InvTax

INVTAX is a business accounting program that computes and compares net income and income tax liability under two inventory costing methods — FIFO (First In, First Out) and LIFO (Last In, First Out) — across three consecutive years. The user inputs sales, three purchase figures, beginning inventory, operating expenses, and ending inventory values for each method and each year; the program then calculates cost of goods sold and pre-tax income accordingly. Tax computation is delegated to a subroutine at line 700 that implements a five-bracket progressive tax schedule with rates of 17%, 20%, 30%, 40%, and 46%, using INT() to truncate fractional cents. Results are presented in a columnar LIFO/FIFO comparison table for all three years.

---

Program Analysis

Program Structure

The program is organized into four logical phases:

1. Input collection (lines 10–118): prompts for sales (S), three purchase amounts (P1–P3), beginning inventory (B), expenses (E), three FIFO ending inventories (F1–F3), and three LIFO ending inventories (L1–L3). Each INPUT is followed by a PRINT echo for verification.
2. Income calculation (lines 130–180): derives six pre-tax net income figures — N1–N3 for FIFO and N4–N6 for LIFO — using the cost-of-goods-sold formula: Sales − (Opening Inventory + Purchases − Closing Inventory) − Expenses.
3. Tax computation (lines 190–360): calls the tax subroutine at line 700 six times (once per income figure), storing results in T1–T6.
4. Output (lines 500–670): prints a side-by-side LIFO/FIFO table for each of the three years, then halts at line 900.

Tax Subroutine (Line 700)

The subroutine at line 700 implements a five-bracket progressive corporate tax schedule. The income variable I is tested with a cascading series of IF…THEN GOTO guards, and the tax T is computed using base amounts plus a marginal rate on the excess. INT() is used throughout to truncate to whole dollars.

Income Range	Base Tax	Marginal Rate
≤ $25,000	$0	17%
$25,001 – $50,000	$4,250	20%
$50,001 – $75,000	$9,250	30%
$75,001 – $100,000	$16,750	40%
> $100,000	$26,750	46%

This bracket structure corresponds roughly to the US Tax Reform Act of 1978 era corporate rates, consistent with the program’s educational accounting context.

Notable Bugs and Anomalies

The program contains two significant bugs in the after-tax income calculation block (lines 440–490):

• Line 450: LET T2=N2-T2 overwrites the tax variable T2 with the after-tax income for FIFO Year 2. The intended variable was almost certainly I2 (as used at line 610: PRINT …;I2). Because I2 is never assigned, it will print as 0, and the true tax amount T2 is destroyed — corrupting both Year 2 output lines (600 and 610).
• Line 670: The Year 3 output block prints pre-tax income and taxes for LIFO and FIFO (lines 640–650) but is missing the final PRINT "NET INCOME AFTER TAX"; I6; I3 line that would mirror the structure of Years 1 and 2. The program jumps directly to GOTO 900 at line 680, so Year 3 after-tax figures are never displayed.

Key BASIC Idioms

• Input echo via PRINT immediately after each INPUT (e.g., lines 21, 32, 35) provides a paper-trail-style confirmation of entered values.
• The tax subroutine uses the classic Sinclair-BASIC pattern of cascading IF I>threshold THEN GOTO tests falling through to a default case, with all branches converging on a single RETURN at line 765.
• GOSUB 700 / RETURN is used repeatedly to avoid duplicating the tax bracket logic for each of the six income scenarios.
• Line numbers are spaced in multiples of 10 (with occasional fills at ×1 and ×5 intervals) to allow later insertion of lines — a common BASIC maintenance practice.

Program Termination

After output, line 680 branches to STOP at line 900, bypassing lines 910–920. Line 920 (RUN) would restart the program if reached, but is never executed in normal flow.