ch09_97

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Structured COBOL Programming, Stern & Stern, 9th Edition

PowerPoint Presentation:

Richard H. Baum, Ph.D.DeVry Institute of Technology

9th Edition

Structured COBOLProgramming

Nancy Stern

Hofstra University

Robert A. Stern

Nassau Community

College

“Copyright @ 2000 John Wiley & Sons, In. All rightsreserved. Reproduction or translation of this work beyond that permitted in Section 117 of the 1976

United States Copyright Act without the expresspermission of the copyright owner is unlawful.Request for further information should be addressedto the permissions Department , John Wily & Sons,Inc. The purchaser may make back-up copies forhis/her own use only and not for distribution orresale. The Publisher assumes no responsibility forerrors, omissions, or damages, caused by the use of these programs or from the use of the informationcontained herein.”

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CHAPTER 9Iteration: Beyond theBasic PERFORM

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OBJECTIVES

• To familiarize you with:

1. The simple PERFORM.

2. How PERFORM statements are usedfor iteration.

3. The various options available with thePERFORM statement.

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CONTENTS

• The Simple PERFORM Reviewed – The Basic Formats

– Modularizing Programs Using PERFORM

Statements – Nested PERFORM: A PERFORM within a

PERFORM

– Executing a Group of Paragraphs with aSimple PERFORM

– The Use and Misuse of GO TO Statements

– The EXIT Statement

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CONTENTS• Iteration Using Other Types of PERFORMs

– PERFORM UNTIL…(A Review)

– Coding a Loop with a PERFORM

– PERFORM...TIMES

– Examples of Loops

– PERFORM VARYING

•

Using Nested PERFORM VARYINGStatements

• The PERFORM WITH TEST AFTER Option

(COBOL 85)

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The Simple PERFORMReviewed

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The Basic Formats

• There are two formats of the basicPERFORM:

1. In-Line PERFORMPERFORM

Statements to

be executedEND-PERFORM

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The Basic Formats

• An in-line PERFORM begins with the wordPERFORM, is followed by statements tobe executed, and ends with an END-

PERFORM scope terminator.

• All instructions within the PERFORM ...END-PERFORM are executed in sequence.

• Periods are optional after any scopeterminator except at the end of aparagraph, when they are required.

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The Basic Formats

2. PERFORM Paragraph-Name

PERFORM [paragraph-name-1]

• The PERFORM paragraph-namestatement will:

1. Execute all instructions in the named

paragraph.2. Transfer control to the next instruction in

sequence, after the PERFORM paragraph-name.

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The Basic FormatsPseudocode

• Pseudocode for In-line PERFORM

PERFORM

. Statements

. to be performed

. go hereEND-PERFORM

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The Basic FormatsPseudocode

• Pseudocode for a PERFORM paragraph-name

PERFORM Paragraph.

.

PARAGRAPH

.

. {Statements to be performed go here

.

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Modularizing Programs UsingPERFORM Statements

• Version 1: Nonmodular Approach

IF AMT1-IN < AMT2-IN

ADD AMT1-IN TO TOTAL-1


ADD 1 TO OK-REC-CTR

ELSE


ADD 1 TO ERR-REC-CTR

END-IF.

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Modularizing Programs UsingPERFORM Statements

Version 2: Modular ApproachIF AMT1-IN < AMT2-INPERFORM 300-OK-RTN

ELSEPERFORM 400-ERR-RTN

END-IF.300-OK-RTN.

ADD AMT1-IN TO TOTAL-1 ADD AMT2-IN TO TOTAL-2 ADD 1 TO OK-REC-CTR.

400-ERR-RTN. ADD AMT2-IN TO TOTAL-3 ADD 1 TO ERR-REC-CTR.

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Executing a Group of Paragraphs with a Simple

PERFORM• Expanded format for the PERFORM

paragraph-name statement:

PERFORM paragraph-name-1

{THROUGH} {THRU} paragraph-

name-2

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Executing a Group of Paragraphs with a Simple

PERFORM• The PERFORM paragraph-name executes

all statements beginning at paragraph-

name-1 until the end of paragraph-name-2 is reached.

• Control is then transferred to the

statement directly following thePERFORM.

Th U d Mi f

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The Use and Misuse of GO TO Statements

Format:

GO TO paragraph-name-1

•

Unlike the PERFORM, which returnscontrol to the statement following thePERFORM, the GO TO permanentlytransfers control to another paragraph.

• However, in well-designed, structuredprograms it is best to avoid the use of GOTO as much as possible.

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The EXIT Statement

• EXIT is a COBOL reserved word thatperforms no operation.

•

It is used to allow execution to pass overother statements or to transfer controlback to the statement following theoriginal PERFORM.

• It is used, when necessary, as an endpoint in a paragraph being performed.

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ITERATION USINGOTHER TYPES OFPERFORMs

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PERFORM UNTIL . . . A Review

The format of a PERFORM UNTIL ... is:

• Format 2

PERFORM [paragraph-name-1][{THROUGH} {THRU} paragraph-

name-2]

UNTIL condition-1

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PERFORM UNTIL . . . A Review

• With COBOL 85, if an in-line PERFORM isused, no paragraph-name is coded.

– The statements within the PERFORM end withan END-PERFORM scope terminator.

• Any simple or compound condition can bespecified in a PERFORM UNTIL ...

• As with a simple PERFORM, COBOL 85permits an in-line PERFORM UNTIL …

– this format also uses END-PERFORM as ascope terminator.

