Transcript Lecture Notes - Chapter 9, Iteration

Chapter 9
Iteration: Beyond the Basic
PERFORM
9-1
Simple PERFORM
Format
PERFORM [paragraph-name-1]
•
•
Executes all instructions in named
paragraph
Then transfers control to instruction
following PERFORM
9-2
Simple PERFORM
• Use to execute a paragraph from
different points in a program
• Use to modularize program
– Write each set of related instructions as
separate module or paragraph
– Use PERFORM paragraph-name to
execute each module as needed
9-3
0000-MAIN-MODULE. all first modules should start with 000-something.
PERFORM 1000-INITIALIZATION
PERFORM 2000-PROCESS-RENTALS
UNTIL F-MORE-RECORDS = "N"
PERFORM 3000-TERMINATION
STOP RUN.
*****************************************************************
* FUNCTION: OPENS FILES, PRINTS HEADINGS, DOES PRIMING READ OF
*
CUSTOMER FILE
* CALLS:
100-GET-DATE-AND-TIME
*
1200-PRINT-HEADINGS
* CALLED BY: N/A
*****************************************************************
1000-INITIALIZATION.
OPEN INPUT CUSTOMER-FILE
OUTPUT PRINT-FILE
PERFORM 1100-GET-DATE-AND-TIME
PERFORM 1200-PRINT-HEADINGS
READ CUSTOMER-FILE
AT END
MOVE "N" TO F-MORE-RECORDS
NOT AT END
ADD 1 TO A-TOTAL-RECORDS-READ
END-READ.
9-4
In-Line PERFORM
Format
PERFORM
.
.
.
Statements to
be executed
END-PERFORM
• Use when only a few statements are to
be executed
• Modularize with PERFORM paragraphname when many statements required
9-5
Nested PERFORM
• PERFORM may be one of instructions
in range of another PERFORM
…
Perform 200-Paragraph
….
.
.
200-Paragraph.
…
Perform 500-Paragraph
Nested
PERFORM
9-6
0000-MAIN-MODULE. all first modules should start with 000-something.
PERFORM 1000-INITIALIZATION
PERFORM 2000-PROCESS-RENTALS
UNTIL F-MORE-RECORDS = "N"
PERFORM 3000-TERMINATION
STOP RUN.
*****************************************************************
* FUNCTION: OPENS FILES, PRINTS HEADINGS, DOES PRIMING READ OF
*
CUSTOMER FILE
* CALLS:
100-GET-DATE-AND-TIME
*
1200-PRINT-HEADINGS
* CALLED BY: N/A
*****************************************************************
1000-INITIALIZATION.
OPEN INPUT CUSTOMER-FILE
OUTPUT PRINT-FILE
PERFORM 1100-GET-DATE-AND-TIME
PERFORM 1200-PRINT-HEADINGS
Nested performs
READ CUSTOMER-FILE
AT END
MOVE "N" TO F-MORE-RECORDS
NOT AT END
ADD 1 TO A-TOTAL-RECORDS-READ
END-READ.
9-7
Nested In-Line PERFORM
• In-line PERFORMs can include nested in-line
PERFORMs or PERFORMs with paragraph-name
Perform
. . .
Perform
. . .
End-Perform
. . .
End-Perform
Big thing to remember:
CAREFULLY indent (always!!!)
Always use scope terminators!!!
9-8
Executing Group of Paragraphs
Format
PERFORM paragraph-name-1
THROUGH
paragraph-name-2
THRU
• Use expanded format to execute all
statements, including other paragraphs, from
paragraph-name-1 through paragraph-name-2
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GO TO Statement
Format
GO TO paragraph-name-1
• Does not return control to following
statement like PERFORM
• Transfers control permanently to
another paragraph
9-10
PERFORM … THRU, GO TO,
AND EXIT STATEMENTS
• Both still permitted but not used a lot in
new COBOL programs
• Provide less control, greater risk of logic
errors than using PERFORMs
• EXIT statement used with these as end
point in PERFORM …THRU paragraph
also should be avoided
9-11
The old way: but such code still clearly lives and breathes!!!
Perform 400-do-it thru 499-do-it-exit.
….
….
400-do-it.
….
perform 800-who-cares.
….
if a > b
Go to 420-do-more
else
move c to d.
