cobol array processing and tables

8/10/2019 Cobol Array Processing and Tables

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Array Processing and Table

Handling


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OBJECTIVES

To familiarize you with:1. How to establish a series of items using an OCCURS clause.

2. How to access and manipulate data stored in an array ortable.

3. The rules for using an OCCURS clause in the DATA DIVISION.4. The use of a SEARCH or SEARCH ALL for a table look- up.


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CONTENTS An Introduction to Single-Level OCCURS Clauses

Why OCCURS Clauses Are Used

Rules for Use of the OCCURS Clause

Processing Data Stored in an Array

Using OCCURS with VALUE and REDEFINES Clauses

Printing Data Stored in an Array


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CONTENTS

Using an OCCURS Clause for Table Handling

Defining a Table

Storing the Table in WORKING-STORAGE

Looking Up Data in a Table: Finding a Match

Use of the SEARCH Statement for Table and Array Processing

Format of the SEARCH Statement

The INDEXED BY Clause and the SEARCH Statement

Modifying the Contents of an Index


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CONTENTS

Using Two WHEN Clauses for an Early Exit from aSEARCH

Searching for Multiple Matches

Internal vs... External Tables

Looking up Table Data for Accumulating Totals

The SEARCH ... VARYING Option for ProcessingParallel Tables


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CONTENTS

The SEARCH ALL Statement Definition of a Serial Search

Definition of a Binary Search

Format of the SEARCH ALL Statement

ASCENDING or DESCENDING KEY with the SEARCH ALL Statement


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CONTENTS

Multiple-Level OCCURS Clause

Accessing a Double-Level or Two-Dimensional Array

Defining a Double-Level or Two-Dimensional Array

Using PERFORM ... VARYING ... AFTER

Using a Double-Level or Two-Dimensional Array for Accumulating

Totals

Performing a Look-Up Using a Double-Level OCCURS


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An Introduction toSingle-Level OCCURS Clauses


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Why OCCURS Clauses Are Used

SOME USES OF OCCURS

1. Defining a series of input or output fields, each with the same format.2. Defining a series of totals in WORKING-STORAGE to which amounts

are added; after all data is accumulated.

3. Defining a table in WORKING-STORAGE to be accessed by eachinput record.

With a table, we use the contents of some input filed to "look up" therequired data in the table.


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Defining a Subscript

Collectively, these 24 fields within the array are calledTEMPERATURE, which is the identifierused to access them in thethe PROCEDURE DIVISION.

We use the identifier TEMPERATURE along with a subscriptthat

indicates which of the 24 fields we wish to access:DISPLAY TEMPERATURE (23)


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SUMMARY OF OCCURS

AND SUBSCRIPTS1. An OCCURSclause is defined in the DATA

DIVISIONto indicate the repeated occurrence ofitems in an array that have the same format.

2. A subscriptis used in the PROCEDURE DIVISIONtoindicate which specific item within the arraywewish to access.


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QUESTIONS?


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SELF-TESTConsider the following for Questions 1 through 5:

01 IN-REC.05 AMT1 PIC 9(5).

05 AMT2 PIC 9(5).

05 AMT3 PIC 9(5).

05 AMT4 PIC 9(5).

05 AMT5 PIC 9(5).


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SELF-TEST

1. An OCCURS clause could be used in place ofdefining each AMT field separately because

____________________________________ .

SOLUTION: all AMTs have the same format or PICclause


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SELF-TEST

2. (T or F) Suppose AMT2 and AMT4 had PIC 9(3). AnOCCURS clause could not be used to define allthe AMT fields.

SOLUTION: T


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SELF-TEST

3. Recode the fields within IN-REC using an OCCURSclause.

SOLUTION:

01 IN-REC.

05 AMT OCCURS 5 TIMES PIC 9(5).


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SELF-TEST

4. To access any of the five items defined with theOCCURS clause, we must use a ____ in thePROCEDURE DIVISION.

SOLUTION: subscript


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SELF-TEST

5. Code a routine to determine the total of all five AMT fields.Assume that a field called SUB has been defined in WORKING-STORAGE to serve as a subscript.

