INSPECT count or replace gives wrong result

Troubleshooting guide for INSPECT count or replace gives wrong result. Logic bugs do not abend — the program runs to completion and produces wrong results, so they are found by control totals, not dumps.

What it means

• COBOL control flow (PERFORM, GO TO, CALL) is paragraph-based, so a misplaced period or wrong paragraph range silently changes what executes.
• Parameter bugs between programs (CALL ... USING vs LINKAGE SECTION) corrupt data without any error message.
• Copybook and name collisions cause the compiler to accept code that does something different from what the programmer intended.

Common causes

• Infinite loop: PERFORM UNTIL condition where the condition field is never updated inside the loop, or the update is in a paragraph that is never reached.
• PERFORM PARA-A THRU PARA-B executes every paragraph in between, including paragraphs the programmer thought were separate.
• Nested PERFORM: an inner PERFORM ... THRU range that overlaps the outer range causes paragraphs to execute twice or be skipped.
• CALL USING passes 3 items but the called program LINKAGE SECTION defines 4 — the 4th item reads whatever follows in memory.
• Copybook updated in one program but not recompiled in the caller — layout mismatch between CALLER and CALLEE.
• Duplicate paragraph or data names: the compiler uses the first definition; the second is silently ignored (warning only).
• 88-level condition name with overlapping values, e.g. 88 VALID VALUE 1 THRU 5 followed by 88 ALSO-VALID VALUE 5 — value 5 sets both.
• INITIALIZE WS-GROUP clears every subordinate field including counters and flags the programmer wanted to keep.
• STRING ... WITH POINTER: pointer not reset to 1 before the next STRING, so data is written past the intended position or overflows.
• UNSTRING with ALL delimiter phrase missing: consecutive delimiters create empty items that shift the receiving fields.

How to diagnose

• For loops: DISPLAY a counter every 1000 iterations, or check whether the loop-exit field is actually modified inside the loop body.
• For PERFORM ranges: list the paragraphs between the THRU endpoints — every one of them executes, including fall-through paragraphs after the last one if there is no GOBACK/STOP RUN.
• For CALL mismatches: compare the USING list against the LINKAGE SECTION item by item — count, order, and PIC sizes must match exactly.
• Compile with LIST and XREF and check for duplicate-name warnings; treat every warning as a bug.
• For 88-levels: DISPLAY the field value in hex when the condition misbehaves — the value is often not what the tester assumed.

How to fix it

• Replace PERFORM ... THRU with explicit PERFORM paragraph-name (no THRU) or use inline PERFORM ... END-PERFORM blocks.
• Make CALL USING and LINKAGE SECTION agree exactly; pass a single group item instead of many fields to reduce mismatch risk.
• Recompile every program that copies a changed copybook — automate this in the build so stale modules cannot ship.
• Reset STRING pointers (MOVE 1 TO WS-PTR) before each STRING, and check ON OVERFLOW.
• Split INITIALIZE into targeted MOVEs, or INITIALIZE only the subgroup that really needs clearing.

Example: Safe loop and parameter patterns

Inline PERFORM keeps loop control visible; passing one group item keeps CALL and LINKAGE SECTION in sync.

Interview tip

• Q: A program produces wrong totals but never abends. It uses PERFORM ... THRU and CALLs a subprogram. Where do you look first?
• A: I check the PERFORM THRU range first, because it executes every paragraph between the endpoints — often more than intended. Then I compare the CALL USING list against the subprogram LINKAGE SECTION field by field, since a count or size mismatch silently corrupts data.

Prevention checklist

• Prefer inline PERFORM ... END-PERFORM over paragraph THRU ranges.
• Pass one group item on CALL USING instead of long field lists.
• Compile with SSRANGE and TEST options in QA to catch subscript and logic errors early.
• Reconcile control totals (record counts, hash totals) between input and output on every run.