Michael Coughlan
Page 16
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88 Single-Room VALUE "SINGLE".
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2. Write an IF statement that uses the SET verb to manually set the condition name
InvalidCode to true if DeptCode contains anything except 1, 6, or 8.
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3. assume the variable DeptCode in question 2 is described as
01 DeptCode PIC 9.
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Write a level 88 condition name called InvalidCode that is automatically set to true when
the statement ACCEPT DeptCode accepts any value other than 1, 6, or 8.
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Chapter 5 ■ Control StruCtureS: SeleCtion
4. in each of the following five groups of skeleton IF statements, state whether the statements
in each group have the same effect (in the sense that they evaluate to true or false). answer
yes or no.
Do these statements have the same effect?
Answer
IF Num1 = 1 OR Num1 NOT = 1...
IF NOT (Num1 = 1 AND Num1 = 2) ...
IF TransCode IS NOT = 3 OR Total NOT > 2550 ...
IF NOT (TransCode IS = 3 OR Total > 2550) ...
IF Num1 = 31 OR Num2 = 12 AND Num3 = 23 OR Num4 = 6 ...
IF (Num1 = 31 OR (Num2 = 12 AND Num3 = 23)) OR Num4 = 6 ...
IF Num1 = 15 OR Num1 = 12 OR Num1 = 7 AND City = "Cork" ...
IF (Num1 = 15 OR Num1 = 12 OR Num1 = 7) AND City = "Cork" ...
IF (Num1 = 1 OR Num1 = 2) AND (Num2 = 6 OR Num2 = 8) ...
IF Num1 = 1 OR Num1 = 2 AND Num2 = 6 OR Num2 = 8 ...
5. Write an EVALUATE statement to implement the decision part of a game of rock, paper, scissors.
Most of the program has been written for you. Just complete the EVALUATE. ADD a WHEN OTHER
branch to the EVALUATE to detect when a player enters a code other than 1, 2, or 3.
IDENTIFICATION DIVISION.
PROGRAM-ID. Listing5-10.
AUTHOR. Michael Coughlan.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 PlayerGuess-A PIC 9 VALUE 1.
88 Rock-A VALUE 1.
88 Paper-A VALUE 2.
88 Scissors-A VALUE 3.
01 PlayerGuess-B PIC 9 VALUE 2.
88 Rock-B VALUE 1.
88 Paper-B VALUE 2.
88 Scissors-B VALUE 3.
PROCEDURE DIVISION.
BEGIN.
DISPLAY "Guess for player A (1=rock, 2=scissors, 3=paper) : "
WITH NO ADVANCING
ACCEPT PlayerGuess-A
DISPLAY "Guess for player B (1=rock, 2=scissors, 3=paper) : "
WITH NO ADVANCING
ACCEPT PlayerGuess-B
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Chapter 5 ■ Control StruCtureS: SeleCtion
EVALUATE _____________________________________________________
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prOGraMMING eXerCISe
listing 4-2 is a program that accepts two numbers from the user, multiplies them together, and then displays the result. Modify the program so that
• it also accepts an operator symbol (+ - / *).
• it uses EVALUATE to discover which operator has been entered and to apply that operator to the
two numbers entered.
• it uses the condition name ValidOperator to identify the valid operators and only displays the
result if the operator entered is valid.
• the Result data item is changed to accommodate the possibility that subtraction may result in a
negative value.
• the Result data item is changed to accommodate the decimal fractions that may result from
division. the result data item should be able to accept values with up to two decimal places (for
example, 00.43 or 00.74).
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Chapter 5 ■ Control StruCtureS: SeleCtion
LaNGUaGe KNOWLeDGe eXerCISeS—aNSWerS
1. For each of the following condition names, which do you consider to be inappropriately
named? Suggest more suitable names for these only.
