Deposit Multiplier

This program calculates how many years it takes for a deposit to grow by a given multiplication factor at a specified annual interest rate. The user inputs an interest rate (as a percentage) and a target growth factor, and the program iterates year by year using compound interest via the exponentiation operator (`**`), counting periods until the accumulated amount exceeds the factor. After displaying the result, it pauses briefly and then reruns automatically. The program uses a simple iterative loop rather than the direct logarithmic formula, recalculating the full power `I**B` on each iteration rather than incrementally multiplying.

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Program Analysis

Program Structure

The program is organized into three logical phases:

1. Input phase (lines 10–45): Prompts the user for an annual interest rate and a growth factor, echoing each entry with a PRINT statement after input.
2. Computation loop (lines 50–130): Converts the interest rate to a multiplier, then iterates year-by-year counting until the compounded amount exceeds the target factor.
3. Output and restart (lines 200–240, 9999): Displays results, pauses, clears the screen, and reruns automatically.

Algorithm

Line 50 transforms the percentage rate I into a compound interest multiplier: I = 1 + (I / 100). The loop then evaluates A = I ** B at each year B, incrementing B until A exceeds the target factor F. This is a brute-force approach — rather than computing B = LOG(F) / LOG(I) directly, it recalculates the full power from scratch on every iteration. For typical interest rates and modest growth factors the loop terminates quickly, so performance is not a practical issue.

Key BASIC Idioms

• PRINT ,, and PRINT ,,,, — use comma-separated print positions to indent output across tab stops, a common layout technique.
• Echo printing after INPUT (lines 25 and 45) — restates the user’s entry, useful on systems where the input value may scroll or be unclear.
• PAUSE 40000 — a long pause (approximately 11 minutes on a 50 Hz machine) effectively waits for user acknowledgement before continuing; combined with the subsequent RUN, it acts as a soft “press any key to restart.”
• RUN at line 240 (and line 9999) restarts the program completely, clearing all variables, creating a continuous-use kiosk loop.

Notable Techniques and Anomalies

• The exponentiation operator ** is used rather than ^, indicating this is an early or non-Spectrum BASIC dialect.
• Variables A and B are explicitly initialised to zero at lines 60–70, which is redundant after a fresh RUN but would matter if execution ever reached line 60 without a full restart — defensive practice.
• Line 9998 is a SAVE command with the filename DEPOSIT MULTIPLIE%R, where %R represents an inverse-video letter R, serving as an auto-run flag embedded in the filename.
• Line 9999 contains a second RUN, which ensures that if the program is loaded without auto-run it still starts immediately.
• The output at line 200 prints A, the first value of A that exceeds F, meaning it reports the compounded amount one year after the target is first surpassed rather than the threshold crossing year itself — a minor off-by-one in the display of A, though B correctly reflects the number of years elapsed.

Variable Summary

Variable	Role
I	Input interest rate (%), later overwritten with the multiplier 1 + I/100
F	Target growth factor supplied by the user
B	Year counter (loop variable)
A	Computed compounded amount I ** B at each iteration