CHAPTER 8 Decision Making Using the IF and EVALUATE Statements

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CHAPTER 8Decision Making Using theIF and
EVALUATEStatements
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OBJECTIVES

• To familiarize you with
• 1. The use of IF statements for selection.
• 2. The variety of formats and options available
  with the conditional statement.
• 3. The use of the EVALUATE statement with COBOL
  85.

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Selection

• Selection Using
• Simple Conditional
• Nested Conditional
• Compound Conditional
• Sign and Class Tests
• Negating Conditionals
• Evaluate
• Condition Names (Named Conditions)

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A REVIEW of LOGICAL CONTROL STRUCTURES

• LOGICAL CONTROL STRUCTURES
• 1. Sequence
• 2. Selection (IF-THEN-ELSE)
• 3. Iteration (PERFORM)
• 4. Case (EVALUATE)

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Basic Conditional Statements

• Format for IF statements
• IF condition-1 THEN
• imperative statement-1 . . .
• ELSE
• imperative statement-2 . . .
• END-IF

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Basic Conditional Statements

• Simple Relational Conditions
• 1. IF identifier-1 IS EQUAL TO identifier-2
• 2. IF identifier-1 IS LESS THAN identifier-2
• 3. IF identifier-1 IS GREATER THAN
  identifier-2

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Basic Conditional Statements

• Illustration of a simple conditional
• IF AMT1 IS EQUAL TO AMT2
• DIVIDE QTY INTO TOTAL
• ELSE
• ADD UNIT-PRICE TO FINAL-TOTAL
• END-IF

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Basic Conditional Statements

• Example of an IF Statement Without an ELSE
  Clause
• MOVE NAME-IN TO NAME-OUT
• MOVE AMOUNT-IN TO AMOUNT-OUT
• IF AMOUNT-IN IS EQUAL TO ZEROS
• MOVE 'NO TRANSACTIONS THIS
• MONTH' TO OUT-AREA
• End-IF
• WRITE PRINT-REC AFTER ADVANCING 2 LINES

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Basic Conditional Statements

• More Than One Operation Can Be Performed When a
  Condition Exists
• The instruction format includes dots or ellipses
  (...) indicating that more than one operation may
  be executed for each condition.
• The following performs two MOVE operations if
  AMT1 is equal to AMT2, and two ADD operations if
  AMT1 is not equal to AMT2

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Basic Conditional Statements

• IF AMT1 IS EQUAL TO AMT2
• MOVE NAME-IN TO NAME-OUT
• MOVE DESCRIPTION-IN TO
• DESCRIPTION-OUT
• ELSE
• ADD AMT1 TO TOTAL1
• ADD AMT2 TO TOTAL2
• END-IF.

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DEBUGGING TIP

• Omitting the scope terminator is permitted for
  all versions of COBOL as long as the IF sentence
  ends with a period.
• However, we recommend that you use scope
  terminators with COBOL 85 and omit periods except
  for the last statement in a paragraph.

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Coding Guidelines Indenting

• Indent statements within the IF instruction to
  make programs easier to read and debug. The
  following is the coding style for conditionals
• IF condition THEN
• imperative statement
• ...
• ELSE
• imperative statement
• ...
• END-IF.

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Basic Conditional Statements

• Using Relational Operators in Place of Words
• The following symbols for simple relational
  conditions are valid within a COBOL statement
• RELATIONAL OPERATORS
• Symbol Meaning
• lt IS LESS THAN
• gt IS GREATER THAN
• IS EQUAL TO
• lt IS LESS THAN OR EQUAL TO
• gt IS GREATER THAN OR EQUAL TO

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Basic Conditional Statements

• Do Not Mix Field Types in a Comparison
• Conditional statements must use fields with the
  same data types to obtain proper results.
• In the statement, IF CODE-IN 123
  MOVE NAME-IN TO NAME- OUT
  END-IF CODE-IN should be
  a nonnumeric field, since it is compared to a
  nonnumeric literal.
• IF CTR1 CTR2 THEN ADD AMT1 TO
  TOTALEND-IF

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Basic Conditional Statements