01 Country-Code PIC XX.
88 UnitedStates VALUE "US".
* Change
* Example of use - IF UnitedStates DISPLAY "We are in America" END-IF
01 Operating-System PIC X(15).
88 Windows VALUE " WINDOWS".
* Change
* Example of use - IF Windows DISPLAY "Windows is best" END-IF
01 Room-Type PIC X(20).
88 Double-Room VALUE "DOUBLE".
88 Single-Room VALUE "SINGLE".
* No change.
* Example of use -IF Double-Room ADD DoubleRoomSurchage TO RoomRent END-IF
2. Write an IF statement that uses the SET verb to manually set the condition name
InvalidCode to true if DeptCode contains anything except 1, 6, or 8.
IF NOT (DeptCode = 1 OR DeptCode = 6 OR DeptCode = 8) THEN
SET InvalidCode TO TRUE
END-IF.
or, using implied subjects:
IF NOT (DeptCode = 1 OR 6 OR 8) THEN
SET InvalidCode TO TRUE
END-IF.
3. assume the variable DeptCode in question 2 is described as
01 DeptCode PIC 9.
88 InvalidCode VALUE 0, 2 THRU 5,7,9.
Write a level 88 condition name called InvalidCode that is automatically set to true when
the statement ACCEPT DeptCode accepts any value other than 1, 6, or 8.
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Chapter 5 ■ Control StruCtureS: SeleCtion
4. in each of the following five groups of skeleton IF statements, state whether the statements
in each group have the same effect (in the sense that they evaluate to true or false). answer
yes or no.
Do these statements have the same effect?
answer
IF Num1 = 1 OR Num1 NOT = 1...
YES
IF NOT (Num1 = 1 AND Num1 = 2)...
in the sense that they
are both always true.
IF TransCode IS NOT = 3 OR Total NOT > 2550 ...
NO
IF NOT (TransCode IS = 3 OR Total > 2550)...
IF Num1 = 31 OR Num2 = 12 AND Num3 = 23 OR Num4 = 6...
YES
IF (Num1 = 31 OR (Num2 = 12 AND Num3 = 23)) OR Num4 = 6...
the brackets only make
explicit what is ordained
by the precedence rules.
IF Num1 = 15 OR Num1 = 12 OR Num1 = 7 AND City = "Cork"...
> NO
IF (Num1 = 15 OR Num1 = 12 OR Num1 = 7) AND City =
in the first num1=7
"Cork"...
anD City=SpaCeS are
anDed together but in
the second City=“Cork”
is anDed with the result
of the expression in the
parentheses
IF (Num1 = 1 OR Num1 = 2) AND (Num2 = 6 OR Num2 = 8) ...
NO
IF Num1 = 1 OR Num1 = 2 AND Num2 = 6 OR Num2 = 8 ...
5. Write an EVALUATE statement to implement the decision part of a game of rock, paper, scissors.
Most of the program has been written for you. Just complete the EVALUATE. ADD a WHEN OTHER
branch to the EVALUATE to detect when a player enters a code other than 1, 2, or 3.
Listing 5-10. Rock, Paper, Scissors Game
IDENTIFICATION DIVISION.
PROGRAM-ID. Listing5-10.
AUTHOR. Michael Coughlan.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 PlayerGuess-A PIC 9 VALUE 1.
88 Rock-A VALUE 1.
88 Paper-A VALUE 2.
88 Scissors-A VALUE 3.
01 PlayerGuess-B PIC 9 VALUE 2.
88 Rock-B VALUE 1.
88 Paper-B VALUE 2.
88 Scissors-B VALUE 3.
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Chapter 5 ■ Control StruCtureS: SeleCtion
PROCEDURE DIVISION.
BEGIN.
