Random Plot

This program draws 25 randomly placed straight lines on screen using Bresenham’s line-drawing algorithm implemented entirely in BASIC. Starting from a random point, it repeatedly selects a new random destination within a 62×42 pixel area and calls the subroutine at line 1000 to plot each line pixel by pixel. The subroutine correctly handles all octants by choosing the major axis (the longer dimension) as the driving axis and using an integer accumulator variable S to decide when to step along the minor axis. The REM statements scattered throughout appear to serve as inline documentation for intermediate values rather than functional code.

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Program Analysis

Program Structure

The program is divided into two parts: a short main loop (lines 10–70) and a line-drawing subroutine (lines 1000–1260). Lines 1270–1290 handle saving and re-running the program.

1. Lines 10–20: Initialise the starting point (A, B) with random coordinates.
2. Lines 25–70: Loop 25 times, each iteration picking a random endpoint (C, D), calling the line-draw subroutine, then advancing the current position to the endpoint.
3. Lines 1000–1260: Bresenham’s line-drawing subroutine, plotting from (A,B) to (C,D).

Coordinate Space

Random coordinates are constrained to a 62×42 pixel region: A and C are in the range 1–62 and B and D in the range 1–42. This keeps all points within the displayable screen area without risk of out-of-bounds PLOT errors.

Bresenham’s Line Algorithm

The subroutine at line 1000 is a faithful BASIC implementation of Bresenham’s integer line algorithm. Key variables are:

Variable	Role
U	Horizontal delta (C−A)
V	Vertical delta (D−B)
D1X, D1Y	Diagonal step direction (both axes)
D2X, D2Y	Primary-axis step direction (one axis only)
M	Length of the major (driving) axis
N	Length of the minor axis
S	Bresenham error accumulator, initialised to INT(M/2)
L	Loop counter, 0 to M inclusive

The algorithm determines which axis is longer (lines 1060–1120) and sets the step vectors accordingly so that it always iterates along the major axis. At each step, N is added to the accumulator S; when S reaches or exceeds M, M is subtracted and a diagonal step (D1X, D1Y) is taken; otherwise only the primary-axis step (D2X, D2Y) is used. This correctly handles all eight octants.

Notable Techniques

• SGN for direction: SGN U and SGN V extract ±1 or 0 cleanly without any IF/THEN branching, keeping the step-vector setup compact.
• Error accumulator initialisation: Setting S = INT(M/2) before the loop centres the error term, producing a more visually symmetric line than initialising to 0.
• Connected line segments: After each subroutine call, A and B are updated to C and D (lines 50–60), so successive lines form a continuous polyline rather than isolated segments.
• REM as commentary: Several REM statements (e.g. lines 1005, 1015, 1035) appear immediately after variable assignments to document what each variable represents, a common documentation practice in BASIC of this era.

Bugs and Anomalies

• Subroutine modifies caller variables: The subroutine uses A and B as its working plot position, overwriting the caller’s start-point values during the drawing loop. The main loop relies on this side-effect by reading the final values of A and B at lines 50–60 — however, because the loop always runs from 0 to M inclusive and ends exactly at (C, D), the final values of A and B after the RETURN will equal the original C and D, so the behaviour is correct if fragile.
• Line 1270 CLEAR: This line is unreachable during normal execution (the RETURN at 1260 sends control back to the main loop, which ends at line 70 with no fall-through). It would only execute if the main loop somehow fell through, suggesting it may be a remnant of development.