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.

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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.