DISPLAY "Guess for player A (1=rock, 2=scissors, 3=paper) : "
WITH NO ADVANCING
ACCEPT PlayerGuess-A
DISPLAY "Guess for player B (1=rock, 2=scissors, 3=paper) : "
WITH NO ADVANCING
ACCEPT PlayerGuess-B
EVALUATE TRUE ALSO TRUE
WHEN Rock-A ALSO Rock-B DISPLAY "Draw"
WHEN Rock-A ALSO Paper-B DISPLAY "Player B wins"
WHEN Rock-A ALSO Scissors-B DISPLAY "Player A wins"
WHEN Paper-A ALSO Rock-B DISPLAY "Player A wins"
WHEN Paper-A ALSO Paper-B DISPLAY "Draw"
WHEN Paper-A ALSO Scissors-B DISPLAY "Player B wins"
WHEN Scissors-A ALSO Rock-B DISPLAY "Player B wins"
WHEN Scissors-A ALSO Paper-B DISPLAY "Player A wins"
WHEN Scissors-A ALSO Scissors-B DISPLAY "Draw"
WHEN OTHER DISPLAY "Evaluate problem"
END-EVALUATE
STOP RUN.
prOGraMMING eXerCISe aNSWer
Listing 5-11. Simple Calculator
IDENTIFICATION DIVISION.
PROGRAM-ID. Listing5-11.
AUTHOR. Michael Coughlan.
*> Accepts two numbers and an operator from the user.
*> Applies the appropriate operation to the two numbers.
DATA DIVISION.
WORKING-STORAGE SECTION.
01 Num1 PIC 9 VALUE 7.
01 Num2 PIC 9 VALUE 3.
01 Result PIC --9.99 VALUE ZEROS.
01 Operator PIC X VALUE "-".
88 ValidOperator VALUES "*", "+", "-", "/".
PROCEDURE DIVISION.
CalculateResult.
DISPLAY "Enter a single digit number : " WITH NO ADVANCING
ACCEPT Num1
DISPLAY "Enter a single digit number : " WITH NO ADVANCING
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Chapter 5 ■ Control StruCtureS: SeleCtion
ACCEPT Num2
DISPLAY "Enter the operator to be applied : " WITH NO ADVANCING
ACCEPT Operator EVALUATE Operator
WHEN "+" ADD Num2 TO Num1 GIVING Result
WHEN "-" SUBTRACT Num2 FROM Num1 GIVING Result
WHEN "*" MULTIPLY Num2 BY Num1 GIVING Result
WHEN "/" DIVIDE Num1 BY Num2 GIVING Result ROUNDED
WHEN OTHER DISPLAY "Invalid operator entered"
END-EVALUATE
IF ValidOperator
DISPLAY "Result is = ", Result
END-IF
STOP RUN.
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Chapter 6
Control Structures: Iteration
The previous chapter dealt with COBOL’s selection constructs: IF and EVALUATE. In this chapter, you examine the last of the classic structured programming constructs: iteration.
In almost every programming job, there is some task that needs to be done over and over again. The job of
processing a file of records is an iteration of this task: get and process record. The job of getting the sum of a stream of numbers is an iteration of this task: get and add number. The job of searching through an array for a particular value is an iteration of this task: get next element and check element value. These jobs are accomplished using iteration constructs.
Other languages support a variety of iteration constructs, each designed to achieve different things. In Modula-2
and Pascal, While..DO and Repeat..Until implement pre-test and post-test iteration. The for loop is used for counting iteration. The many C-language derivatives use while and do..while for pre-test and post-test iteration, and again the for loop is used for counting iteration.
COBOL supports all these different kinds of iteration, but it has only one iteration construct: the PERFORM verb (see Table 6-1). Pre-test and post-test iteration are supported by PERFORM WITH TEST BEFORE and PERFORM WITH TEST
AFTER. Counting iteration is supported by PERFORM..VARYING. COBOL even has variations that are not found in other languages. PERFORM..VARYING, for instance, can take more than one counter, and it has both pre-test and post-test variations. Whereas in most languages the loop target is an inline block of code, in COBOL it can be either an inline block or a named out-of-line block of code.
