Car Payment

This program calculates monthly car loan payments using the standard amortisation formula. It prompts the user for purchase price, down payment, loan term in months, and annual interest rate, then computes the monthly payment rounded to the nearest cent. The interest rate conversion at line 210 contains a notable typo: `0.0$` should be `0.01`, meaning the division by 100 to convert the percentage to a decimal is broken as written. The program loops continuously, allowing recalculation by pressing any key after displaying the result.

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Program Analysis

Program Structure

The program is organised into a simple sequential flow with a loop back to the start for repeated calculations:

1. Lines 5–60: Title banner with a decorative row of asterisks
2. Lines 100–201: Four INPUT prompts collecting purchase price (T), down payment (R), number of months (N), and annual interest rate (I)
3. Lines 210–260: Interest conversion, amortisation calculation, rounding, and display
4. Lines 300–320: Keypress wait loop and restart via GOTO 20
5. Lines 400–500: SAVE with auto-run flag and RUN sentinel

The Amortisation Formula

Line 230 implements the standard fixed-rate loan payment formula:

P = (T - R) * I / (1 - 1 / (1 + I) ** N)

where T - R is the principal (purchase price minus down payment), I is the monthly interest rate, and N is the number of months. The ** operator is used for exponentiation, which is standard BASIC syntax here.

Key Bug: Broken Interest Conversion

Line 210 reads:

LET I=(0.0$*I)/12

The token 0.0$ is clearly a typo for 0.01, the multiplier needed to convert an annual percentage (e.g. 12) to a decimal (0.12) before dividing by 12 to obtain the monthly rate. As written, this line would cause a syntax error at runtime. The correct form should be:

LET I=(0.01*I)/12

Notable Techniques

• Echo-printing inputs: Lines 111, 141, 171, and 201 each PRINT the just-entered value immediately after INPUT. On ZX81/TS1000 hardware, INPUT erases its own prompt line on acceptance, so reprinting confirms the value entered.
• Penny rounding: Line 240 uses INT(100*P+0.5)/100 to round the payment to the nearest cent — a common fixed-point rounding idiom in BASIC.
• Keypress polling loop: Lines 310–320 use IF INKEY$="" THEN GOTO 310 to spin-wait for any key before restarting, a typical ZX81 idiom.
• Asterisk separator: Lines 30–50 use a FOR loop to print 11 asterisks as a decorative divider, avoiding a long literal string.

Variable Summary

Variable	Purpose
T	Total purchase price
R	Down payment (deposit)
N	Number of monthly payments
I	Annual interest rate (input), then converted to monthly decimal rate
P	Computed monthly payment amount
L	FOR loop counter for asterisk banner

Anomalies and Notes

• The REM at line 5 contains the program title in inverse video characters, a common technique for making the listing title stand out.
• The initial LET P=0 at line 10 is unnecessary since P is always assigned at line 230 before use, but it is harmless.
• The SAVE "1003%4" at line 400 uses an inverse 4 character as an auto-run flag encoding the starting line number.