• As in MOVE operations, the literal should have
  the same format as the data item.
• If CODE-OUT has a PICTURE of 9's, the following
  would be appropriate
• IF CODE-OUT 123 THEN MOVE AMT-IN TO
  AMT-OUTEND-IF

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Basic Conditional Statements

• Numeric Fields Should Not Contain Blanks
• Suppose we code IF AMT-IN IS EQUAL TO 10
  ADD 1 TO COUNTER
  END-IF
• If AMT-IN were a field defined as numeric, but
  actually contained all blanks, the instruction
  would result in a data exception error, which
  causes a program interrupt.
• This error will occur because blanks are not
  valid numeric characters.
• Be certain, then, that if a field is defined as
  numeric, it actually contains numbers.

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ASCII and EBCDIC Collating Sequences

• When performing an alphanumeric comparison, the
  hierarchy of the comparison, called the collating
  sequence, depends on the computer being used.

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ASCII and EBCDIC Collating Sequences

• The two types of internal codes that are most
  commonly used for representing data are
• EBCDIC is found on IBM and IBM-compatible
  mainframes.
• ASCII is used on most micros and many minis and
  mainframes.
• The collating sequences for these differ
  somewhat.

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ASCII and EBCDIC Collating Sequences

• COLLATING SEQUENCES
• EBCDIC ASCII
• Low Spaces Spaces
  Special characters Special
  characters a-z 0-9
  A-Z A-Z
• High 0-9 a-z

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DEBUGGING TIP

• Do not mix upper- and lowercase letters when
  entering data in fields. This reduces the risk
  that comparisons might give problematic results.
  
• As a convention, we recommend you use uppercase
  letters in all input fields as well as in
  instructions.
• Use lowercase letters only for comments.
• If you do mix case be sure to use TOUpper etc.

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The CONTINUE or NEXT SENTENCE Clause

• There are times when you might want to execute a
  series of steps only if a certain condition does
  not exist.
• The COBOL expression CONTINUE (COBOL 85) or
  NEXT SENTENCE (COBOL 74) will enable you
• (1) to avoid performing any operation if a
  condition exists
• (2) to execute instructions only if the ELSE
  condition is met.

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COBOL 2000 CHANGES

• Both CONTINUE and NEXT SENTENCE can be used
  interchangeably in the new standard.
• That is, NEXT SENTENCE will be permitted even if
  an END-IF scope terminator is used.

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SELF-TEST

• What is wrong with the following statements (1-6)
  ?
• 1. IF A IS LESS THAN B
  
  GO TO CONTINUE
  ELSE
  
  ADD 1
  TO XX
  END-IF

Solution You cannot say GO TO CONTINUE
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SELF-TEST

• 3. IF A EQUALS B
  MOVE 1
  TO A
  END-IF

Solution This should be IF A IS EQUAL TO B ....
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SELF-TEST

• 4. IF A IS LESS THEN B
  MOVE
  2 TO CODE1 END-IF

Solution When the words GREATER and LESS are
used, the COBOL word that follows is THAN and not
THEN.
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SELF-TEST

• 5. IF C D
  
  MOVE 0 TO COUNTER.
  ELSE
  
  
  MOVE 100 TO COUNTER
  END-IF

Solution There should be no period after MOVE 0
TO COUNTER.
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SELF-TEST

• 6. IF C D
  
  MOVE 0 TO COUNTER
  ELSE
  
  NEXT
  SENTENCE.

Solution ELSE NEXT SENTENCE, although not
incorrect, is unnecessary. Note that END-IF
cannot be used with NEXT SENTENCE (unless your
compiler has an enhancement that permits it) but
can always be used with CONTINUE.
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Selection Using Other Options of the IF Statement
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NESTED IFs
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NESTED IFs

• IF condition-1 IF condition 2
  imperatives ELSE
  imperatives END-IFELSE
  imperativesEND-IF

• IF condition-1 IF condition 2
  imperatives ELSE
  imperatives END-IFEND-IF

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NESTED IFs

• Using these hierarchy rules to evaluate
• Given A 2, B 2, C 3, D 4
• IF C D THEN IF A B THEN
  PERFORM 600-PARA-1 ELSE
  PERFORM 500-PARA-2
• END-IFELSE PERFORM
  400-PARA-3END-IF