420-do-more.
if some-condition
go to 499-do-it-exit.
430-do-still-more.
….
499-do-it-exit.
exit.
Notice the performs;
Note also the falling
into paragraphs;
potential bouncing
around within a
performed group,
etc….
Note: this code still
works. Handle with
care!!!
9-12
PERFORM UNTIL (traditional)
Format
 PERFORM [paragraph-name-1]
UNTIL condition-1
• Repeats statements in paragraph until
condition is true
• Called iteration or loop
9-13
In-Line PERFORM UNTIL
• No paragraph name follows PERFORM
• Instead statements to be repeated
placed between PERFORM UNTIL …
END-PERFORM
9-14
PERFORM para UNTIL
(traditional PERFORM)
• A paragraph name is included in the
PERFORM
• Instead statements to be repeatedly
executed in the PERFORMed
paragraph until some condition
becomes true.
• a MainStay of modern structured
programming.
9-15
0000-MAIN-MODULE..
PERFORM 1000-INITIALIZATION

PERFORM 2000-PROCESS-RENTALS
UNTIL F-MORE-RECORDS = “N”
PERFORM 3000-TERMINATION
9-16
Looping Principles
Coding a Loop with PERFORM
• Often want to perform some action a
certain number of times
• Use a field as a counter to count
number of times action is repeated
• Set field to zero initially, then increment
it by 1 each time action repeated
• When field equals number of times
action is to be repeated, condition is
met and loop ends
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 ALL LOOPS MUST:
• Have their loop control variable / condition
initialized prior to entering loop or as part of
the loop control itself.
• Controlling variable’s value / values must be
altered in each iteration of the loop
• There must be a test to determine when to
exit the loop.
• Most loops have a single pre-test and
multiple post-tests.
– Implies one may never enter the loop, but once in,
control will be checked EACH iteration looking to
terminate the loop.
9-18
Loop Example
• Display the message 'Hello' 3 times
Move Zeros To Count
Perform Until Count = 3
Display 'Hello'
Add 1 To Count
End-Perform
Note: initialization outslide of loop, pre-test,
body of loop, adjusting control variable,
post-test…
9-19
Loop Example
• Count initialized to zero so not equal to 3
when condition checked first time (pre-test)
• Hello displayed on screen and Count
incremented to 1 (variable adjusted)
Move Zeros To Count
Perform Until Count = 3
Display 'Hello'
Add 1 To Count
End-Perform
Repeated post-tests, increments, and executions.
9-20
Loop Example
• Condition checked again and still not
met (1 not equal to 3) so in-line
statements repeated again
• ‘Hello’ displayed second time and Count
incremented to 2
Move Zeros To Count
Perform Until Count = 3
Display 'Hello'
Add 1 To Count
End-Perform
9-21
Loop Example
• Count not equal to 3 so ‘Hello’ displayed
third time and Count incremented to 3
• Condition met when checked again so
loop ends
Move Zeros To Count
Perform Until Count = 3
Display 'Hello'
Add 1 To Count
End-Perform
9-22
Coding a Loop
• Precede loop by instruction to initialize
field to be tested
• Include PERFORM UNTIL …that
repeats until field tested reaches
desired value
• Include instruction in loop to change
value of field tested so that condition is
eventually met
9-23
Condition Tested First
• Condition tested before paragraph or inline statements executed even once
• If condition met on first test, paragraph
or statements executed zero times
Example
Move 6 To X
Perform 300-Process-Rtn
Until X > 5
Paragraph
executed 0 times
Showing pre-test.
9-24
Ending PERFORM UNTIL
• Loop stops when condition is true
• One of instructions in loop should
change identifier used in condition
Move 0 To Y
Example
Perform Until Y > 10
. . .
Add 1 To Y
Changes Y so condition
End-Perform
eventually met
In-line PERFORM MUST have scope
terminator!
