English-like languages used to write programs • Easier than writing programs in machine language • Must be translated or compiled into machine language to run on computer

7. Program Development Process • Determine Program Specifications • Design Program Using Program Planning Tools • Code and Enter Program • Compile Program • Test Program • Document Program

8. 1. Program Specifications • Systems analysts, users and programmers develop specifications • Specifications include: • Description of input and output data • Step-by-step processing required to convert input to output

9. 1. Program Specifications Record layout forms describe format of input and output data • Data names of each data item in record • Location of each data item in record • Size of each data item • Number of decimal positions (for numeric data items) • Note that this is different than what you are used to in C++

10. 1. Program Specifications Printer spacing charts describe • Spacing • Layout of printed output • Heading, detail, total lines • Error messages

11. 2. Design the Program Program planning tools used to map out structure and logic of program • Flowcharts use block diagrams to represent logic • Pseudocode uses English-like statements • Hierarchy charts show relationships among sections of program

12. 3. Code and Enter Program • Programmer writes and enters program into computer • Program written in symbolic language (like COBOL) • Called source program • You all already know this

13. 4. Compile Source Program Compiler is program that • Checks source program for rule violations • Translates source program into object program • Thursday we’ll meet in Dundee and try this out Source program in symbolic language Translated by compiler Object program in machine language

14. 5. Test Program • Test or debug program to ensure it contains no errors • Check for two types of errors • Compile-Time Errors • Execution Errors

15. Compile-Time Errors • Errors detected by compiler during translation from COBOL to machine language • Detects violations of programming rules • Misspelled reserved words • Missing punctuation • Also called syntax errors

16. Execution Errors • Detected when program is run • Logic error causes incorrect output • Sequence of instructions incorrect • Wrong instruction coded • Run-time error if computer cannot execute instruction • Attempt to divide by zero • Attempt to read a file that cannot be found

17. Debugging Techniques • Desk checking • Correcting syntax errors • Program walkthroughs • Executing the program

18. 6. Document the Program • Documentation - formal set of procedures and instructions to specify how to use program • Written for • Those working with output • Computer operators who run program • Maintenance programmers who make modifications to program • This was a big part of the design of COBOL, Self documenting code

19. Nature of COBOL • Business-oriented language • Standard language • English-like language • Relatively easy to understand

20. History of COBOL • Developed in 1959 as standard language to meet needs of business • Committee to develop language convened by Department of Defense • Included representatives from academia, business, computer manufacturers

21. Standard Versions of COBOL • 1960s • 1968 • 1974 • 1985 • wide variations in COBOL compilers • first COBOL standard set by American National Standards Institute (ANSI) • second ANSI standard to make COBOL more efficient, standardized • this ANSI standard incorporated structured programming techniques

22. Current and Future Standards • 1985 currently the most widely used • 2008 is next standard • Approval expected in 2008 or so • Information on 2008 COBOL standard at http://www.ansi.org

23. Future of COBOL Likely to remain important language • Older COBOL programs need to be updated • Still used by many for new program development • Mostly maintaining existing code, that’s why you’re here.

24. Use of COBOL • About 200 billion lines of COBOL source code in use • 5 billion new lines added each year • Used by 42.7% of application programmers in medium to large U.S. companies • $200 million in expected revenues for 2001 • Y2K Problem

25. Improving Program Design Two techniques used to develop programs that are easier to understand, test, debug and modify • Structured Programming • Top-Down Programming

26. Structured Programming • Eliminates use of GO TO statements • Allowed skipping to different sections of program without returning to starting point • Program logic easier to follow with "GO-TO-less" programming

27. Structured Programming Program divided into paragraphs • Main paragraph or module controls logic flow using PERFORM statements • Main module "performs" other modules when instructions in that module required • Each module can be written and tested independently of others

28. Top-Down Programming • Another technique to make programs easier to understand, test, debug and modify • Develop program like term paper • Develop outline first • Add details for each of main steps • Add further refinement for more complex steps • Use of stubs For COBOL program • Code main modules or routines first • Code intermediate modules next • Details deferred to minor modules and coded last

29. Interactive vs Batch Programs • Cobol suited for developing both types of programs Interactive programs • Accept input data from keyboard • Input data processed immediately • Output (results) displayed on screen immediately

