Things To Do

This program implements a simple to-do list manager, storing up to 22 items of up to 30 characters each in a two-dimensional string array. It presents a menu-driven interface with six options: adding items, setting priorities (reordering), deleting items, clearing the file (via RUN), viewing the file, and saving to tape. Priority setting works by prompting the user to re-enter items in desired order into a second array B$, then copying back to A$. The program uses POKE 16418 to control the system’s display mode flag and exploits INKEY$ key codes (29–34, corresponding to digits 1–6) for menu navigation, dispatching via GOTO VAL Y$*1000.

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Program Structure

The program is organised around a central menu loop at line 100–380 that dispatches to one of six routines based on the user’s keypress. Each routine ends with GOTO 100 (or GOTO VAL "1"*1000 etc.) to return to the menu. The overall flow is:

1. Initialisation (10–40): Sets FAST mode, dimensions both string arrays, initialises record counter R1=0.
2. Menu (100–380): Displays six options, waits for a keypress in range 1–6, dispatches via computed GOTO.
3. Routines at ×1000: Lines 1000, 2000, 3000, 4000, 5000, 6000 implement each menu option.
4. Display subroutine (2500–2540): Shared CLS + list-print used by options 2, 3, and 5.

Data Storage

Two DIMed arrays are used:

Variable	Size	Purpose
A$(22,30)	22 rows × 30 chars	Primary to-do list storage
B$(22,30)	22 rows × 30 chars	Temporary buffer during priority reordering

The variable R1 tracks how many items have been entered. Items are 1-indexed; the maximum list length is 22 entries.

Menu Dispatch Technique

The menu waits for a keypress at line 330 with LET Y$=INKEY$ inside a loop. Line 340 validates that CODE Y$ is between 29 and 34, which correspond to the ZX81 character codes for digits 1 through 6. Line 380 then uses GOTO VAL Y$*1000, converting the digit character to an integer and multiplying by 1000 to reach the correct routine — a compact dispatch idiom that avoids a chain of IF statements.

POKE 16418 Usage

Address 16418 (CDFLAG / the “slow/fast” flag in ZX81 RAM) is manipulated directly:

• POKE 16418,0 at line 300 — switches the display mode before entering the SLOW input loop.
• POKE 16418,2 at line 370 — restores the flag after menu selection, before dispatching.

This is used alongside the FAST/SLOW keywords to control display flickering and input responsiveness.

Priority Reordering (Option 2)

The reorder routine (lines 2000–2130) works as follows:

1. Display current list via subroutine at 2500.
2. For each position I from 1 to R1, ask user to input a source index A.
3. Copy A$(A) into B$(I) and blank out A$(A).
4. After all selections, copy B$ back to A$ and clear B$.

This approach has a notable bug: if the user enters the same source index twice, the second pick will copy a blank string (since A$(A) was cleared at line 2040 on the first pick). There is also no bounds checking on the entered index A.

Delete Routine (Option 3)

Deletion at lines 3000–3070 shifts all elements from position A+1 downward by one, then decrements R1. This is a correct in-place array compaction. After deletion, it falls through to the view routine at 5000 rather than going directly back to the menu.

Clear and Save

Option 4 (4000) simply executes RUN, which reinitialises all variables and restarts the program — an efficient way to clear all data without explicit array-zeroing code. Option 6 (6000) uses INPUT F$ to obtain a filename, then SAVE F$ to write the program and all current variable data (including the list arrays) to tape.

Copy / Print Option

Lines 2120 and 5040 both implement the same pattern: PAUSE 40000 followed by IF INKEY$="Z" THEN COPY. This gives the user approximately 40 seconds to press Z to trigger a printer copy of the displayed list before returning to the menu.