Guess It

This program implements a two-player Memory (Concentration) card-matching game on a 6×8 grid. Pairs of 12 different block-graphic symbols are randomly placed in the array A(6,8), and players take turns entering row and column coordinates to reveal and match hidden tiles. The grid uses inverse-video question marks (%?) as face-down tile covers, and matched tiles are replaced with a blank space. Scores for Player 1 (B) and Player 2 (C) are tracked, and the game ends when all 24 pairs (B+C=24) have been matched. The program uses FAST mode during initialization and switches to SLOW mode before gameplay begins.

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Program Analysis

Program Structure

The program divides cleanly into three phases: initialization, board display, and the main game loop.

1. Lines 5–120: Setup — runs in FAST mode, randomises, dimensions the board array, defines the symbol string, and places pairs randomly.
2. Lines 130–185: Board rendering — prints the title, column headers, row labels, and the initial grid of hidden tiles; then switches to SLOW mode.
3. Lines 190–520: Game loop — alternates players via GOSUB 230, handles input, reveals tiles, checks for matches, updates scores, and detects the end of the game.

Data Representation

The 6×8 array A(6,8) stores integer values 1–12, each appearing exactly twice. These values index into the 24-character string A$ (line 40), where each pair of characters encodes one block-graphic symbol using zmakebas two-character escapes. The 12 distinct symbols are drawn from the ZX81 block-graphic character set: ▘ ▝ ▖ ▄ ▌ ▗ ▀ ▐ ▟ ▙ ▛ ▘.

Because each symbol occupies exactly 2 characters in A$, the PRINT AT statement on line 270 correctly prints a two-character graphic with A$(F) — this works because ZX81 BASIC A$(F) returns a single character at position F, so actually only one character is printed per reveal. The pairing of characters in A$ is therefore decorative rather than functional; each match index selects one character.

Random Placement Algorithm

Lines 50–120 use a collision-rejection loop to place pairs. The outer loop runs I from 1 to 12 (one symbol value per pair), and the inner loop runs J from 1 to 4 — meaning four copies of each symbol are placed, not two. This is a bug: with 12 symbols × 4 placements = 48 required slots but only 48 cells (6×8=48), the board is completely filled, but players must find groups of four identical tiles rather than pairs, making the game harder than a standard Concentration game.

Turn Management

The current player is held in variable Z (1 or 2). After a failed match, lines 350–380 toggle Z between 1 and 2. On a successful match, the scoring branch at lines 460–510 increments B (Player 1’s score) or C (Player 2’s score) and prints them at fixed screen positions. The player does not switch after a successful match, giving the matching player another turn — consistent with standard Concentration rules.

Key BASIC Idioms

• LET X=INT(RND*6+1) / INT(RND*8+1) — standard ZX81 integer range idiom for 1–6 and 1–8.
• IF A(X,Y)<>0 THEN GOTO 70 — collision rejection; retries until an empty cell is found.
• PAUSE 100 at line 330 — gives players time to see both revealed tiles before they are hidden again on a mismatch.
• Inverse-video characters in PRINT strings (e.g. %G%U%E%S%S) render the title and hidden-tile marker in inverse video.

Screen Layout

Screen row	Content
1	Title: “GUESS BACK” (inverse video)
3	Column numbers 1–8
5, 7, 9, 11, 13, 15	Grid rows for board rows 1–6
11, col 3	Player 1 score (B)
11, col 29	Player 2 score (C)
17	Current player indicator
19	Input prompts / match result