Color Show

Color Show is a random color display program that plots colored spaces across the screen using a constrained random walk. The program uses RANDOMIZE to seed the random number generator, then continuously selects a random paper color (0–8) and performs up to 20 steps of a two-dimensional random walk, printing colored spaces at each position. Variables `a` and `b` track the current screen position, clamped to the ranges 1–20 (rows) and 1–30 (columns) respectively to keep output within the display area. A subroutine at line 400 draws a rectangular border using PLOT and DRAW commands before the main loop begins.

Program Structure

The program is divided into two functional sections: an initialization subroutine (lines 410–460) that draws a screen border, and the main loop (lines 110–380) that performs the random color walk. Control flows as follows:

1. Line 50 calls the border-drawing subroutine at line 400 (which begins executing at line 410).
2. Line 110 seeds the random number generator with RANDOMIZE.
3. Lines 120 sets the initial screen coordinates: row a=10, column b=20.
4. Lines 160–380 form an infinite outer loop via GO TO 160, picking a new color and walk length each iteration.
5. Lines 210–370 are the inner FOR loop executing up to n steps of the random walk.

Border Subroutine

The subroutine at lines 410–460 draws a closed rectangle using PLOT and four DRAW commands covering the full 256×176 pixel screen area. The GO SUB 400 at line 50 targets line 400, which does not exist — execution falls through to line 410, a well-known BASIC technique to save memory by avoiding an otherwise unnecessary line.

Random Walk Mechanics

Each outer loop iteration picks a random paper color c in the range 0–8 and a random step count n in the range 0–19. The inner loop moves the cursor by incrementing or decrementing a (row) or b (column) based on a random direction d (0–3):

• d=0: move down (a+1)
• d=1: move up (a-1)
• d=2: move right (b+1)
• d=3: move left (b-1)

After each move, boundary clamping is applied to keep a within 1–20 and b within 1–30, preventing PRINT AT from causing an out-of-range error.

Key BASIC Idioms and Techniques

• PAPER c followed by PRINT AT a,b;" " — printing a space with a set paper color is the standard method for painting colored character cells without using INK.
• The boundary clamp uses four successive IF statements (lines 280–310) rather than structured min/max functions, which is typical for the BASIC dialect in use.
• Large line-number gaps (e.g., 50, 110, 120, 160…) leave room for future insertions without renumbering.

Bugs and Anomalies

   1 REM RANDOM COLORS
  50 GO SUB 400
 110 RANDOMIZE 
 120 LET a=10:LET b=20
 160 LET c= INT (RND*9)
 170 LET n= INT (RND*20)
 210 FOR x=0 TO n
 220 LET d= INT (RND*4)
 230 IF d=0 THEN LET a=a+1
 240 IF d=1 THEN LET a=a-1
 250 IF d=2 THEN LET b=b+1
 260 IF d=3 THEN LET b=b-1
 280 IF b >=30 THEN LET b=30
 290 IF b <=1 THEN LET b=1
 300 IF a >=20 THEN LET a=20
 310 IF a <=1 THEN LET a=1
 350 PAPER c
 360 PRINT AT a,b;" "
 370 NEXT x
 380 GO TO 160
 410 PLOT 0,0
 420 DRAW 255,0
 430 DRAW 0,175
 440 DRAW -255,0
 450 DRAW 0,-175
 460 RETURN