SOLUTION: COBOL 85 (With an In-line PERFORM)

MOVE ZEROS TO TOTAL

PERFORM VARYING SUB FROM 1 BY 1 UNTIL SUB > 5

ADD AMT (SUB) TO TOTAL

END-PERFORM . . .


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Rules for Use of the

OCCURS Clause Levels 02--49

An OCCURS clause may be used on levels 02-- 49only.

That is, the OCCURS is not valid for the 01 level sinceit must be used for defining fields, not records.

Elementary or Group Items may also be definedwith an OCCURS Clause.


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Using OCCURS with VALUE

and REDEFINES Clauses Sometimes we want to initialize elements in a

table or an array with specific values.

We have seen that with COBOL 85 you can use a

VALUE clause to set an entire array to zero:

01 ARRAY-1 VALUE ZERO.05 TOTALS OCCURS 50 TIMES PIC 9(5).


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Using an OCCURS Clause forTable Handling


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Defining a Table

A tableis a list of stored fields that are looked upor referenced by the program.

Tables are used in conjunction with table look-ups,

where a table look-upis a procedure that finds aspecific entry in the table.

Thus, an arraystores data or totals to beoutputted, whereas a tableis used for looking upor referencing data.


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Storing the Table in

WORKING-STORAGE Storing data in a table file rather than in each transaction

record is more efficient because it minimizes data entryoperations.

It is also more efficient because the data can be more easilymaintained, or updated, in a separate table, as needed.

Any time input records need to be saved for future processing,use a READINTO to store the data in WORKING-STORAGE.


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Use of the SEARCH Statement forTable and Array Processing


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Format of the SEARCH

StatementSEARCH identifier-1

[AT END imperative-statement-1]

WHEN condition-1 {imperative- statement-2} {NEXT

SENTENCE} ...

[END-SEARCH]*

*COBOL 85 only. If END-SEARCHis used, NEXT SENTENCEmust bereplaced with CONTINUEunless your COBOL 85 compiler has an

enhancement that permits it.


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Using the SEARCH ... AT END forData Validation

With the SEARCH statement, the AT END clause specifies whatshould be done if the table has been completely searched andno match is found.

Since it is possible for input errors to occur, we strongly

recommend that you always use this optional clause. Without it, the ``no match'' condition would simply cause the

program to continue with the next sentence; thus producingincorrect results or even a program interrupt.


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The INDEXED BY Clause andthe SEARCH Statement When using a SEARCHstatement, table entries must be specified

with an index rather than a subscript.

An indexis similar to a subscript, but it is defined along with thetable entries as part of the OCCURS description:

01 SALES-TAX-TABLE.05 TABLE-ENTRIES OCCURS1000 TIMES

INDEXED BYX1.10 WS-ZIPCODE PIC 9(5).10 WS-TAX-RATE PIC V999.

H I d Diff F S b i t


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How an Index Differs From a Subscript

Indexes are processed differently and more efficiently than

subscripts. When you define an index, the computer sets up an internal

storage area called an index register.

Registers use the displacement values determined by the index toaccess table addresses.

This is faster than working with subscripts.

We recommend that you use indexes and SEARCH statements fortable look-ups.

iff i


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How an Index Differs From a Subscript

Because an index refers to a displacement and not just anoccurrence value, its contents cannot be modified with a MOVE,ADD, or SUBTRACT like a subscript can.

To change the contents of an index, then, we use either:

(1) a PERFORM ... VARYING, which can vary the values in eithersubscripts or indexes.

(2) a SETstatement, which can move, add, or subtract values in anindex.


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Modifying the Contents of anIndex: The SET StatementBasic Format:

SET index-name-1 {TO} {UP BY} {DOWN BY} integer-1

EXAMPLES:

Statement Meaning1. SET X1 TO 1 Move 1 to the X1 index.

