---
title: "Random Bar Chart"
id: 57014
type: "computer_media"
slug: "random-bar-chart"
url: "http://localhost/computer_media/random-bar-chart/"
markdown_url: "http://localhost/computer_media/random-bar-chart.md"
published_at: "2024-10-02T07:38:08+00:00"
modified_at: "2026-03-30T21:20:10+00:00"
author: "David Anderson"
featured_image:
  url: "http://localhost/wp-content/uploads/2024/10/38_RndBr.png"
excerpt: "Fifty random bars plotted pixel by pixel, then a calculated mean line drawn across them — a compact statistics visualiser in under 130 lines of BASIC."
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: "Graphics"
    slug: "graphics"
    taxonomy: "genre"
    url: "http://localhost/type/graphics/"
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/2024/10/38_RndBr.png"
media_type_tags: "Graphics"
---

# Random Bar Chart

This program generates a random bar chart on screen, plotting 50 vertical bars of random height (1–40 pixels each) using the ZX81/TS1000’s PLOT command. After drawing all bars, it calculates and overlays a horizontal mean line across the chart by replaying the column range. The mean value is then printed below the chart using TAB formatting.

***

## Program Analysis

### Program Structure

The program divides into four logical phases:

1. **Initialisation (lines 1–5):** Prints the title in inverse video and sets the running total `T` to zero.
2. **Bar drawing (lines 10–70):** Iterates `J` from 0 to 49 (50 columns). For each column a random height `R` is chosen and accumulated into `T`; an inner loop plots each pixel of that bar vertically.
3. **Mean line overlay (lines 80–110):** After a 100-frame pause, a second pass over the same 50 columns plots a single pixel at row `T/50` for each column, drawing a horizontal mean line.
4. **Result display (lines 120–130):** Prints the calculated mean value with `TAB 5` indentation, then stops.

### Key Variables

| Variable | Purpose |
| --- | --- |
| `T` | Running total of all random heights |
| `J` | Column index (0–49, used as X coordinate) |
| `R` | Random bar height for the current column (1–40) |
| `K` | Row counter for inner bar-drawing loop |

### BASIC Idioms and Techniques

- **Random integer generation:**`INT (RND*40+1)` produces a value in the range 1–40 inclusive, a standard Sinclair BASIC idiom.
- **Pixel-level graphics:**`PLOT J,K` uses the ZX81’s low-resolution (64×44) graphics to draw each bar one pixel at a time in the inner `FOR K` loop.
- **Accumulator pattern:**`T` is incremented by `R` on each of the 50 iterations so that `T/50` gives the arithmetic mean without needing a separate summation pass.
- **Deferred annotation:** The mean line is drawn in a separate loop (lines 90–110) after a pause, providing a visual reveal effect.
- **Inner loop starting at 0:**`FOR K=0 TO R` actually plots `R+1` pixels (rows 0 through R), making bars one pixel taller than the raw random value; a minor off-by-one to be aware of.

### Notable Anomalies

- **Off-by-one in bar height:** Because `K` runs from 0 to `R` inclusive, each bar is `R+1` pixels tall rather than `R`. This also means the mean line position (`T/50`) slightly underestimates the visual midpoint of the bars.
- **Screen coordinate limits:** The ZX81 PLOT area is 64 wide by 44 tall. With `J` reaching 49 and `R` reaching 40, both axes stay safely within bounds, but bars at maximum height approach the top of the usable area.
- **Lines 200–210 (SAVE/RUN):** These are a loader stub; the inverse-video digit in the filename acts as an auto-run marker. They are not reached during normal execution due to the `STOP` at line 130.

## Source Code

```
   1 PRINT "%R%A%N%D%O%M% %B%A%R% %C%H%A%R%T"
   5 LET T=0
  10 FOR J=0 TO 49
  20 LET R=INT (RND*40+1)
  30 LET T=T+R
  40 FOR K=0 TO R
  50 PLOT J,K
  60 NEXT K
  70 NEXT J
  80 PAUSE 100
  90 FOR J=0 TO 49
 100 PLOT J,T/50
 110 NEXT J
 120 PRINT TAB 5;"MEAN R = ";T/50
 130 STOP 
 200 SAVE "1003%8"
 210 RUN
```