This program calculates the maximum home purchase price a buyer can afford based on their down payment, maximum monthly payment, interest rate, and loan term. It uses the standard mortgage present-value formula: the loan amount is derived from the monthly payment divided by the monthly interest factor, rounded down to the nearest $100. The result is displayed alongside a decorative double-rectangle border drawn with PLOT and DRAW commands. Input validation is applied to all fields, rejecting negative values, out-of-range interest rates, and loan terms outside 1–30 years.
Program Structure
The program is organized into three logical phases:
- Initialization / title display —
GO SUB 2000at line 20 draws a decorative border and title before input begins. - Input collection — Lines 30–98 gather four values: down payment, maximum monthly payment, annual interest rate, and loan term in years, each with inline validation loops.
- Calculation and output — Lines 100–240 compute the affordable home price and display a formatted summary;
STOPat line 1999 halts execution.
The subroutine at lines 2000–2010 draws two concentric rectangles using PLOT/DRAW and prints a centred title, acting as a reusable banner called both at startup and after the CLS at line 150.
Mortgage Calculation
The core financial logic implements the present-value annuity formula for a fixed-rate mortgage:
f1 = 1 - (1 + rate/12)^-(12*ye)— the annuity numerator.f2 = f1 / (rate/12)— the full present-value factor.lamount = month * f2— maximum loan principal, then truncated to the nearest $100 viaINT (lamount/100)*100.price = lamount + down— total affordable purchase price, rounded to the nearest dollar usingINT ((price+.5)*100/100).
Rounding lamount down (floor) to $100 keeps the monthly payment safely within budget; rounding price to the nearest dollar is cosmetic.
Input Validation
| Variable | Condition to retry | Line |
|---|---|---|
down | down < 0 | 40 |
month | month < 0 | 70 |
rate | rate < .001 OR rate > 100 | 90 |
ye | INT ye < 1 OR INT ye > 30 | 96–97 |
Note that a down payment or monthly payment of exactly zero is accepted (only negative values are rejected), allowing the program to handle edge cases such as zero-down or interest-only scenarios.
Notable BASIC Idioms
- Boolean string concatenation — Line 220 uses
("s" AND ye>1)to pluralise “year” without a conditional branch, a common Sinclair BASIC idiom where a true condition evaluates to 1 and false to 0 (empty string). - INVERSE 1 highlight — Line 155 prints the calculated price in inverse video for visual emphasis before reverting implicitly.
- Echoing input — After each
INPUT, the accepted value is printed at a fixed screen position (lines 41, 71, 91, 98) to provide a clean summary layout rather than relying on the INPUT prompt remnants.
Image Gallery
Source Code
0
1 REM QUALIFIER (HOMES)
2 REM RESET 1987 BYTE POWER
3 REM BY K. BOISVERT
4
20 GO SUB 2000
30 PRINT AT 5,0;"Maximum down payment:";
40 INPUT "Max:";down: IF down<0 THEN GO TO 40
41 PRINT "$";down
60 PRINT AT 7,0;"Max. monthly payment:";
70 INPUT "Max:";month: IF month<0 THEN GO TO 70
71 PRINT "$";month
80 PRINT AT 9,0;"Interest rate:";
90 INPUT "Interest:";rate: IF rate<.001 OR rate>100 THEN GO TO 90
91 PRINT rate;"%"
95 LET rate=rate/100: PRINT AT 11,0;"In how many years (30 max):";
96 INPUT "Year(s):";ye
97 LET ye=INT ye: IF ye<1 OR ye>30 THEN GO TO 96
98 PRINT ye
100 LET f1=(1-((1+(rate/12))^-(12*ye)))
110 LET f2=f1/(rate/12)
120 LET lamount=month*f2: LET lamount=INT (lamount/100)*100
140 LET price=lamount+down: LET price=INT ((price+.5)*100/100)
150 CLS : GO SUB 2000: PRINT AT 5,0;"You can afford homes that are selling for:";
155 PRINT INVERSE 1;"$";price
160 PRINT '"Mortgage of $";lamount
190 PRINT '"Down payment of $";down
200 PRINT '"Monthly payments of $";month
220 PRINT '"For a period of ";ye;" year"+("s" AND ye>1)
230 PRINT '"At an interest of ";rate*100;"%"
240 PRINT AT 20,0;"'RUN' TO GO BACK TO PROGRAM"
1999 STOP
2000 PLOT 65,156: DRAW 116,0: DRAW 0,14: DRAW -116,0: DRAW 0,-14
2001 PLOT 68,158: DRAW 110,0: DRAW 0,10: DRAW -110,0: DRAW 0,-10: PRINT AT 1,9;"The Qualifier"
2010 RETURN
9999 SAVE "QUALIFIER" LINE 1Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.