Demos2

Demos2 is a two-part graphical demonstration program offering a Lissajous-style spiral plotter and a filled-circle renderer. The spiral routine accepts four integer parameters (A, B, C, D) that control the sine-wave frequencies along each axis, continuously plotting line segments to trace parametric curves as T increments by 0.01 per iteration. The circle-fill routine uses the Pythagorean relationship (Z = R², ZZ = n², XX = √(Z−ZZ)) to compute chord half-lengths at each horizontal offset, then draws vertical lines across the circle’s interior using paired PLOT/DRAW calls for both the positive and negative x sides. A menu at line 220 routes the user to either demo via RUN targeting specific line numbers.

Program Analysis

Program Structure

The program is organized as three largely independent routines sharing a single file, with a menu dispatcher acting as the entry point at line 220. Execution flow is managed by RUN targeting specific line numbers rather than GO SUB, which means each demo effectively restarts the interpreter context. The layout is:

1. Lines 10: Bootstrap — immediately jumps to the menu at line 220.
2. Lines 20–100: Spirals demo.
3. Lines 110–210: Circle fill demo.
4. Lines 220–250: Menu and dispatcher.
5. Line 300: SAVE with auto-run.

Spirals Routine (Lines 20–100)

This routine generates Lissajous figures by parameterizing two points on sine curves and drawing line segments between them. Variables A and B control the frequencies of the first point (X1, Y1), while C and D control the second point (X2, Y2). The canvas is centered at (125, 87), matching the approximate midpoint of the 256×175 graphics area. The parameter T starts at 0 and increments by 0.01 each iteration in an infinite loop via GO TO 50.

Offsetting the center to (125, 87) rather than (128, 87) is a minor inaccuracy — the true horizontal midpoint of a 256-pixel-wide display is 128 — but the visual effect is negligible. There is no termination condition; the loop runs until the user breaks out manually.

Circle Fill Routine (Lines 110–210)

The fill algorithm draws the circle outline first using the built-in CIRCLE command, then fills it by iterating over each horizontal offset n from 0 to R. For each n, the chord half-length XX is computed as SQR(R↑2 - n↑2), derived from the Pythagorean theorem. Two vertical strokes are drawn symmetrically: one at X+n and one at X-n, each starting at the top of the chord (YY = Y + XX) and drawing downward by -XY where XY = 2*XX.

A subtle naming collision exists: the INPUT variable on line 130 uses lowercase x and y, but subsequent lines refer to X and Y. On this platform BASIC variable names are case-insensitive for numeric scalars, so this causes no runtime error. However, the local variable xy (line 188) shadows the earlier use of y only in appearance — they are distinct variables (XY vs Y) and do not conflict.

Menu and Dispatch (Lines 220–250)

The menu uses PRINT with multiple apostrophes for blank-line spacing and TAB 9 for crude centering of the title. Input validation on line 230 rejects values outside 1–2 with a self-referential GO TO 230. Dispatch is done with RUN 20 and RUN 110 rather than GO SUB, meaning no return path exists to the menu once a demo starts — the user must break and restart. The CLEAR on line 220 resets variables before each menu display.

Notable Techniques

• Use of RUN <line> as a dispatch mechanism avoids the need for a GO SUB/return structure but sacrifices the ability to return to the menu automatically.
• The Pythagorean chord calculation in the fill routine is a standard scan-line fill approach adapted for BASIC’s line-drawing primitives.
• Intermediate values Z and ZZ in lines 160–170 store R↑2 and n↑2 to avoid recomputing the squares inside the SQR call, a minor efficiency optimization.
• The spiral loop has no exit condition, which is intentional for a continuous animated demo but means the only way to return to the menu is via a manual BREAK.

Potential Issues

  10 RUN 220
  20 REM  Spirals 
  30 CLEAR :INPUT "ENTER 4 NUMBERS BETWEEN 1 AND 20";a,b,c,d
  40 LET T=0
  50 LET X1=125+125* SIN (A*T)
  60 LET Y1=87+87* SIN (B*T)
  70 LET X2=125+125* SIN (C*T)
  80 LET Y2=87+87* SIN (D*T)
  90 PLOT x1,y1:DRAW X2-X1,Y2-Y1
 100 LET t=t+.01:GO TO 50
 110 REM  Circle fill 
 120 CLS 
 130 INPUT "x(0-255) ";X,"y(0-255) ";y,"radius ";r
 140 CIRCLE X,Y,R
 150 FOR n=0 TO R
 160 LET Z=R↑2:LET ZZ=n↑2
 170 LET XX= SQR (Z-ZZ)
 180 LET YY=Y+XX:LET xy=2*XX
 190 PLOT X+n,YY:DRAW 0,-XY
 200 PLOT X-n,YY:DRAW 0,-XY
 210 NEXT n
 220 CLEAR :PRINT '''''' TAB 9;"2 Screen Demos"'''"1. Spirals","2. CIRCLE fill"
 230 INPUT "Select 1 2 ";z:IF z<1 OR z>2 THEN GO TO 230
 240 IF z=1 THEN RUN 20
 250 RUN 110
 300 SAVE "Demo's*2" LINE 1