Flag

This program draws a stylized flag graphic on screen using PRINT AT positioning and block graphics characters. It works in three phases: first rendering alternating striped rows across columns 1–26 (rows 5–16), then overlaying a solid region in the upper-left canton (rows 6–10, columns 1–13), and finally using UNPLOT to punch circular or dot patterns into the flag in a grid arrangement. The program uses the ZX81’s UNPLOT command to clear individual pixels, creating a decorative pattern over the stripes. A SAVE command at line 300 stores the program before RUN at line 310 re-executes it.

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Program Analysis

Program Structure

The program is divided into four logical sections separated by line-number gaps:

1. Lines 5–70: Draw the main body of the flag — a horizontal stripe at row 5 and alternating filled/blank rows from 6 to 16 across all 26 columns.
2. Lines 100–140: Overlay the canton (upper-left rectangle, rows 6–10, columns 1–13) with solid block characters, overwriting the stripe pattern.
3. Lines 200–270: Use UNPLOT in nested loops to clear individual pixels, creating a decorative dot or star-like pattern across the flag.
4. Lines 280–310: STOP halts execution; SAVE and RUN follow but are not reached in normal flow.

Flag Rendering — Phase 1 (Lines 10–70)

The outer loop FOR I=1 TO 26 iterates over columns. At each column, line 20 prints a space character ("% " — inverse space, i.e. a filled block) at row 5. The inner loop FOR J=6 TO 16 STEP 2 then prints a block graphic "\##" (a solid 2×2 block character) at even rows and an inverse space at odd rows (J+1), producing a horizontal stripe effect down the flag body.

Canton Overlay — Phase 2 (Lines 100–140)

A second nested loop fills rows 6, 8, and 10 (STEP 2) in columns 1–13 with inverse spaces, creating a solid rectangular canton in the top-left quadrant. This overwrites the stripe pattern laid down in phase 1 for that region.

UNPLOT Decoration — Phase 3 (Lines 200–270)

The outer loop FOR J=23 TO 31 STEP 2 and inner loop FOR I=5 TO 21 STEP 4 address pixel coordinates (note ZX81 pixel addressing). UNPLOT clears individual pixels to create a spaced dot grid. Line 230 conditionally skips line 240 when J=23; line 240 contains a likely bug: UNPLOT I=2,J-1 — the expression I=2 evaluates as a boolean (0 or 1 in ZX81 BASIC) rather than I-2 as presumably intended. This means the second UNPLOT always plots at pixel column 0 or 1 rather than offset from I. Line 260 adds an additional UNPLOT at a fixed column 25 for each J value.

Notable Techniques

• Inverse space as filled block: "% " (inverse space character) is used as a solid fill — a common ZX81 idiom for drawing filled rectangles without block graphics.
• Block graphic character: "\##" represents a fully-filled 2×2 block graphic, used for the stripe rows.
• STEP 2 loops: Both the stripe loop (J=6 TO 16 STEP 2) and the UNPLOT loop (J=23 TO 31 STEP 2) use STEP 2 to address alternating rows/pixels, a space-efficient way to produce patterns.
• Conditional branching within inner loop: IF J=23 THEN GOTO 250 skips the second UNPLOT on the first pass of the outer loop only.

Bugs and Anomalies

Line	Issue	Effect
240	UNPLOT I=2,J-1 — I=2 is a boolean expression	Always UNPLOTs at x-coordinate 0 (false) or 1 (true when I=2), not at I-2 as likely intended

The STOP at line 280 means lines 300 (SAVE) and 310 (RUN) are never executed during normal program flow and appear to be remnants or setup lines.