2. SET X1 UP BY 1 Add 1 to the X1 index.

3. SET X1 DOWN BY 1 Subtract 1 from the X1 index.

Initializing an Index Before Using the


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Initializing an Index Before Using theSEARCH

A SEARCHstatement does not automatically initialize the

index at 1 because sometimes we may want to beginsearching a table at some point other than the beginning.

Initializing an index at 1 must be performed by a SETstatementprior to the SEARCHif we want to begin each table look-up withthe first entry; for example: SETX1TO1

DIFFERENCES BETWEEN


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DIFFERENCES BETWEENSUBSCRIPTS AND INDEXES

Subscript represents an occurrence of an array or table element.

Defined in a separate WORKING-STORAGE entry.

To change a subscript's value, use a PERFORM ... VARYINGor any

of the following:MOVE 1 TO SUB ADD 1 TO SUB

SUBTRACT 1 FROM SUB


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THE SEARCH ... VARYING OPTION FORPROCESSING PARALLEL TABLES

Format:SEARCH identifier-1 VARYING

{identifier-2} {index-name-1}[AT END imperative-statement-1]

{WHEN condition-1 {imperative-statement-2}{NEXT SENTENCE}[END-SEARCH]*

*COBOL 85 only. The NEXT SENTENCEclause is not

permitted with the END-SEARCHunless yourcompiler has an enhancement that allows it.


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Definition of a Serial

Search1. The first entry in the table is searched.2. If the condition is met, the table look-up is completed.

3. If the condition is not met, the index or subscript is incremented by

one, and the next entry is searched.4. This procedure is continued until a match is found or the table has

been completely searched.


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Definition of a Serial Search

A sequentialor serial search, as described here, is best usedwhen either:

1. The entries in a table are not in either ascending or descendingsequence; that is, they are arranged randomly; or,

2. Table entries are organized so that the first values are the onesencountered most frequently;

in this way, access time is minimized because you are apt to end thesearch after the first few comparisons.


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Definition of a Binary

Search When table entries are arranged in sequence by

some field, such as T- CUSTOMER-NO, the mostefficient type of look-up is a binary search.

The following is the way the computer performs abinary search:


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Definition of a Binary Search

1. Begin by comparing CUST-NO of the inputcustomer record to the middle table argumentfor T-CUSTOMER-NO. For example, this might be the twenty-fifth entry in

the table.2. Since T-CUSTOMER-NOs are in sequence, if

CUST-NO-IN > T- CUSTOMER-NO (25) - which isthe middle entry in our table - this eliminates theneed for searching the first half of the table. In such a case, we compare CUST-NO-IN to T-

CUSTOMER-NO (37), the middle table argument ofthe second half of the table (rounding down), andcontinue our comparison in this way.


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Definition of a Binary

Search3. If CUST-NO-IN < T-CUSTOMER-NO (25), we compare CUST-NO-IN to

T-CUSTOMER-NO (12);

that is, we divide the top half of the table into two segments and

continue our comparison.4. The binary search is complete either (a) when a match has been

found, that is, CUST-NO-IN = T-CUSTOMER-NO (X1), or (b) whenthe table has been completely searched.


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Uses of a Binary Search

1. When table entries are arranged in some sequence - eitherascending or descending.

2. When tables with a large number of sequential entries (e.g., 50 ormore) are to be looked up or searched.

For small tables or those in which entries are not arranged in asequence, the standard serial search look-up method previouslydescribed is used.

Format for SEARCH ALL


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Format for SEARCH ALL

SEARCH ALL identifier-1

[AT END imperative-statement-1]WHEN {data-name-1 { IS EQUAL TO} {IS =}

{identifier-2} {literal-1} {arithmetic-expression-1}}

{condition-name-1

[AND {data-name-2 {IS EQUAL TO} {IS =}{identifier-3}{literal-2}{arithmetic-expression-2}}]

{condition-name-2} . . .

{imperative-statement-2} {NEXT SENTENCE}

[END-SEARCH]* Cobol 85 only. NEXT SENTENCE cannottypically be used with END-SEARCH.


