Spelling

This program is a spelling quiz that displays words one at a time and asks the player to type each one back from memory. It uses two fixed-length string arrays: W$(100,6) holds up to 100 six-character words, and I$(1,6) captures the player’s input for comparison. A GOSUB at line 1000 populates all 100 vocabulary entries, covering themed groups such as food, clothing, animals, and art supplies. The quiz tracks attempts with variable T and correct answers with C, printing a score after every 10 words. A repeat-prevention check at line 68 compares the current word against the previously shown word (stored in S$(1)) to avoid immediate repetition.

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Program Analysis

Program Structure

The program divides cleanly into three parts: initialisation (lines 10–35), the main quiz loop (lines 65–150), and a word-list data subroutine (lines 1000–2000). Control flows from the setup into an infinite loop that picks a random word, displays it briefly, clears the screen, prompts for input, evaluates the answer, and either increments the correct-answer counter or shows the right spelling before looping back.

Data Structures

• W$(100,6) — a 100-element string array, each element 6 characters wide, holding the word list. All words in the list fit within six characters.
• I$(1,6) — a single-element string array used to capture the player’s typed answer via INPUT I$(1).
• S$(1,6) — a single-element string array used solely to remember the most recently shown word, preventing immediate repetition.
• T — attempt counter, reset to 0 after every 10 questions.
• C — correct-answer counter, reset alongside T.

Quiz Loop Logic

1. Line 65: pick a random index N in range 1–100 using INT(RND*101).
2. Line 67: reject 0 (possible because RND can return 0, making INT(0*101)=0).
3. Line 68: reject the index if it matches the previously shown word, stored in S$(1).
4. Line 69: record the chosen word in S$(1) for next-round duplicate prevention.
5. Lines 70–90: display the word, pause for about 4 seconds (PAUSE 100 at ~25 fps on a ZX81 = ~4 s), then clear the screen.
6. Line 100: collect the player’s attempt.
7. Lines 110–119: on a correct answer, increment both counters; after 10 attempts, print the score, reset, pause, and continue.
8. Lines 120–150: on a wrong answer, show the correct spelling, pause, increment T, and redisplay the word for another attempt.

Notable Techniques and Idioms

Using a one-element DIM array (DIM I$(1,6)) rather than a simple string variable is a common ZX81 idiom to obtain a fixed-width string buffer for INPUT, ensuring the comparison with the equally-dimensioned W$(N) works correctly with trailing-space padding.

The PAUSE 100 at line 80 serves as the “memorisation window”: the word is visible just long enough to read, then CLS at line 90 forces the player to rely on memory when typing. This is the core mechanic of the quiz.

Bugs and Anomalies

• Duplicate word at indices 35 and 90: Both W$(35) and W$(90) are set to "POLICE". This means one slot is wasted and the word appears twice as often in the random draw.
• Repeat prevention only checks the last word: Line 68 only prevents the immediately preceding word from being repeated; the same word can reappear after any intervening question.
• Wrong-answer path increments T but not C: On a wrong answer, T is incremented at line 133 and the loop jumps back to line 70 to redisplay. If the player eventually gets it right, T is incremented again at line 111 and C is incremented, but T has now been incremented twice for that word, potentially shortening the round unexpectedly.
• Score display resets unconditionally: Lines 115–116 reset both T and C whether reached from a correct answer (line 113) or a wrong-answer path (line 135), which is the intended behaviour, but the wrong-answer double-increment of T noted above means the 10-question boundary may be hit mid-retry.
• INT(RND*101) produces values 0–100: Line 67 correctly guards against 0, but the range is otherwise fine since the array is dimensioned to 100 and the maximum value produced is 100.

Word List

The 100 vocabulary entries are organised into loose thematic clusters loaded sequentially in the subroutine at lines 1000–1990: fruits and animals, kitchen items, tableware, seafaring, medical, clothing, zoo animals, art supplies, musical instruments, street scenes, and miscellaneous. All words are at most six letters, matching the fixed array width.

Indices	Theme
1–9	Fruits, animals, household
10–30	Food and kitchen items
31–50	Tableware, sea, nature
51–66	Medical, clothing
67–73	Zoo animals
74–83	Art supplies, music
84–100	Street scenes, outdoors, misc.