Appointment Calendar

This program implements a monthly appointment calendar allowing up to eight appointments per day across 31 days. It stores appointments in eight parallel string arrays (A$ through H$), each dimensioned 31×32, using a 32-character blank string as a sentinel value to detect empty slots. The program supports entering, viewing, editing, and deleting appointments interactively, as well as saving the entire calendar to tape and printing individual days or the full month via LPRINT. Navigation uses INKEY$ polling for menu selection and INPUT for text entry, with option 9 in the day-view triggering immediate printer output.

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Program Analysis

Program Structure

The program is organized into several distinct functional regions:

1. Lines 10–100: Variable and array initialization — numeric array X(31) (unused after setup), blank string U$, and eight 31×32 string arrays.
2. Lines 110–380: Title/splash screen, month input, and main menu with INKEY$ dispatch.
3. Lines 390–900: Day selection and appointment entry loop (slots A$–H$).
4. Lines 910–1010: Tape save routine.
5. Lines 1020–1500: Day viewer with inline edit/delete and optional LPRINT output.
6. Lines 1510–1570: Entry prompt subroutine and blank-entry guard.
7. Lines 1580–1700: Full-month print loop.
8. Lines 1710–2020: Eight individual LPRINT subroutines, one per appointment slot.
9. Lines 2500–2510: Auto-save and restart block.

Data Storage Design

Appointments are held in eight parallel string arrays A$() through H$(), each dimensioned (31,32), giving 32 characters per appointment slot per day. The 32-character string U$ (32 spaces) serves as a sentinel: a slot equal to U$ is considered empty. This is a straightforward but memory-hungry approach — eight arrays of 31×32 characters consume approximately 7,936 bytes of string storage alone, which is a significant portion of the 16 KB RAM available on an unexpanded machine.

The numeric array X(31) declared at line 10 is never actually used for data storage; the day number is instead held in the numeric variable X entered by the user. The array allocation is therefore wasted memory.

Menu and Navigation

The main menu at lines 220–380 uses an INKEY$ polling loop (lines 320–330) rather than INPUT, avoiding the need for the user to press ENTER. Invalid key presses fall through to GOTO 0220, re-drawing the menu. The leading zeros in GOTO 0320, GOTO 0390, etc. are cosmetic — BASIC ignores them — but they were a common stylistic convention to pad line number references to a consistent width.

Appointment Entry Flow

Lines 460–850 implement a sequential slot-filling loop. Each of the eight slots is checked in order: if the slot is not blank (i.e., already contains an appointment), it is skipped; if it is blank, the entry subroutine (line 1510) is called and user input is stored. The guard subroutine at line 1560 checks whether the entered string I$ is empty, and if so jumps back to the day-selection prompt at line 390, effectively allowing the user to stop entering appointments early by pressing ENTER with no text. If all eight slots fill up, line 870 prints a “IS FULL” message.

Day View, Edit, and Print

The day viewer at lines 1020–1500 displays all eight slots with numeric labels and waits for an INKEY$ keypress. Pressing 9 reprints the screen while simultaneously sending each non-blank slot to the printer via LPRINT. Pressing 0 returns to the menu. Pressing digits 1–8 prompts for a replacement string at lines 1390–1410; pressing ENTER with an empty input effectively deletes the appointment because an empty INPUT on a string variable returns a zero-length string, which will no longer equal U$ — meaning the slot will display as blank but will still not be treated as empty by the sentinel check. This is a subtle bug: deleted entries are replaced with an empty string "" rather than U$, so the “slot full” logic and the print-all loop will incorrectly treat them as occupied.

Print-All Subroutine Redundancy

Lines 1710–2020 consist of eight near-identical LPRINT subroutines, each printing the month name, day number, one appointment string, and a blank line. This block could have been collapsed into a single parameterized subroutine, but the ZX81 BASIC dialect makes passing parameters to subroutines awkward, so the repetition is understandable given the constraints. Together these subroutines account for over 300 lines of essentially duplicated code.

Notable Techniques and Idioms

• INKEY$ polling loop (IF M$="" THEN GOTO 0320) used for single-keypress menu selection without requiring ENTER.
• Blank string sentinel (U$ = 32 spaces) for detecting empty array slots, a common ZX81 idiom since arrays are pre-filled with spaces.
• Option M$="2" (Look at a date) shares the same day-entry input block (line 390) as option 1, with the branch to the viewer at line 450 — the code path diverges only after the day number is entered.
• The variable I$ is reused for multiple purposes: appointment text during entry, a print-trigger flag ("9") in the viewer, and a dummy INPUT variable at line 890 for a “press ENTER to continue” pause.
• SAVE T$ at line 1000 saves the entire program and variable space (including all calendar data) under the name entered by the user, providing simple persistence.

Bugs and Anomalies

Location	Issue
Lines 460–850 (skip logic)	Slots already containing data are skipped rather than appended after, meaning re-entering a day skips to the first empty slot but also skips editing existing entries in the entry path.
Lines 1420–1490 (delete)	Replacing an appointment with an empty INPUT sets the slot to "", not U$. The sentinel check (<>U$) will then treat the slot as occupied, causing it to appear in print-all output as a blank line.
Line 10	DIM X(31) allocates a numeric array that is never used, wasting approximately 128 bytes of RAM.
Line 1040	On first entry to the viewer, I$ may hold stale content from a previous interaction, potentially triggering unexpected LPRINT output.