1. 9-1 Iteration: Beyond the Basic PERFORM Chapter 9

2. 9-2 Simple PERFORM PERFORM [paragraph-name-1] Executes all instructions in named paragraph Then transfers control to instruction following PERFORM Format

3. 9-3 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

4. 9-4 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.

5. 9-5 In-Line PERFORM PERFORM. END-PERFORM Use when only a few statements are to be executed Modularize with PERFORM paragraph- name when many statements required Statements to be executed Format

6. 9-6 Nested PERFORM PERFORM may be one of instructions in range of another PERFORM … Perform 200-Paragraph …... 200-Paragraph. … Perform 500-Paragraph Nested PERFORM

7. 9-7 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. Nested performs

8. 9-8 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. 9-9 Executing Group of Paragraphs PERFORM paragraph-name-1 THROUGH THRU paragraph-name-2 Use expanded format to execute all statements, including other paragraphs, from paragraph-name-1 through paragraph-name-2 Format

10. 9-10 GO TO Statement GO TO paragraph-name-1 Does not return control to following statement like PERFORM Transfers control permanently to another paragraph Format

11. 9-11 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

12. 9-12 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. The old way: but such code still clearly lives and breathes!!! 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!!!

13. 9-13 PERFORM UNTIL (traditional)  PERFORM [paragraph-name-1] UNTIL condition-1 Repeats statements in paragraph until condition is true Called iteration or loop Format

14. 9-14 In-Line PERFORM UNTIL No paragraph name follows PERFORM Instead statements to be repeated placed between PERFORM UNTIL … END-PERFORM

15. 9-15 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.

16. 9-16 0000-MAIN-MODULE.. PERFORM 1000-INITIALIZATION  PERFORM 2000-PROCESS-RENTALS UNTIL F-MORE-RECORDS = “N” PERFORM 3000-TERMINATION

17. 9-17 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

18. 9-18  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.

19. 9-19 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…

20. 9-20 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.

21. 9-21 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

22. 9-22 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

23. 9-23 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

24. 9-24 Condition Tested First Condition tested before paragraph or in- line statements executed even once If condition met on first test, paragraph or statements executed zero times Move 6 To X Perform 300-Process-Rtn Until X > 5 Example Paragraph executed 0 times Showing pre-test.

25. 9-25 Ending PERFORM UNTIL Loop stops when condition is true One of instructions in loop should change identifier used in condition Move 0 To Y Perform Until Y > 10... Add 1 To Y End-Perform In-line PERFORM MUST have scope terminator! Example Changes Y so condition eventually met

26. 9-26 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

27. 9-27 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…’

28. 9-28 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 '='

29. 9-29 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

30. 9-30 PERFORM … TIMES Executes a sequence of steps a fixed number of times No counter needed Loop below executes paragraph 300- Print-Rtn 5 times Perform 300-Print-Rtn 5 Times

31. 9-31 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

32. 9-32 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

33. 9-33 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

34. 9-34 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 Count when loop terminates??? (test questions.)

35. 9-35 Nested PERFORMs One of statements in PERFORM loop may be another PERFORM loop A loop within another loop is called a nested loop

36. 9-36 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

37. 9-37 Nested PERFORM Pseudocode Perform 5 Times Perform 10 Times 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). Outer loop Inner loop

38. 9-38 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

39. 9-39 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…

40. 9-40  PERFORM VARYING PERFORM VARYING identifier-1 identifier-2identifier-3 FROM BY integer-1integer-2 UNTIL condition-1 statement-1 … END-PERFORM This guy does it all! (super for array processing!) Format

41. 9-41 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.)

42. 9-42 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 Ctr when loop terminates?

43. 9-43 PERFORM VARYING Execution CTRCondition Output 11 > 5 falseCtr = 1 22 > 5 falseCtr = 2 33 > 5 falseCtr = 3 44 > 5 falseCtr = 4 55 > 5 falseCtr = 5 66 > 5 true(loop ends)

44. 9-44 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 Display 'Total = ', Total Output: Total = 169 Value of control variable at end? Number of iterations?

45. 9-45 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

46. 9-46 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.

47. 9-47 Print Times Tables Perform Varying N1 From 1 By 1 Until N1 > 9 Perform Varying N2 From 1 By 1 Until N2 > 9 Compute Product = N1 * N2 Display N, ' * ' M ' = ', Product End-Perform Notice indentation and alignment! Outer loop Inner loop

48. 9-48 Print Times Tables Execution N1N2 Output 111 * 1 = 1 121 * 2 = 2......... 191 * 9 = 9 212 * 1 = 2 222 * 2 = 4......... 292 * 9 = 18 Outer loop first time Inner loop repeats 9 times Outer loop second time Inner loop repeats 9 times

49. 9-49 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

50. 9-50 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

51. 9-51 PERFORM WITH TEST AFTER PERFORM [paragraph-name-1] BEFORE [WITH TEST ] AFTER UNTIL condition-1 Same as accustomed Perform, but with no pre-test! Format

52. 9-52 TEST AFTER 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? Example

53. 9-53 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

54. 9-54 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

55. 9-55 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

56. 9-56 Chapter Summary Formats of PERFORM Statement –PERFORM VARYING Automatically initializes and changes value of loop counter Nested PERFORMS (PERFORM statements within PERFORM statements) allowed

57. 9-57 Chapter Summary In-line PERFORMs permitted with all PERFORM options –Code does not need to be in separate paragraph –Terminated with END-PERFORM

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