30. Interactive vs Batch Programs Batch programs • Process large volumes of input at periodic intervals • Input data read in from files • Output written to files

31. Overview of the Four Divisions • Every COBOL program contains up to four separate divisions in the following order: IDENTIFICATION DIVISION ENVIRONMENT DIVISION DATA DIVISION PROCEDURE DIVISION • Specific to COBOL

32. Overview of the Four Divisions • IDENTIFICATION DIVISION • Identifies program to operating system • Provides documentation about program • ENVIRONMENT DIVISION • Defines file-names • Describes devices used to store them • Not included in fully interactive programs

33. Overview of the Four Divisions • DATA DIVISION • Describes input and output format of data in files • Defines any constants and work areas • PROCEDURE DIVISION • Contains instructions to read input, process it and create output

34. Sample Interactive Program • Purpose • to compute employee WAGES • Input from keyboard • HOURS and RATE • Processing • compute WAGES as HOURS x RATE • Output displayed on screen • WAGES • Page 17 of book

35. Sample Interactive Program • IDENTIFICATION DIVISION • One required entry, PROGRAM-ID • Names the program • DATA DIVISION • Describes and defines storage for all data • Data defined in WORKING-STORAGE SECTION for interactive program

36. Data Defined in Sample Program • Keyed input fields (HOURS, RATE) • Output fields (WAGES) • Other fields used for processing (MORE-DATA)

37. PICTURE Clause • 01 level begins definition of each field • PICTURE or PIC clause describes • Type of data • Numeric (PIC 9) • Nonnumeric (PIC X) • Size of field - determined by number of 9’s or X’s

38. PICTURE Clauses • RATE with PIC 99V99 includes V to show assumed decimal point position • User enters data with decimal point • Program uses V to align data • WAGES includes actual decimal point • Shown when value displayed on screen

39. Giving Field Initial Value • MORE-DATA with PIC XXX is nonnumeric field • Assigned initial contents of YES by use of VALUE clause • Value must be in quotation marks since MORE-DATA is nonnumeric field

40. PROCEDURE DIVISION • Set of instructions to be executed by program • Organization of instructions planned before coding begins • Pseudocode, an English-like description of program instructions, used for planning • Describes program logic and order in which instructions will be executed

41. PROCEDURE DIVISION • PROCEDURE DIVISION includes one paragraph 100-MAIN • List of instructions that follow make up paragraph • Period follows last statement in paragraph (STOP RUN.) • Main processing controlled by PERFORM … END-PERFORM loop

42. PERFORM … END-PERFORM • Repeats set of instructions as long as user enters YES in response to prompt "IS THERE MORE DATA (YES/NO)?" • MORE-DATA initially contains YES so instructions in loop executed first time

43. PERFORM … END-PERFORM • When user enters NO as response • MORE-DATA set to "NO" and loop ends • After loop, STOP RUN is executed, ending program

44. PERFORM … END-PERFORM Statements in loop executed in order they are listed • DISPLAY displays value in quotes or value of field on screen • ACCEPT stores value user enters from keyboard in field • MULTIPLY performs calculation to find WAGES

45. Sample Batch Program • In batch mode, data comes from input file instead of keyboard • Data for each employee stored in a record in file on disk • Employee name, hours and rate data called fields • Page 30

46. Sample Batch Program • Calculated results (Wages) stored in file instead of displayed on screen • For each input record • Record created and stored in output file • Includes employee name, hours, rate and computed wages • File intended for printing so spacing added between fields for readability

47. COBOL Divisions • All four divisions included for batch programs • IDENTIFICATION DIVISION first with required PROGRAM-ID paragraph • ENVIRONMENT DIVISION • INPUT-OUTPUT SECTION assigns input and output files to specific devices

48. DATA DIVISION • FILE SECTION describes format of input and output files • Fields in records described using PICTURE clause • Decimal point not stored in input records • Use V for implied decimal for alignment • Use actual decimal point for fields in output record so it is printed

49. PROCEDURE DIVISION • Contains instructions to be executed by computer • Instructions executed in order they appear • Includes two paragraphs with period at end of each

50. 100-MAIN-MODULE • OPENs files to be used by program • Repeatedly READs in records (PERFORM … END-PERFORM) until there are no more • Calls second paragraph 200-WAGE-ROUTINE to process each record • CLOSEs files after all records read • Ends program (STOP RUN)