Credit Rating Interest Rate

This program calculates a recommended loan interest rate by scoring an applicant across eight weighted criteria: loan amount, repayment term, salary, job history, job security, credit rating, existing debts, and the prevailing prime interest rate. Each criterion contributes a penalty point value to a running total stored in variable I, and the final score maps to one of four interest rate bands: 14%, 17%, 19%, or 22%. Input values are echoed back to the screen immediately after each INPUT statement using a paired PRINT, a common ZX81/TS1000 idiom to confirm keyboard entry on the display. The branching logic uses a cascade of IF/GOTO statements rather than structured conditionals, with separate lines setting each sub-score variable (A1 through H1) before they are summed at line 840.

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Program Analysis

Program Structure

The program is organised as a linear sequence of input-and-score blocks, one per financial criterion, followed by a summation and decision phase. Each block follows the same pattern: prompt, INPUT, echo PRINT, then a chain of IF/GOTO tests that branch to a line setting a sub-score variable, which then falls through or GOTOs the next block’s entry point.

1. Lines 10–79: Loan amount (variable A, score in A1)
2. Lines 80–179: Repayment term selection (variable B, score in B1)
3. Lines 180–309: Applicant salary (variable C, score in C1)
4. Lines 310–409: Number of jobs in last 5 years (variable D, score in D1)
5. Lines 410–509: Job security rating (variable E, score in E1)
6. Lines 510–639: Credit rating (variable F, score in F1)
7. Lines 640–739: Other debts (variable G, score in G1)
8. Lines 740–839: Prime interest rate (variable H, score in H1)
9. Lines 840–1000: Summation, output, and STOP

Scoring Model

Each criterion contributes a penalty point value; higher points indicate higher risk. The eight sub-scores are summed into I at line 840. The total is then tested against three thresholds to select an interest rate:

Score range	Interest rate
≤ 33	14 PCT.
34 – 59	17 PCT.
60 – 99	19 PCT.
≥ 100	22 PCT.

The sub-score ranges for each criterion are summarised below:

Criterion	Variable	Low penalty	High penalty
Loan amount	A1	6 (>5000)	12 (≤5000)
Repayment term	B1	4 (12 mo)	12 (24 mo)
Salary	C1	4 (≥20000)	16 (zero)
Job count (5 yr)	D1	3 (1 job)	15 (≥3 jobs)
Job security	E1	3 (excellent)	12 (poor)
Credit rating	F1	4 (excellent)	16 (none)
Other debts	G1	3 (none)	12 (large)
Prime rate	H1	3 (<0.12)	12 (≥0.15)

Key BASIC Idioms

Every INPUT statement is immediately followed by a PRINT of the same variable (e.g. lines 30/31, 100/101). This is a standard idiom on this platform to redisplay the entered value on-screen after the INPUT line scrolls away, giving the user confirmation of what was typed.

The string "INTEREST RATE" is stored in Z$ at line 850 and reused across four PRINT statements (lines 930, 950, 970, 990), avoiding repetition of the literal string and saving a small amount of memory.

The prime interest rate input at line 750 expects a decimal fraction (e.g. .12 for 12%), while all other numeric inputs are integers. No validation or guidance on this convention is given to the user, which could cause confusion.

Notable Techniques and Anomalies

The conditional chain at lines 880–910 contains a logic gap: line 880 tests I<=33, line 890 tests I<60, and line 900 tests I<=99. The boundary between the first and second band is therefore 34–59 (exclusive of 60), but the boundary between the second and third bands is ambiguous: a score of exactly 60 falls through line 890’s test (since 60 is not <60) and is caught by line 900. This is consistent, but the asymmetric use of <= versus < could mislead a reader.

Line 830 carries the REM comment TOTAL UP POINTS but execution reaches line 840 directly regardless, since line 820 sets H1=6 and falls through. The REM at 830 is never branched to intentionally; it simply occupies space between the last H1 assignment and the summation line.

The cascade at lines 330–400 for job count has no ELSE or default: if D is zero or negative, none of the three IF tests match and D1 is never set, leaving it at its previous value or zero. Similarly, lines 430–500 for job security and 540–630 for credit rating can leave their variables unset if the user enters an out-of-range value, which would silently corrupt the total.

Line 1020 (LIST) is unreachable after the SAVE at line 1010 and the STOP at line 1000; it has no effect during normal execution.