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NESTED IFs

• Using these hierarchy rules to evaluate
• Given A 2, B 2, C 3, D 4
• IF A B THEN IF C D THEN
  PERFORM 600-PARA-1 ELSE
  PERFORM 500-PARA-2
• END-IFELSE PERFORM
  400-PARA-3END-IF

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NESTED IFs

• Using these hierarchy rules to evaluate
• Given A 2, B 2, C 3, D 4
• IF C D THEN IF A B THEN
  PERFORM 600-PARA-1 ELSE
  PERFORM 500-PARA-2
• END-IFEND-IF

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Compound Conditional

• We have seen that selection and iteration
  structures provide programs with a great deal of
  logical control capability.
• The compound conditional offers even greater
  flexibility for selection and enables the IF
  statement to be used for more complex problems.
• With the compound conditional, the programmer can
  test for several conditions with one statement.

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Compound ConditionalOR

• By using OR in a compound conditional, if any of
  the conditions specified is true will cause
  execution of the statement(s).

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OR
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Compound ConditionalOR

• Examples
• 1. IF AMT1 AMT2 OR AMT2 gt AMT3 (IF AMT1
  AMT2 OR gt AMT3)
• PERFORM 500-TOTAL-RTN
• END-IF.
• 2. IF AMT1 lt AMT3 OR AMT1 AMT4
• ADD AMT1 TO TOTAL
• ELSE
• PERFORM 600-ERR-RTN
• END-IF.
• If none of the conditions is met, the computer
  executes either the ELSE clause, if coded, or the
  next sentence.
• Any number of conditions separated by ORs may be
  coded in a single statement.

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AND in a Compound Conditional

• If a statement or statements are to be executed
  only when all of several conditions are met, use
  the word AND in the compound conditional.

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AND
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Compound ConditionalAND

• Examples
• 1. IF AMT1 AMT2 AND AMT2 gt AMT3 (IF AMT1
  AMT2 AND gt AMT3)
• PERFORM 500-TOTAL-RTN
• END-IF.
• 2. IF AMT1 lt AMT3 AND AMT1 AMT4
• ADD AMT1 TO TOTAL
• ELSE
• PERFORM 600-ERR-RTN
• END-IF.
• If either of the conditions is not met, the
  computer executes either the ELSE clause, if
  coded, or the next sentence.
• Any number of conditions separated by ANDs may be
  coded in a single statement.

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Compound Conditional Format

• IF condition-1 OR AND condition-2 THEN
• statement-1 . . .
• CONTINUE
• ELSE
• statement-2 ...
• END-IF

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HIERARCHY RULES FOR COMPOUND
CONDITIONALS

• 1. Conditions surrounding the word AND are
  evaluated first.
• 2. Conditions surrounding the word OR are
  evaluated last.

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HIERARCHY RULES FOR COMPOUND
CONDITIONALS

• 3. When there are several AND or OR connectors,
  the AND conditions are evaluated first, as they
  appear in the statement, from left to right. Then
  the OR conditions are evaluated, also from left
  to right.
• 4. To override Rules 1-3, use parentheses around
  conditions you want to be evaluated first.

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Using AND and OR in the Same Statement

• Using these hierarchy rules to evaluate
• Given A 2, B 2, C 3, D 4, E 5, and F
  6
• IF C D AND E F or A B THEN
  PERFORM 600-PARA-1
• END-IF

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SOLUTION
FALSE TRUE IF (C
D and E F) or A B THEN PERFORM
600-PARA-1END-IFFALSE or TRUE
true
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Sign and Class Tests

• Sign Test
• We can test whether a field is POSITIVE,
  NEGATIVE, or ZERO with a sign test.
• Example
• IF AMT IS POSITIVE
• PERFORM 200-CALC-RTN
• END-IF.
• We can also test to see if AMT IS NEGATIVE or
  ZERO.

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Sign and Class Tests

• Class Test
• We can test for the type of data in a field by
  coding IF identifier- 1 IS NUMERIC
  or IF identifier-1 IS ALPHABETIC.
• If the ELSE option is executed with the NUMERIC
  class test, then either the field contains
  alphabetic data ) or it contains alphanumeric
  data, meaning any possible characters.