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Limitations of the SEARCH ALLStatement

1. The condition following the word WHEN can testonly for equality:

Valid: WHEN T-CUSTOMER-NO (X1) = CUST-NO-IN

Invalid: WHEN T-WEIGHT-MAX (X1) < WEIGHT-MAILED


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Limitations of the SEARCH ALLStatement

2. If the condition following the word WHEN is a compoundconditional:

a. Each part of the conditional can only consist of a relational test that

involves an equal condition.

b. The only compound condition permitted is with ANDs, not ORs.

Valid: WHEN S-AMT (X1) = AMT1 AND TAX-AMT (X1) = AMT2

Invalid: WHEN SALES-AMT (X1) = AMT3 OR AMT4 = AMT5


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Limitations of the SEARCH

ALL Statement3. Only one WHEN clause can be used with a SEARCH ALL.

4. The VARYING option may not be used with the SEARCH ALL.

5. The OCCURS item and its index, which define the table argument,must appear to the left of the equal sign.

Valid: WHEN S-AMT (X1) = AMT1...

Invalid: WHEN AMT1 = S-AMT (X1)...


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ASCENDING or DESCENDING KEYwith the SEARCH ALL Statement

1. To use the SEARCH ALLstatement, we must indicate which tableentry will serve as the key field.

2. That is, we specify the table entry that will be in sequence so thatthe binary search can be used to compare against that field.

3. We must indicate whether that KEYis ASCENDINGorDESCENDING.


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ASCENDING or DESCENDING KEYwith the SEARCH ALL Statement

ASCENDING KEY - Entries are in sequence and increasing in value.

DESCENDING KEY - Entries are in sequence and decreasing in value.Format:

(level-number 02--49) identifier-1

OCCURS integer-1 TIMES{ASCENDING} {DESCENDING} KEY IS

data-name-2INDEXED BY index-name-1


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Differences Between the SEARCH


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Differences Between the SEARCHand the SEARCH ALLSEARCH ALL

Performs a binary search

Tables entries must be in sequence by the table argument or eventhe table function. The field that is in sequence is specified in anASCENDING or DESCENDING KEY clause as part of the OCCURS entry

Does not need a SET prior to the SEARCH ALL

Can only have a single = condition tested with the WHEN clause

May only have one WHEN clause


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MULTIPLE-LEVEL OCCURS

CLAUSE When describing an area of storage, more than one level of

OCCURS may be used.

As many as seven levels of OCCURS are permitted with COBOL

85, and as many as three levels are permitted with COBOL 74. Like a single-level OCCURS, multiple levels of OCCURS may be

used for:

(1) accumulating totals in an array.

(2) storing a table for ``look-up'' purposes.

RULES FOR USING A DOUBLE-LEVEL


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RULES FOR USING A DOUBLE LEVELOCCURS

1. If an item is defined by a double-level OCCURS clause, it must beaccessed by two subscripts.

2. The first subscript refers to the higher-level OCCURS; the secondsubscript refers to the lower-level OCCURS.

3. The subscripts must be enclosed in parentheses.

RULES FOR USING A DOUBLE-LEVEL


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RULES FOR USING A DOUBLE LEVELOCCURS

4. Subscripts may consist of positive integers or data-names withpositive integer contents.

5. On most systems, the left parenthesis must be preceded by atleast one space; similarly, the right parenthesis must be followed

by a period, if it is the end of a sentence, or at least one space. The first subscript within parentheses is followed by a comma and a

space.

Expanded Format for Accessing a


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Expanded Format for Accessing aDouble-Level orTwo-Dimensional Array

PERFORM [procedure-name-1 [{THROUGH} {THRU} procedure-name-2]

[WITH TEST {BEFORE} {AFTER}]

VARYING {identifier-2} {index-name- 1} FROM{identifier-3}{index-name-2} {literal-1}

BY {identifier-4} {literal-2} UNTIL condition-1

[AFTER {identifier-5}{index-name-3}

FROM {identifier-6}{index-name-4} literal-3}BY {identifier-7} {literal-4} UNTIL condition-2]

[END-PERFORM]*

cobol array processing and tables

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