Ice Cream Cone

This program draws a two-scoop ice cream cone using ZX81/TS1000 block graphics and lets the user choose from seven flavours for each scoop. The cone itself is built with CHR$ 136 (a solid block graphic character) across rows 11–20 in a widening pattern, creating a triangular silhouette. Each scoop is drawn as an approximate circle by printing the chosen flavour character (mapped from the menu number via VAL R$+127) in nested rectangular bands of decreasing width, stacking the second scoop above the first. The program loops indefinitely, clearing the screen and returning to the flavour menu after any keypress.

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Program Analysis

Program Structure

The program divides into four logical phases:

1. Menu display (lines 15–100): prints the title and seven flavour options.
2. Input loop (lines 110–240): a FOR X=1 TO 2 loop collects one flavour choice per scoop, storing the result as I$ (bottom scoop) and J$ (top scoop).
3. Drawing (lines 250–540): clears the screen, draws the cone body, then the two scoops.
4. Repeat loop (lines 550–600): pauses, waits for a keypress, and jumps back to line 20 for another cone.

Flavour Encoding

The user presses a digit key 1–7. The code at line 170 validates by checking CODE R$ is between 29 and 35, which are the ZX81 internal codes for the characters ‘1’ through ‘7’. Line 190 computes R = VAL R$ + 127, mapping the digit to a character code in the 128–134 range (block graphic / inverse video territory). Two special cases patch the result:

• Line 200: if R=129 (choice 2), set R=137 — selects a different block graphic for visual contrast.
• Line 210: if R=132 (choice 5), set R=10 — uses the newline character code, which on the ZX81 displays as a blank/space, giving a hollow appearance for “Molasses Lace”.

The resulting character is stored via CHR$ R into I$ or J$ and later used as the fill character for each scoop.

Cone Drawing Algorithm

The cone is drawn in lines 280–350. A FOR R=20 TO 11 STEP -2 loop iterates upward from the bottom of the screen. Variables A and B start at column 15 (a single point) and widen by one column on each side per iteration, building a triangular shape. Each row prints CHR$ 136 (a solid block) from column A to B on both the current row and the one above it, effectively doubling the vertical density.

Scoop Drawing Algorithm

Each scoop is approximated as a filled circle using three concentric rectangular bands of decreasing width, printed with the flavour character. The bottom scoop (I$) occupies roughly rows 5–10, and the top scoop (J$) rows 0–4. The technique for each scoop is identical:

Band	Column range	Rows (bottom scoop)	Rows (top scoop)
Outer	9–21 / 10–20	7, 8	2, 3
Middle	10–20 / 11–19	6, 9	1, 4
Inner	11–19 / 12–18	5, 10	0

The conditional jumps at lines 420 and 450 (and their equivalents 520 and 540 for the top scoop) skip inner-band printing when the column is outside the narrower range, efficiently controlling which cells are filled without nested loops.

Key BASIC Idioms

• LET R$=INKEY$ followed by IF R$="" THEN GOTO is the standard busy-wait input idiom on this platform.
• SLOW at line 10 switches to slow (display-sync) mode, ensuring the graphics render visibly rather than flashing.
• PAUSE 123 at line 550 provides a brief enforced display pause before the “press any key” prompt.
• Line 800 contains a bare RUN, which has no effect when reached normally but serves as a safety net if execution somehow falls through.

Notable Techniques and Anomalies

• Using CHR$ 10 (newline) as a flavour tile character for choice 5 is an unorthodox trick; on the ZX81 this may produce unexpected display artefacts rather than a clean pattern.
• The loop variable R is reused: first as the decoded character code (lines 190–210), then as the row counter in the cone-drawing loop (lines 280–350). This is safe because I$ and J$ have already captured the character by line 240, but it is a potential source of confusion.
• The cone step is STEP -2 and prints on both row R and R-1, so rows 11–20 are covered, but the widening increments once per two-row pair, giving the cone a somewhat blocky profile.
• Line 700 contains a SAVE command with an inverse-video zero in the filename, which encodes auto-run behaviour on load.