Table 6-1. Iteration Constructs and Their COBOL Equivalents
C, C++, Java
Modula-2, Pascal
COBOL
Pre-test
while {}
While..DO
PERFORM WITH TEST BEFORE UNTIL
Post-test
do {}
Repeat..Until
PERFORM WITH TEST AFTER UNTIL
while
Counting
for
For..DO
PERFORM..VARYING..UNTIL
Paragraphs Revisited
In the PROCEDURE DIVISION, a paragraph is a block of code to which you have given a name. A paragraph begins with the paragraph name (see Example 6-1) and ends when the next paragraph or section name is encountered or when the end of the program text is reached. The paragraph name must always be terminated with a period (full stop).
There may be any number of statements and sentences in a paragraph; but there must be at least one sentence, and the last statement in the paragraph must be terminated with a period. In fact, as I mentioned in the previous chapter, there is a style of COBOL programming called the minimum-period style1-2, which you should adopt. This style suggests that there should be only one period in the paragraph. It is particularly important to adhere to this style when coding inline loops, because a period has the effect of delimiting the scope of an inline PERFORM.
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Chapter 6 ■ Control StruCtureS: IteratIon
Example 6-1. Two Paragraphs: ProcessRecord Ends Where ProcessOutput Begins
ProcessRecord.
DISPLAY StudentRecord
READ StudentFile
AT END MOVE HIGH-VALUES TO StudentRecord
END-READ.
ProduceOutput.
DISPLAY "Here is a message".
The PERFORM Verb
Unless it is instructed otherwise, a computer running a COBOL program processes the statements in sequence, starting at the first statement of the PROCEDURE DIVISION and working its way down through the program until the STOP RUN, or the end of the program text, is reached. The PERFORM verb is one way of altering the sequential flow of control in a COBOL program. The PERFORM verb can be used for two major purposes;
• To transfer control to a designated block of code
• To execute a block of code iteratively
>
Whereas the other formats of the PERFORM verb implement iteration of one sort or another, this first format is used to transfer control to an out-of-line block of code—that is, to execute an open subroutine. You have probably have come across the idea of a subroutine before. A subroutine is a block of code that is executed when invoked by name.
Methods, procedures, and functions are subroutines. You may not have realized that there are two types of subroutine:
• Open subroutines
• Closed subroutines
If you have learned BASIC, you may be familiar with open subroutines. If you learned C, Modula-2, or Java, you are probably familiar with closed subroutines.
Open Subroutines
An open subroutine is a named block of code that control (by which I mean program statement execution) can fall into, or through. An open subroutine has access to all the data items declared in the main program, and it cannot declare any local data items.
Although an open subroutine is normally executed by invoking it by name, it is also possible, unless you are careful, to fall into it from the main program. In BASIC, the GOSUB and RETURN commands allow you to implement open subroutines. Example 6-2 is a short BASIC program that illustrates the fall-through problem. Two outputs are provided: one where the EXIT statement prevents fall-through and the other where control falls through into OpenSub because the EXIT statement has been removed.
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Chapter 6 ■ Control StruCtureS: IteratIon
Example 6-2. Open Subroutine in Yabasic3 Showing Output With and Without the EXIT Statement REM Demonstrates Open subroutines in Yabasic
REM When the EXIT is removed, control falls
REM through into OpenSub
REM Author. Michael Coughlan
PRINT "In main"
GOSUB OpenSub
PRINT "Back in main"
EXIT
LABEL OpenSub
PRINT "In OpenSub"
RETURN
In some legacy COBOL programs, falling through the program from paragraph to paragraph is a deliberate
strategy. In this scheme, which has been called gravity-driven programming, control falls through the program until it encounters an IF and GO TO combination that drives it to a paragraph in the code above it; after that, control starts to fall through the program again. Example 6-3 provides an outline of how such a program works (P1, P2, P3, and P4 are paragraph names).