9-25
Avoid Loop Errors (infinite loop)
• Consider this loop
Move Zeros To Count
Perform Until Count = 5
Display Out-Message
End-Perform
• Error occurs because no instruction
included to change Count from zero
• DISPLAY executed over and over again
because condition never met
9-26
Avoid Loop Errors
• Loop that executes repeatedly without
end called infinite loop
• On mainframe program automatically
terminated after fixed period of time
• On PCs press interrupt keys (e.g.,
Escape key, Ctrl + Break keys) to
terminate program
• May see program freeze or scroll
‘forever…’
9-27
Alternative Loop
Move 1 To Count
Perform Until Count > 3
Display 'Hello'
Add 1 To Count
End-Perform
• Initialization value for Count now 1
• Condition uses '>' instead of '='
9-28
Alternative Loop
• Loop still displays Hello 3 times when
Count = 1, 2 and 3
• When Count = 4, loop condition met
• Testing for '>' or '>=' less prone to
infinite loops than testing for '='
• If value of Count exceeds 3 but is never
exactly equal to 3, loop will still
terminate
9-29
PERFORM … TIMES
• Executes a sequence of steps a fixed
number of times
• No counter needed
• Loop below executes paragraph 300Print-Rtn 5 times
Perform 300-Print-Rtn 5 Times
9-30
PERFORM … TIMES
• May use field whose value represents
number of times to repeat loop
• Field must be numeric, containing only
positive integers or 0
• Loop below performs 300-Print-Rtn ten
times
Move 10 To How-Many
Perform 300-Print-Rtn (perform para)
How-Many Times
9-31
PERFORM … TIMES
• Also used with in-line loop
• Loop below executes MULTIPLY
statement 3 times
Move 2 To Num
Perform 3 Times (in-line perform)
Multiply 2 By Num
End-Perform
• Num equals 16 when loop ends
9-32
Loop Example
Sum even integers from 2 through 10
• Initialize a field to first number to be
added (2)
• Increment field by 2 so it equals even
numbers (2, 4, 6, 8, 10)
• Use this field's value to
– Test in condition
– Add to a total field to find sum
9-33
Code for Loop Example
• Sum even integers from 2 through 10
Move 0 To Total
Move 2 To Count
Perform Until Count > 10
Add Count To Total
Add 2 To Count
End-Perform
Display 'Total=', Total
Initialize field to be tested
Test field until it
reaches desired value
Change field tested so
condition eventually met
Result: Total = 30
How many times does loop iterate? What is value of Count9-34
when loop terminates??? (test questions.)
Nested PERFORMs
• One of statements in PERFORM loop
may be another PERFORM loop
• A loop within another loop is called a
nested loop
9-35
Nested PERFORM Example
• Assume 50 records will be read in as 5
groups of 10 records
• Amount fields of each group are to be
added and a total printed
• Five totals, one for each group of 10
records will be printed
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Nested PERFORM Pseudocode
Perform 5 Times
Outer loop
Perform 10 Times
Inner loop
Read record from file and
add its amount to group total
End-Read
End-Perform
Perform Print-Group-Total
End-Perform
Used a lot for ‘control breaks’ (chapter 10).
9-37
Nested PERFORM Example
•  Outer loop repeats these steps 5 times
– Performs inner loop to read in 10 records
– Prints group total
•  Inner loop repeated 50 Times or 10 times
each time outer loop is repeated
•  Notice that step to print group total is not
part or inner loop
– Executed only 5 times or once each time outer
loop executed
9-38
TIMES vs UNTIL
• Use PERFORM … TIMES if you know in
advance the number of times loop statements
are to be executed
• Use PERFORM … UNTIL if number of times
loop repeated is needed for output or
calculations
• Very VERY oftentimes, you do NOT know the
number of times you will need to loop,
though…
9-39
 PERFORM VARYING
Format
PERFORM VARYING identifier-1
identifier-2
identifier-3
FROM
BY
integer-1
integer-2
UNTIL condition-1
statement-1 …
END-PERFORM
This guy does it all! (super for array
processing!)
9-40
PERFORM VARYING
• Repeatedly executes statements in loop while
varying value of a field
• First identifier-1 is given FROM value
– That is, initialization!
• Condition then tested (pre-test first time)
• Executes statements in loop if condition not
met
• Then adds BY value to identifier-1 (that is,
increments/adjusts control variable) and
repeats condition test (post test – may be
executed many times.)
9-41
PERFORM VARYING Example
Perform Varying Ctr From 1 By 1
Until Ctr > 5
Display 'Ctr = ', Ctr
End-Perform
• Sets Ctr to 1, since Ctr > 5 not true, executes
DISPLAY statement
• Increments Ctr by 1, tests condition again
• How many times does loop iterate? Value of
9-42
Ctr when loop terminates?
