Catch the Alien

This is a simple arcade-style game in which the player manoeuvres a cursor character around the screen to catch a randomly placed alien before a countdown timer expires. The alien (%A, displayed in inverse video as an “A”) is placed at a random row and column using INT(22*RND) and INT(27*RND), while the player’s marker (%+, an inverse “+”) is moved with the keys J, I, M, and K — a common four-direction layout. The score is derived from the remaining loop counter X, counting down from 100, so catching the alien quickly yields a higher score; a high-score variable H is maintained across rounds. The game loop uses CLS on every iteration to redraw the screen, and collision detection is a simple two-step coordinate comparison at lines 170–180.

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Program Analysis

Program Structure

The program is organised into a small number of logical phases:

1. Title / init (lines 1–5): Displays the title, pauses briefly, and initialises the high-score variable H to zero.
2. Round setup (lines 10–30): Resets the player’s row (L=10) and column (C=15) to the centre of the screen, then places the alien at a random position (M, N).
3. Main game loop (lines 50–200): Counts X down from 100 to 1. Each iteration reads a keypress, moves the player cursor, redraws the screen, and checks for a catch.
4. Failure path (lines 219–230): Prints “LOST” and loops back to a new round.
5. Success path (lines 300–310): Updates the high score if appropriate, displays both scores, then falls through to the pause/restart shared at line 220.

Variables

Variable	Role
H	High score (persists across rounds)
L	Player row (0–21)
C	Player column (0–31)
M	Alien row (random each round)
N	Alien column (random each round)
X	FOR loop counter / score (100→1)
L$	Current keypress

Key BASIC Idioms

• Inverse-video characters as sprites: %A (inverse “A”) represents the alien and %+ (inverse “+”) represents the player. This is a typical ZX81/TS1000 technique for making characters stand out without any separate graphics data.
• Countdown loop as timer and score: The FOR X=100 TO 1 STEP -1 loop simultaneously limits round duration and provides the score — the sooner the catch, the larger the remaining value of X.
• INKEY$ polling inside FOR loop: Line 100 reads INKEY$ on every iteration without waiting, giving a near-real-time movement feel, though the loop speed is bounded by the redraw cost of CLS and two PRINT AT calls each cycle.
• Two-condition collision detection (lines 170–180): Row and column matches are checked in sequence. Line 170 uses IF L<>M THEN GOTO 200 to skip the column check entirely when the row does not match, avoiding a compound condition.

Movement and Controls

Four keys provide cardinal movement using a layout common on rubber-key machines:

Key	Effect
I	Up (decrement row L)
M	Down (increment row L)
J	Left (decrement column C)
K	Right (increment column C)

Bugs and Anomalies

• No boundary clamping: L and C are never checked against screen edges. Moving the player off the 22×32 display area will cause an “Out of screen” error and crash the program.
• Alien not erased on miss: Because the alien is drawn once at line 60 (inside the loop) and CLS is called at line 150, the alien is actually redrawn every iteration — this works correctly, but it means the alien always appears at the fixed position drawn before the loop, and any flickering is inherent to the CLS-then-redraw cycle.
• Line 60 is inside the FOR loop but the alien position is fixed: M and N are not updated inside the loop, so the alien does not move; it is simply redrawn at the same spot each frame, which is the intended behaviour.
• High-score condition uses IF H<X: This correctly updates H only when the current score exceeds it.
• Lines 320–330: These are utility/maintenance lines (SAVE and RUN) that are unreachable during normal program execution and do not affect gameplay.