Selection

1
Selection Iteration

• DT266/2 Information System
• COBOL

2
Conditions in COBOL

• Conditions - punctuation ().
• lt, , gt, lt or , less than, ..., alphabetic,
  numeric, negative.
• Level 88s
• owning group
• condition name
• Value
• true values
• individual, range, selection
• Truth, falseness

3
Level 88s

• Level 88
• Condition Name
• Specifies that a condition is true when value of
  data item changes to that defined in the
  associated picture clause
• 01 MONTH-IN PIC 99.
• 88 VALID-MONTH value 1 through 12.
• 88 FEB value 2.
• 88 30-DAY value 4, 6, 9,11.
• 01 DATE-INDICATOR PIC 9 value 0.
• 88 VALID-DATE value 1.

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Selection

• 3 types of selection
• IF ltconditiongt code/paragraph-name
• END-IF
• IF ltconditiongt code/paragraph-name
• ELSE code/paragraph-name
• END-IF
• Evaluate ...When ...When Other...End-Evaluate
• Examples - Select1, Select2

5
Identification Division. Program-Id. PGMSLECT
initial. Data Division. Working-Storage
Section. 01 Amount-In pic 99. Procedure
Division. Display "Age eligibility for free
schooling". Display "Enter a 2-digit age"
with no advancing. Accept Amount-In. If
Amount-In lt 4 Display "This child is too
young" else if Amount-In lt 19
Display "This person is eligible for free
schooling" else Display
"This person is not automatically eligible"
end-if end-if Stop run.
6
Evaluating against a value

• Evaluate TRUE
• When A gt B
• code-1
• When C gt D
• code-2
• When other
• code-3
• End-Evaluate

• This construct allows several conditions to be
  checked sequentially. As soon as a true
  condition is met, the code relating to that
  condition is executed and control skips past
  End-evaluate.
• A period at any point will terminate the evaluate.

7

• Identification Division.
• Program-Id. PGMSLECT2 initial.
• Data Division.
• Working-Storage Section.
• 01 Amount-In pic 99.
• Procedure Division.
• Display "Age eligibility for free
  schooling".
• Display "Enter a 2-digit age" with no
  advancing.
• Accept Amount-In.
• Evaluate Amount-In
• When 0 through 3
• Display "This child is too
  young"
• When 4 through 19
• Display "This person is
  eligible for free schooling"
• When other
• Display "This person is not
  automatically eligible"
• End-evaluate
• Stop run.

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Evaluating with condition names
Evaluate true When 30-day
If day-in lt 31 set
valid-date to true End-if
When not Feb Set valid-date to
true When other Perform
Validate-feb End-evaluate.
In this example, a data item containing the month
is declared as follows- 01 MONTH-IN PIC
99. 88 VALID-MONTH value 1 through 12.
88 FEB value 2. 88
30-DAY value 4, 6, 9,11. 01
DATE-INDICATOR PIC 9 value 0. 88 VALID-DATE
value 1.
9
Evaluating menu options
Evaluate true When Add-new-record
perform ENTER-TOY When
View-a-record perform VIEW-a-TOY When
Delete-a-record perform Delete-toy
When Amend-a-record perform Change-toy
End-evaluate.
In this example, the option has been chosen from
either a drop-down or numeric menu. 01
menu-option pic 9. 88 add-new-record value
1. 88 view-a-record value 2. 88
delete-a-record value 3. 88 amend-a-record
value 4. 88 exit-menu value 5.
10
Performing paragraphs

• Paragraphs are pieces of code that can be placed
  anywhere in the program.
• A displaced paragraph occurs after the stop run
  and is performed from elsewhere in the program by
  the statement
• Perform ltparagraph-namegt
• Paragraphs end when a new one begins
• When the paragraph is executed, control returns
  to the statement after the perform

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Example of displaced paragraph
Procedure Division. Main Section.
Housekeeping-paragraph. Perform
Get-Radius. Multiply R by R giving
R-Squared. Multiply R-Squared by PI
giving D-Area. Display 'Area is ' with
no advancing. Display D-Area.
Stop run. Get-Radius. Display
'Enter 2-digit integer radius' with no
advancing. Accept R.
12
Iteration

• PERFORM statement
• LEADING EDGE
• While Loop
• Test Before
• TRAILING EDGE
• Repeat Loop
• Test After
• VARYING
• For Loop

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Use of PERFORM for iteration

• All forms of iteration use PERFORM
• The leading edge or test before loop is a while
  loop
• The condition is tested before the code is
  executed.
• PERFORM ltloop-paragraphgt until ltconditiongt.
• PERFORM until ltcondition
• ltcode statementsgt
• END-PERFORM.

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Sample leading edge loop
Procedure Division. Display "Age
eligibility for free schooling".
Perform enter-age until amount-in gt3 and
amount-in lt 19. Stop run.
Enter-age. Display "Enter a 2-digit
age" with no advancing Accept
Amount-In If not (amount-in gt 3 and
amount-in lt 19) Display "Not an
eligible age - retry!" End-if.
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Embedding paragraphs

• Sometimes it is preferable to be able to see the
  code in the line of the program. In this case,
  the code is embedded between
• PERFORM until ltconditiongt
• embedded code
• END-PERFORM.
• There should be no period within this code.

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• Identification Division.
• Program-Id. iter1 initial.
• Data Division.
• Working-Storage Section.
• 01 Amount-In pic 99 value 0.
• Procedure Division.
• Display "Age eligibility for free
  schooling".
• Perform until amount-in gt3 and
  amount-in lt 19
• Display "Enter a 2-digit age"
  with no advancing
• Accept Amount-In
• If not (amount-in gt 3 and
  amount-in lt 19)
• Display "Not an eligible age -
  retry!"
• End-if
• End-perform
• Stop run.

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Trailing-edge loops

• The trailing-edge loop or repeat loop is
  implemented in Cobol by the with test after
  clause.
• This loop forces the code to be executed at least
  once
• Iteration ceases subject to the falseness of the
  condition.

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Sample trailing edge loop

• Perform accept-menu-option with test after
  until option gt 0 and option lt 6.
• Perform Carry-out-option.
• Stop run.
• Accept-menu-option.
• Accept menu-option.
• If menu-option lt 1 or gt 5
• display Invalid option - try again
• end-if.
• Carry-out-option.

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Embedded trailing edge loop

• Perform with test after until option gt 0 and
  option lt 6
• Accept menu-option
• If menu-option lt 1 or gt 5
• display Invalid option - try again
• end-if
• end-perform.
• Perform carry-out-option.
• Stop run.
• Carry-out-option.