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

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

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

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.

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

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

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

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

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++

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

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

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

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

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

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

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

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

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

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

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

Standard Versions of COBOL 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

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

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

READ Statement Reads one record into program storage area Record must be in storage to use it Takes one of two actions depending on whether record was read

READ Statement PERFORM instruction after NOT AT END executed when record read Statements in paragraph 200-WAGE-ROUTINE executed to process record If no more records to be read, MOVE instruction after AT END executed 'NO ' moved to ARE-THERE-MORE-RECORDS, ends loop

200-WAGE-ROUTINE First MOVE initializes PRINT-REC to blanks Then MOVEs name, hours, wages to output fields Calculates WAGES with MULTIPLY statement, MOVES it to output field WRITEs data in employee output record to print file

Entering & Running a Program To type in and run a COBOL program on your computer system, you need to know how to: Log on and off of the computer Name COBOL files on the computer Use a text editor to key in, modify and save files Compile a COBOL source program to translate it into machine language Link or load the object program Run the object program

Chapter Summary The Nature of COBOL English-like language Used widely for business programming With new standard coming out soon, it is likely to remain important language

Chapter Summary COBOL programs divided into four divisions IDENTIFICATION DIVISION identifies program to operating system ENVIRONMENT DIVISION required for batch programs that operate on files DATA DIVISION defines formats for all input, output and work areas needed FILE SECTION required for batch programs that operate on files WORKING-STORAGE SECTION for work fields or fields used in interactive programs

Chapter Summary PROCEDURE DIVISION Contains instructions to process input and produce output Divided into paragraphs or modules