This program continuously draws randomly positioned hollow squares on screen using block graphics characters stored in a 4×4 string array. Each square is four rows tall and four characters wide, constructed from Spectrum block graphics to create a box outline with solid corners and edges. The screen position is chosen randomly each iteration: column in the range 0–28 and row in the range 0–16, ensuring the 4-row square fits within the display. The loop runs indefinitely, accumulating squares on screen until the user presses the “A” key to stop.
Program Analysis
Program Structure
The program is compact and self-contained, falling into three logical phases:
- Initialisation (lines 10–60): A 4×4 string array
A$is declared and loaded with four rows of block graphics forming a hollow square outline. - Main loop (lines 70–130): Random screen coordinates are chosen, the square is printed at that position row by row, a keypress is checked, and execution loops back unconditionally.
- Save/run stub (lines 140–150): Handles program persistence.
Block Graphic Square Construction
The four strings stored in A$ use ZX Spectrum block graphic characters to draw a 4×4 hollow box. Decoded, the rows are:
| Array row | Line | Visual role | Characters used |
|---|---|---|---|
A$(1) | 30 | Top edge | ▙▀▀▟ |
A$(2) | 40 | Middle (upper) | ▌ ▐ |
A$(3) | 50 | Middle (lower) | ▌ ▐ |
A$(4) | 60 | Bottom edge | ▛▄▄▜ |
The two middle rows are identical, giving the square a hollow interior two character-cells tall. The overall shape is 4 columns wide and 4 rows tall in character-cell terms.
Random Placement
Line 70 computes S=INT(RND*29), giving a column in 0–28, and line 80 computes T=INT(RND*17), giving a row in 0–16. Since the square is 4 rows tall and 4 columns wide, the maximum safe top-left position on a 32×22 display would be column 28 and row 18. The column range is therefore safe, but the row range (0–16) is conservatively constrained to keep the square fully on screen with plenty of margin.
Key BASIC Idioms
- DIM A$(4,4): Declares a two-dimensional string array with 4 strings each of length 4, matching the 4-character-wide square exactly. Each row of the graphic maps directly to one string element.
- FOR H=0 TO 3 / PRINT AT T+H,S: Iterates over the four graphic rows, offsetting the row coordinate by
Heach time to print them consecutively. - IF INKEY$=”A” THEN STOP: Polls the keyboard each iteration without pausing, so the program runs at full speed and only halts when “A” is held. This is a non-blocking keypress check, unlike the
PAUSE 0/INKEY$idiom which waits for any key. - GOTO 70: Returns to the random-coordinate generation rather than the top of the print loop, ensuring fresh coordinates for every new square.
Notable Techniques
Because the screen is never cleared between iterations, squares accumulate on top of one another, producing an increasingly dense pattern. The program exploits the transparency of the block graphic approach — each new square simply overwrites the character cells it occupies, creating an organic layering effect rather than explicit animation.
Potential Anomalies
The DIM A$(4,4) declaration allocates strings of exactly 4 characters. Each graphic row is also exactly 4 characters, so there is no padding issue. However, if a future edit extended any row beyond 4 characters, the excess would be silently truncated by the fixed-length string array. Additionally, lines 140–150 (SAVE and RUN) are unreachable during normal execution because the program loops indefinitely via GOTO 70 and exits only via STOP; they exist solely for program loading/saving purposes.
Content
Image Gallery
Source Code
10 REM %R%A%N%D%O%M% %S%Q%U%A%R%E%S% % % % % % % % %
20 DIM A$(4,4)
30 LET A$(1)="\:'\''\''\':"
40 LET A$(2)="\: \ :"
50 LET A$(3)="\: \ :"
60 LET A$(4)="\:.\..\..\.:"
70 LET S=INT (RND*29)
80 LET T=INT (RND*17)
90 FOR H=0 TO 3
100 PRINT AT T+H,S;A$(H+1)
110 NEXT H
120 IF INKEY$="A" THEN STOP
130 GOTO 70
140 SAVE "1008%6"
150 RUN Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.