Miles Per Gallon

This program calculates and prints a monthly miles-per-gallon report for a car, taking inputs for miles driven, gallons used, cost per gallon, business miles, and IRS mileage allowance. It computes MPG, total fuel cost, and a tax deduction figure, then sends a formatted summary to a printer via LPRINT. An optional branch handles fuel and oil additive costs, calculating per-use cost by dividing bottle price by a fixed 12-use assumption for gas additives and by oil-change interval in miles for oil additives.

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Program Analysis

Program Structure

The program is divided into a clear sequence of functional blocks:

1. Lines 10–50: Title screen with a brief delay loop (FOR T=1 TO 200).
2. Lines 60–184: Main input section — month label, miles, gallons, cost per gallon, business miles, IRS allowance. Core calculations (MPG, TCG, DED) are performed here.
3. Lines 185–189: Branch point asking whether additives are used; “Y” routes to the additive subroutine at line 400, “N” falls through to line 190.
4. Lines 190–270: LPRINT block producing the printed monthly report.
5. Lines 275–395: Secondary oil-additive question and cost summary via LPRINT, with two branches (additive used or not).
6. Lines 400–690: Subroutine for additive cost calculations — gas additive (line 500) and oil additive (line 600).
7. Lines 1000–1060: Reprint option, housekeeping (CLEAR), and auto-run SAVE.

Calculations

Variable	Formula	Meaning
MPG	M/G	Miles per gallon
TCG	C*G	Total cost of fuel
DED	A*R	IRS mileage deduction
CO	AA/BB (BB=12)	Gas additive cost per use (assumes 12 uses per bottle)
DD	G/12.5	Number of gas additive doses used
CM	DD*CO	Monthly gas additive cost
MO	CC/EE	Oil additive cost per mile
MT	MO*M	Monthly oil additive cost
TCA	CM+MT	Total additive cost (both types)
TOC	TCA+TCG	Total operating cost with both additives
TB	CM+TCG	Total operating cost with gas additive only

Control Flow and Notable Idioms

The program uses paired IF … THEN GOTO / IF … THEN GOSUB constructs for all branching — there is no ELSE. The additive section at line 400 is entered via GOSUB 400 (line 188) but returns indirectly: rather than RETURN, every path eventually executes GOTO 190 (lines 470, 690), effectively jumping back into the main body. This means the subroutine call stack is never unwound cleanly, and the GOSUB return address is silently abandoned.

Input echoing is performed after every INPUT statement (e.g., lines 101, 131, 162) with a bare PRINT of the variable, a common technique on systems where the input cursor does not automatically leave a clear line.

Bugs and Anomalies

• Variable reuse — A: A is first used at line 172 for business miles driven, then reused as the loop variable in the delay FOR loop — except that loop uses T, so this collision does not actually occur. However, A is later overwritten implicitly if any loop used it; as written it is safe.
• Undefined CM on oil-only path: At line 350, if the user selected no gas additive but went through the oil-additive path, CM is printed as the total additive cost. But if only an oil additive was used (path via line 600 then 190 then 280 then 295→350), CM will be 0 or uninitialized rather than MT. The oil-additive cost MT is computed at line 675 but never added into CM on this path — only TCA=CM+MT at line 300 handles that case.
• Logic gap at lines 188–189: If the user enters neither “Y” nor “N” at line 185, neither branch fires and execution falls through to line 190 silently, skipping any prompt for retry.
• Lines 1040–1060: After the reprint loop, CLEAR is called followed by SAVE and RUN. The SAVE filename contains an inverse-video digit acting as an auto-run flag. RUN at line 1060 restarts the program, making this block reachable only if the user chose not to reprint.

Printed Output

The report sent to the printer via LPRINT includes the month label, miles driven, gallons used, MPG, cost per gallon, total fuel cost, business miles, and the IRS deduction. Additive costs and total operating cost are appended depending on the user’s choices. Blank LPRINT lines are used throughout for spacing, a standard technique for thermal printer formatting.

Display Formatting

The title and introductory screens use inverse-video characters (rendered in the source as %M%P%G etc.) to produce highlighted banner text. This is a purely cosmetic effect achieved without any POKE or machine code — just the inverse character codes built into the character set.