Easytrieve DO Control Flow

DO WHILE Versus DO UNTIL in Practice

Counted Loop Pattern

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CTR = 1
DO WHILE CTR LE 12
  ADD AMT(CTR) TO WS-YEAR-TOTAL
  CTR = CTR + 1
END-DO

Twelve pay periods summed without twelve ADD statements. CTR is INDEX or numeric working storage. Initialize before DO; increment inside; condition uses LE for inclusive upper bound. Off-by-one bugs use LT 12 instead of LE 12—drops last period.

Array Walk With INDEX

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DEFINE SLOT-IX W 3 N 0
DEFINE FLAGS   W 1 A OCCURS 50 INDEX SLOT-IX
 
SLOT-IX = 1
DO WHILE SLOT-IX LE 50
  IF FLAG(SLOT-IX) EQ 'Y'
    ADD 1 TO ACTIVE-CNT
  END-IF
  SLOT-IX = SLOT-IX + 1
END-DO

Set SLOT-IX before referencing FLAG element. Each pass tests one slot. IF inside DO filters without separate loops. ACTIVE-CNT aggregates result after full walk.

Nested DO for Row and Column

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ROW-IX = 1
DO WHILE ROW-IX LE 5
  COL-IX = 1
  DO WHILE COL-IX LE 10
    PROCESS-CELL
    COL-IX = COL-IX + 1
  END-DO
  ROW-IX = ROW-IX + 1
END-DO

Inner END-DO closes column loop before outer ROW-IX increments. Five times ten iterations. Reset COL-IX inside outer body each row. Deep nesting beyond three levels suggests procedure extraction.

DO UNTIL Validation Pattern

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DO UNTIL WS-CODE NUMERIC
  DISPLAY 'ENTER NUMERIC CODE'
  ACCEPT WS-CODE
END-DO

Body runs at least once—user always sees prompt. UNTIL WS-CODE NUMERIC exits when class test passes. Batch jobs use UNTIL less than WHILE but SCREEN-style logic maps here. Online ACCEPT details vary by environment—pattern is bottom-tested retry.

Early Exit With GOTO

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SLOT-IX = 1
DO WHILE SLOT-IX LE 100
  IF KEY(SLOT-IX) EQ TARGET
    MOVE DESC(SLOT-IX) TO WS-RESULT
    GOTO DONE-WALK
  END-IF
  SLOT-IX = SLOT-IX + 1
END-DO
DONE-WALK. CONTINUE

Linear search through OCCURS when TABLE not used. GOTO DONE-WALK after END-DO label skips remaining iterations when key found early. For large sorted data prefer TABLE SEARCH over DO linear walk—performance differs sharply.

DO With IF — Structure Rules

END-DO must close DO before END-IF closes outer IF when IF wraps DO. Correct nesting: IF ... DO ... END-DO END-IF. Placing END-IF before END-DO is a compile error. Indent consistently.

Infinite Loop Prevention

• Every DO WHILE must change a variable in the condition somewhere in the body—or exit via GOTO.
• Verify increment direction matches condition—IDX = IDX + 1 with WHILE IDX LT max.
• Test empty array: WHILE may run zero times if INDEX starts above OCCURS—often correct.
• QA with small OCCURS before production million-iteration mistake.

DO Versus JOB INPUT

Primary file processing uses JOB INPUT name which implies read loop. DO does not replace GET for sequential extracts. Use DO for working storage iteration, retry blocks, and algorithmic steps after record is already in buffer.

Common DO Mistakes

Explain It Like I'm Five

DO is doing something again and again. WHILE is checking the rules before each turn—if the music is still playing, dance another spin. UNTIL is dancing once then checking if the bell rang—so you always dance at least once. END-DO is when the music stops. INDEX is which numbered hopscotch square you jump on each turn. If you forget to move to the next square, you hop forever on the same one—that is an infinite loop.

Exercises

1. Write DO WHILE summing ten OCCURS amounts with CTR.
2. Convert to DO UNTIL when first slot must be read before test.
3. Add nested DO 3×4 grid with ROW-IX and COL-IX.
4. Insert GOTO early exit when TARGET found in array walk.
5. Identify off-by-one error in DO WHILE CTR LT 12 versus LE 12.