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Sign and Class Tests

• Suppose we code the following
• IF AMT-IN IS NUMERIC
• PERFORM 300-CALC-RTN
• ELSE
• PERFORM 400-ERROR-RTN
• END-IF
• If the field contains 123AB, for example, the
  ELSE clause will be executed since the contents
  of the field are not strictly numeric.

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Sign and Class Tests

• Using Class Tests for Validating Data
• A class test is a useful tool for minimizing
  program errors.
• Suppose we wish to add AMT-IN to TOTAL, where
  AMT-IN is an input field.
• Since input is always subject to data-entry
  errors, it is possible that the field might be
  entered erroneously with nonnumeric data or
  spaces.
• In such a case, ADD AMT-IN TO TOTAL can cause
  the computer to abort the run.

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Sign and Class Tests

• The following test may be used to minimize such
  errors
• IF AMT-IN IS NUMERIC
• ADD AMT-IN TO TOTAL
• ELSE
• PERFORM 500-ERR-RTN
• END-IF
• It is a good practice to validate the AMT-IN
  field, as in the preceding, before performing
  the arithmetic.
• As noted, periods are optional when using END-IF
  unless you are at the end of a paragraph.

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Sign and Class Tests

• ALPHABETIC class with COBOL 85
• COBOL 85 has eliminated the ambiguity over
  uppercase/lowercase when making the ALPHABETIC
  class test.
• Either uppercase or lowercase, or any blank is
  considered ALPHABETIC.
• Moreover, two new class tests have been added
• ALPHABETIC-UPPER and ALPHABETIC-LOWER

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Sign and Class Tests

• The three alphabetic class tests for COBOL 85
  are
• Reserved Word Meaning
• ALPHABETIC A-Z, a-z, and
  blank
• ALPHABETIC-UPPER A-Z and blank
• ALPHABETIC-LOWER a-z and blank

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Sign and Class Tests

• The following is an example of the Alphabetic
  Class Test
• IF NAME-IN IS ALPHABETIC-LOWER THEN
• PERFORM 600-LOWER-CASE-RTN
• END-IF

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CONDITION-NAMES

• A condition-name (named-condition) is a
  user-defined word established in the DATA
  DIVISION that gives a name to a specific value
  that an identifier can assume.
• An 88-level entry coded in the DATA DIVISION is a
  condition-name that denotes a possible value for
  an identifier, which then can be tested to be
  either True or False.
• A condition-name is always coded on the 88 level
  and has only a VALUE clause associated with it
  and will not contain a PICTURE clause.

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CONDITION-NAMES

• Format for 88-level items
• 88 condition-name VALUE literal
• The condition-name must be unique and its VALUE
  must be a literal consistent with the data type
  of the field preceding it
• 05 CODE-IN PIC XX.
• 88 STATUS-OK VALUE '12'.IF
  STATUS-OK is equivalent toIF CODE-IN
  12

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CONDITION-NAMES

• For readability, we indent each 88-level item to
  clarify its relationship to the data-name
  directly preceding it.
• Any elementary item on level numbers 01--49 in
  the FILE SECTION or in the WORKING-STORAGE may
  have a condition-name associated with it.

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The COBOL 85 EVALUATE Statement Using the Case
Structure as an Alternative to Selection
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The COBOL 85 EVALUATE Statement

• Format
• EVALUATE identifier-1
• expression-1
• WHEN condition-1 imperative-
  statement-1 . . .
• WHEN OTHER imperative-statement-2
• END-EVALUATE

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EVALUATE

• EVALUATE STATE-CODE WHEN IL
  imperatives WHEN IA
  imperatives WHEN WI
  imperatives WHEN OTHER
  imperatives END-EVALUATE

• IF STATE-CODE IL imperativesEND-IF
  IF STATE-CODE IA imperativesEND-IF
  IF STATE-CODE WI imperativesEND-IF

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EVALUATE (format 2)

• EVALUATE TRUE WHEN GENDER M WHEN
  MARRIED S WHEN AGE gt 21 WHEN
  OTHEREND-EVALUATE
• alternative to nested IFs or a series of IF
  statements.

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QUESTIONS?