---
title: "Savings Projection"
id: 57009
type: "computer_media"
slug: "savings-projection"
url: "http://localhost/computer_media/savings-projection/"
markdown_url: "http://localhost/computer_media/savings-projection.md"
published_at: "2024-10-02T07:33:31+00:00"
modified_at: "2026-03-30T21:20:14+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/33_Sav.png"
excerpt: "Enter a starting balance, interest rate, and time horizon to watch compound interest build year by year in this simple savings projection tool."
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/"
  - name: "Home"
    slug: "home"
    taxonomy: "genre"
    url: "http://localhost/type/home/"
media_contents:
  - id: 56732
    title: "Timex Sinclair Public Domain Library Tape 1001"
    type: "computer_media"
    url: "http://localhost/computer_media/timex-sinclair-public-domain-library-tape-1001/"
media_type: "Program"
mediadate: "198x"
images:
  - url: "http://localhost/wp-content/uploads/2024/10/33_Sav.png"
media_type_tags: "Finance, Home"
---

# Savings Projection

This program calculates a savings projection by computing compound interest year by year over a user-specified number of years. The user inputs a present balance, an annual interest rate (as a percentage), and a number of years; the program then prints each year’s running total. The accumulator variable Z tracks cumulative interest separately from the principal B, with the formula Z = Z + I*(Z+B)/100 applied each iteration. After displaying results, the program waits for a keypress and loops back to restart, allowing fresh calculations without re-running the program.

***

## Program Analysis

### Program Structure

The program is organised into three logical phases that cycle continuously:

1. **Initialisation and header** (lines 10–19): clears the screen, prints a title, and draws a row of 18 asterisks as a separator using a `FOR`/`NEXT` loop.
2. **Input collection** (lines 20–110): prompts for and reads the present balance (`B`), interest rate (`I`), and number of years (`Y`), echoing each value back to the screen.
3. **Projection loop** (lines 120–165): iterates `Y` times, updating the accumulated interest in `Z` and printing the year number alongside the total balance.

### Interest Calculation

The core formula at line 130 is:

`LET Z=Z+I*(Z+B)/100`

Here `B` is the original principal (never modified) and `Z` holds the cumulative interest earned to date. Each year, `I` percent is applied to the current total balance `(Z+B)` and added to `Z`. This correctly models annual compound interest without altering the original principal variable. Line 140 then prints the year index `L` and the total balance `Z+B` in two tab-separated columns.

### Key BASIC Idioms

- **Keypress wait**: Line 160 uses the classic busy-wait idiom `IF INKEY$="" THEN GOTO 160` to pause until any key is pressed before looping.
- **Separator banner**: Lines 16–18 build the asterisk border programmatically rather than embedding a literal string, saving a few bytes and making the width easy to change.
- **Continuous loop**: Line 170 jumps back to line 15 (rather than line 10), redrawing the header and banner but skipping the `CLS` — a minor quirk that means subsequent runs do not clear the screen before reprinting the title.

### Notable Techniques

The accumulator design (`Z` for interest, `B` for principal) is a neat separation of concerns: the original balance is always available for the compound calculation without needing to re-enter it. Initialising `Z=0` at line 20 ensures the accumulator is reset correctly on the first pass, but because line 170 returns to line 15 rather than line 20, `Z` is *not* reset on subsequent calculations — see the anomaly note below.

### Bugs and Anomalies

- **Z not reset on repeat**: Line 20 (`LET Z=0`) is only reached on the very first run. When the program loops back via line 170 to line 15, it skips line 20, so `Z` retains its value from the previous projection. This will produce incorrect totals on any second or subsequent calculation.
- **Incomplete FOR statement**: Line 165 reads `FOR` with no variable, range, or body. This appears to be a corrupt or truncated line — likely a remnant or editing accident. On execution it will cause a syntax error before it is ever reached (it sits between the keypress wait and the `GOTO`, so control flow skips it, but it remains in the listing as dead, malformed code.
- **CLS skipped on repeat**: As noted, the loop target is line 15, not line 10, so the screen is never cleared between successive projections, causing output to accumulate.

### Variable Summary

| Variable | Purpose |
| --- | --- |
| `B` | Initial (present) balance entered by user |
| `I` | Annual interest rate (percent) |
| `Y` | Number of years to project |
| `Z` | Accumulated interest (running total above principal) |
| `L` | Loop counter / year index |
| `G` | Loop counter for asterisk separator |

## Source Code

```
   5 REM %S%A%V%I%N%G%S
  10 CLS 
  15 PRINT "SAVINGS PROJECTION"
  16 FOR G=1 TO 18
  17 PRINT "*";
  18 NEXT G
  19 PRINT 
  20 LET Z=0
  30 PRINT "PRESENT BALANCE: ";
  40 INPUT B
  50 PRINT B
  60 PRINT "INTEREST RATE: ";
  70 INPUT I
  80 PRINT I
  90 PRINT "NUMBER OF YEARS: ";
 100 INPUT Y
 110 PRINT Y
 120 FOR L=1 TO Y
 130 LET Z=Z+I*(Z+B)/100
 140 PRINT L,Z+B
 150 NEXT L
 159 PRINT "FOR MORE,PRESS ANY KEY"
 160 IF INKEY$="" THEN GOTO 160
 165 FOR 
 170 GOTO 15
 200 SAVE "1003%3"
 300 RUN
```