---
title: "Deposit Multiplier"
id: 57607
type: "computer_media"
slug: "deposit-multiplier"
url: "http://localhost/computer_media/deposit-multiplier/"
markdown_url: "http://localhost/computer_media/deposit-multiplier.md"
published_at: "2024-10-06T00:23:29+00:00"
modified_at: "2026-04-03T07:58:16+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/deposit-multiplier.png"
excerpt: "Enter an interest rate and a target growth factor to find out exactly how many years your deposit takes to multiply — using a brute-force compound interest loop."
category:
  - name: "Archived Media"
    slug: "archived-media"
    taxonomy: "category"
    url: "http://localhost/category/archived-media/"
post_tag:
  - name: "Downloadable"
    slug: "downloadable"
    taxonomy: "post_tag"
    url: "http://localhost/tag/downloadable/"
  - name: "TS 1000"
    slug: "ts1000"
    taxonomy: "post_tag"
    url: "http://localhost/tag/ts1000/"
model:
  - name: "Timex/Sinclair 1000"
    slug: "ts-1000"
    taxonomy: "model"
    url: "http://localhost/model/ts-1000/"
genre:
  - name: "Finance"
    slug: "finance"
    taxonomy: "genre"
    url: "http://localhost/type/finance/"
media_contents:
  - id: 56724
    title: "Synchro-Sette June 1983"
    type: "computer_media"
    url: "http://localhost/computer_media/synchro-sette-june-1983/"
media_type: "Program"
mediadate: "June 1983"
images:
  - url: "http://localhost/wp-content/uploads/2024/10/deposit-multiplier.png"
media_type_tags: "Finance"
---

# 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.

***

## 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 |

## Source Code

```
  10 PRINT ,,"WHAT IS THE INTEREST RATE PER   YEAR?  ";
  20 INPUT I
  25 PRINT I
  30 PRINT "WHAT IS THE INCREASE FACTOR?  ";
  40 INPUT F
  45 PRINT F
  50 LET I=1+(I/100)
  60 LET A=0
  70 LET B=0
 100 LET A=I**B
 110 IF A>F THEN GOTO 200
 120 LET B=B+1
 130 GOTO 100
 200 PRINT ,,,,"THE AMOUNT WOULD BE ";A
 210 PRINT "ITS SIZE IN ";B;" YEARS"
 220 PAUSE 40000
 230 CLS 
 240 RUN 
 9998 SAVE "DEPOSIT MULTIPLIE%R"
 9999 RUN
```