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Coding a Loop with aPERFORM

• The loop should:

1. Be preceded by an instruction that

initializes the field to be tested (e.g.,MOVE 0 TO COUNTER1).

2. Include a PERFORM UNTIL ... that is

executed repeatedly UNTIL the field to betested reaches the desired value

(e.g., PERFORM UNTIL COUNTER1 = 5 orPERFORM UNTIL COUNTER1 >= 5).

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Coding a Loop with aPERFORM

3. Include, as one of the instructions withinthe PERFORM UNTIL, a statement thatincreases (or decreases) the value of the

field being tested (e.g., ADD 1 TOCOUNTER1).

– The contents of the counter is used to control

the number of times that the loop isperformed.

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PERFORM TIMES

• Format


[{THROUGH} {THRU} paragraph-name-2]

{integer-} {identifier-1} TIMES

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PERFORM TIMES

• When using the TIMES format --

PERFORM paragraph-name-1 identifier-1

TIMES:(1) the identifier must be specified in the

DATA DIVISION;

(2) it must have a numeric PICTURE clause;and

(3) it must contain only integers or zeros.

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PERFORM TIMES

• The THRU clause is optional with anyPERFORM paragraph-name statement.

•

When using the integer option with thePERFORM TIMES format, only the actualnumber is acceptable.

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DEBUGGING TIP

• Use PERFORM TIMES rather thanPERFORM UNTIL if you know in advancethe specific number of times a paragraphis to be executed.

• If the number of times a paragraph is tobe executed is variable use a PERFORMUNTIL.

• It is also best to use a PERFORM UNTIL if the number of times a paragraph is beingexecuted needs to be used for output.

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PERFORM VARYING

• The last format for a PERFORM statementis the most comprehensive:


[{THROUGH} {THRU} paragraph-name-2]

VARYING identifier-1 FROM

{identifier-2}{integer-1} BY

{identifier-3} {integer-2}

UNTIL condition-1

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USING NESTED PERFORM VARYING Statements

Rules for using a Nested PERFORM VARYING

1. The innermost PERFORM VARYING loop is executed first.

2. The next outer PERFORM VARYING

loop in sequence is then executed.

O S

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THE PERFORM WITH TEST AFTER OPTION (COBOL 85)

• Format


[WITH TEST {BEFORE} {AFTER}]UNTIL condition-1

• With COBOL 85, if an in-line PERFORM isused, no paragraph-name is coded,

– Statements within the PERFORM end with anEND-PERFORM scope terminator.

THE PERFORM WITH TEST

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THE PERFORM WITH TEST AFTER OPTION (COBOL 85)

• The WITH TEST AFTER clause can also beused with the VARYING option of thePERFORM.

– The format is:PERFORM WITH TEST AFTER VARYING ....

• Note that PERFORM UNTIL (with no WITH

TEST clause) and PERFORM WITH TESTBEFORE UNTIL do the same thing.

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CHAPTER SLIDES END HERE

CHAPTER SUMMARY COMES NEXT

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CHAPTER SUMMARY

A. Formats of the PERFORM Statement

1. Simple PERFORM statements:

a. In-line PERFORM:PERFORM ... END-PERFORM

b. PERFORM paragraph-name-1

[THRU paragraph-name-2] – Cause execution of the instructions at the

named paragraph(s).

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CHAPTER SUMMARY

–

After execution of the instructions withineither the PERFORM ... END-PERFORM or inthe PERFORM paragraph-name, control istransferred to the statement directly

following the PERFORM.

2. The PERFORM UNTIL statement

a. The identifier(s) used in the UNTIL clause

must be altered within the paragraph(s)being performed; otherwise, the paragraphswill be performed indefinitely.

CHAPTER SUMMARY

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CHAPTER SUMMARY

b. If the condition in the UNTIL clause is met at

the time of execution, then the namedparagraph(s) will not be executed at all.

– With COBOL 85, the WITH TEST AFTER clausecan be used to test the condition after the

paragraph has been executed once.

3. The PERFORM TIMES statement

• A numeric identifier or an integer canprecede the word TIMES; with COBOL 85,an arithmetic expression can be used aswell.

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CHAPTER SUMMARY

4. The PERFORM VARYING statementa. The counter or loop control field must be

defined in the DATA DIVISION, typically inWORKING-STORAGE. An initial VALUE for the

loop control field is not required.

b. The PERFORM VARYING automatically doesthe following:

(1) Initializes the counter with the valuespecified in the FROM clause.

(2) Tests the counter for the condition specifiedin the UNTIL clause.

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CHAPTER SUMMARY

(3) Continues with the statement directlyfollowing the PERFORM if the conditionspecified in the UNTIL clause is satisfied.

(4) Executes the named paragraph(s) if the

condition specified in the UNTIL clause is notmet.

(5) After execution of the named paragraph(s),increases (or decreases) the counter by the

value of the integer or identifier specified inthe VARYING clause.

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CHAPTER SUMMARY

B. Additional Considerations

1. The THRU option can be included with

all versions of the PERFORM but werecommend that you avoid this option.

2. PERFORM statements within PERFORM

statements are permissible. These arecalled nested PERFORMs.

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St t d COBOL P i St & St 9th Editi

CHAPTER SUMMARY 3. EXIT is a reserved word that can be used

to indicate the end point of paragraph(s)being performed.

– EXIT must be the only entry in a paragraph

when it is used with COBOL 74.4. In-line PERFORMs are permitted in

COBOL 85 with all PERFORM options;

with in-line PERFORMs it is not necessaryto code separate paragraphs.

– The PERFORM is terminated with an END-PERFORM.

ch09_97

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