PERFORM VARYING Execution
CTR
1
2
3
4
5
6
Condition
1 > 5 false
2 > 5 false
3 > 5 false
4 > 5 false
5 > 5 false
6 > 5 true
Output
Ctr = 1
Ctr = 2
Ctr = 3
Ctr = 4
Ctr = 5
(loop ends)
9-43
PERFORM VARYING Examples
• Finds sum of odd numbers from 1 to 25
Move 0 To Total
Perform Varying Ctr From 1 By 2
Until Ctr > 25
Add Ctr To Total
End-Perform
Output:
Display 'Total = ', Total
Total = 169
Value of control variable at end? Number of iterations?
9-44
PERFORM VARYING Examples
• Statements to be repeated may also be
in separate paragraph
Perform 300-Process-Rtn
Varying Ctr From 1 By 1
Until Ctr > 20
• Executes 300-Process-Rtn 20 Times
9-45
Nested PERFORM VARYING
• May include a PERFORM VARYING loop as
one of statements in another PERFORM
VARYING loop
• Each time outer loop is repeated, inner loop
is executed until its condition is met
• Following example prints the times tables for
numbers 1 to 9
• Nested performs and perform…varyings
are used extensively in array processing.
9-46
Print Times Tables
Perform Varying N1 From 1 By 1
Outer loop
Until N1 > 9
Perform Varying N2 From 1 By 1Inner loop
Until N2 > 9
Compute Product = N1 * N2
Display N, ' * ' M ' = ', Product
End-Perform
End-Perform
Notice indentation and alignment!
9-47
Print Times Tables Execution
N1
1
1
...
1
2
2
...
2
N2
1
2
...
9
1
2
...
9
Output
1*1=1
1*2=2
...
1*9=9
2*1=2
2*2=4
...
2 * 9 = 18
Outer loop
time
first
Inner loop
repeats 9 times
Outer loop
second time
Inner loop
repeats 9 times
9-48
Print Times Tables Execution
• Outer loop repeated seven more times
• Each time, statements in inner loop are
repeated 9 times
– N2 initialized to 1 and incremented by 1
each time through inner loop until N2 > 9
• Outer loop ends after printing 9's table
9-49
PERFORM UNTIL loop
• Condition tested before statements in
loop executed first time
• If condition met on first test, statements
not executed at all
• Can specify that condition be tested
after instructions executed first time
• Then instructions always executed at
least once
9-50
PERFORM WITH TEST AFTER
Format
PERFORM [paragraph-name-1]
BEFORE
[WITH TEST
]
AFTER
UNTIL condition-1
Same as accustomed Perform, but with
no pre-test!
9-51
TEST AFTER Example
Example
Perform With Test After
Until Opt-Num >=1 And <= 5
Display 'Select option (1-5)'
Accept Opt-Num
End-Perform
See where this might be used?
9-52
TEST AFTER Example
• Condition is not checked before loop
begins
• DISPLAY and ACCEPT for user to enter
Opt-Num always executed at least once
• Checks Opt-Num after user types in
value for first time
• If Opt-Num not a value from 1 to 5, loop
is repeated
9-53
Chapter Summary
• Formats of PERFORM Statement
– Simple PERFORM
• In-Line PERFORM
PERFORM … END-PERFORM
• PERFORM paragraph-name-1
[THRU paragraph-name-2]
• Causes execution of instructions in named
paragraph(s)
• After paragraph executed, control returned to
statement after PERFORM
9-54
Chapter Summary
• Formats of PERFORM Statement
– PERFORM UNTIL repeats instructions until
a condition is met
• Condition may be tested before or after
instructions are executed
– PERFORM … TIMES
• Use when you know exact number of times
loop statements are to be executed
9-55
Chapter Summary
• Formats of PERFORM Statement
– PERFORM VARYING
• Automatically initializes and changes value of
loop counter
• Nested PERFORMS (PERFORM
statements within PERFORM
statements) allowed
9-56
Chapter Summary
• In-line PERFORMs permitted with all
PERFORM options
– Code does not need to be in separate
paragraph
– Terminated with END-PERFORM
9-57
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9-58