The Utility Billing Loop That Skips a Customer

Defects visible in the test run

1. Off-by-one loop bound. count starts at 0 and the condition is count <= numberOfCustomers, so the body runs n+1 times. With three customers it reads a fourth, non-existent record - the nonsense bill - and then exits.
2. Average divides by the wrong count. After the loop count equals n+1, and total contains the garbage bill, so the average is wrong twice over.
3. Division by zero. With zero customers the loop still executes once and then divides by a zero count; the design needs an explicit guard for an empty route.

Defects the test run did not expose

1. Integer rate. Declaring rate as an integer truncates 0.45, 0.60 and 0.85 to 0, so every bill is just the standing charge. Consumption and money should be real (or integer cents), never integer units.
2. No validation of readings. If the current reading is lower than the previous one - a replaced meter, a transposed digit, an estimated reading - consumption is negative and the bill becomes a credit. The program must detect and flag rather than compute.
3. No rounding policy. Repeated floating-point addition produces values like 47.300000000000004. Round each bill to two decimal places at the point of printing, and state whether totals use rounded or unrounded values.
4. Ambiguous tier semantics. The spec does not say whether the rate applies to all units or progressively. The student's code applies the rate to the whole consumption; if the intention is marginal tiers, three separate calculations are needed.
5. No handling of non-numeric or missing input, and no error path for a customer who should be skipped.

Diagnosis discipline: a test that shows one symptom usually hides others; the boundary values in the next task are what surface them.

SET total ← 0
SET billed ← 0
READ numberOfCustomers

IF numberOfCustomers <= 0 THEN
    OUTPUT "No customers on this route"
ELSE
    FOR i ← 1 TO numberOfCustomers
        READ customerName, previous, current

        IF current < previous THEN
            OUTPUT customerName, "Invalid reading: current below previous, bill withheld"
        ELSE IF current or previous is not numeric THEN
            OUTPUT customerName, "Invalid input, bill withheld"
        ELSE
            SET used ← current - previous

            IF used <= 100 THEN
                SET rate ← 0.45
            ELSE IF used <= 200 THEN
                SET rate ← 0.60
            ELSE
                SET rate ← 0.85
            ENDIF

            SET bill ← ROUND(used * rate + 12.00, 2)
            OUTPUT customerName, used, bill
            SET total ← total + bill
            SET billed ← billed + 1
        ENDIF
    NEXT i

    IF billed = 0 THEN
        OUTPUT "No bills produced"
    ELSE
        OUTPUT "Average bill: ", ROUND(total / billed, 2)
    ENDIF
ENDIF

Reasoning for the changes

• A counted FOR i ← 1 TO n loop removes the off-by-one error by construction; the bound is stated once.
• billed counts only accepted bills, so invalid records do not distort the average, and the empty-route case is handled before any division.
• Validation precedes calculation: a meter reading that goes backwards is a data problem, and producing a negative bill silently hides it from the office.
• Rates are real numbers; rounding happens once, at output, so printed bills and the printed average agree.
• The tier logic still applies one rate to all units. If the utility intends marginal tiers, replace the single calculation with three: charge the first 100 units at 0.45, units 101–200 at 0.60, and the remainder at 0.85, summing the parts. That is a requirements decision to confirm with the client, not something the programmer should assume.

Boundary and typical cases (single customer route unless stated)

Previous	Current	Units	Purpose	Expected result
1000	1050	50	Ordinary case inside the lowest band	50 × 0.45 + 12.00 = 34.50
1000	1099	99	Just below the first boundary	56.55
1000	1100	100	Exactly on the first boundary	57.00
1000	1101	101	Just above the first boundary	72.60
1000	1200	200	Exactly on the second boundary	132.00
1000	1201	201	Just above the second boundary	182.85
1000	1000	0	Zero consumption; standing charge only	12.00
1500	1400	negative	Current below previous (meter replacement or typo)	Bill withheld, message printed
-	-	non-numeric	Text in a reading field	Bill withheld, message printed
route with 0 customers			Empty input	"No customers on this route", no division
route with 3 valid + 1 invalid			Average must ignore the rejected record	Average of the three valid bills
1000.5	1100.25	fractional	Real readings and rounding check	Bill rounded to 2 dp

Justification

• Boundary values (99, 100, 101, 200, 201) are where conditional logic changes branch. Errors cluster at the comparison operator - <= versus < - and a test at 150 would never reveal it.
• Zero units proves the standing charge is applied independently of consumption.
• Invalid readings test the error path, which is the part most often written but never executed.
• Empty route tests the division-by-zero guard.
• Mixed valid and invalid records checks that billed rather than numberOfCustomers drives the average - the defect the original test run hid.
• Fractional readings check rounding. When reporting, state the expected output for each row before running, so the test is a comparison rather than an interpretation of whatever appears.