Security Code

This program generates and prints daily security codes for a full week, outputting results to both screen and printer via LPRINT. It prompts the user to enter a day/week reference, then calls a subroutine seven times (lines 100–220) to produce a four-digit random code for each day of the week. The code generation subroutine at line 300 uses INT(10000*RND) with a rejection loop at line 310 to ensure the result is always a four-digit number (1000–9999).

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Program Analysis

Program Structure

The program is divided into three logical sections:

1. Initialisation and input (lines 5–18): Prints a heading in inverse video to both screen and printer, prompts for a day/week label, and makes the first GOSUB call to the code generator.
2. Code output loop (lines 100–230): Seven sequential GOSUB 300 calls, each followed by an LPRINT statement labelling the day of the week and the generated code stored in C. Execution ends at STOP on line 230.
3. Code generator subroutine (lines 300–320): Generates a random integer and validates it before returning.

Lines 375, 400, and 500 are utility lines sitting beyond the STOP at line 230 and beyond the subroutine — they are never reached during normal execution.

Code Generation Subroutine

The subroutine at line 300 uses a rejection-sampling pattern to guarantee a four-digit result:

• Line 300: LET C=INT(10000*RND) — produces an integer in the range 0–9999.
• Line 310: IF C<1000 THEN GOTO 300 — rejects any value below 1000, looping until a value in the range 1000–9999 is obtained.
• Line 320: RETURN — passes the validated code back in variable C.

This is a clean and correct approach; the probability of rejection on any single attempt is 10%, so the average number of iterations per code is approximately 1.11, making it efficient in practice.

Input and Output Strategy

The program mirrors output to both the display (via PRINT) and the printer (via LPRINT). The heading on lines 6–7 uses inverse-video characters to produce a bold title effect on screen. The user-supplied day/week label D is echoed back on line 11 (screen) and incorporated into the LPRINT output on line 12, giving the printed sheet a header that identifies the week.

Note that lines 9 and 12 use a trailing semicolon after the prompt string — line 9 to suppress a newline before the INPUT cursor, and line 12 to place D on the same LPRINT line as the label text.

Variable Usage

Variable	Purpose
D	User-entered day/week identifier (numeric INPUT)
C	Current generated security code (set in subroutine, read by LPRINT)

Notable Techniques and Idioms

• The inverse-video title string (e.g. "%D%A%I%L%Y%...") produces a highlighted heading without any POKE or attribute manipulation.
• The rejection loop in the subroutine is a standard Monte Carlo technique adapted for constrained random number generation in BASIC.
• The seven GOSUB calls are written sequentially rather than using a FOR–NEXT loop with a DATA/READ structure for day names; this is simpler but more verbose and harder to maintain.

Utility Lines and Save Block

Line 375 contains a standalone CLEAR and line 500 a standalone RUN; neither is reachable during normal execution. Line 400 uses SAVE "1001%8", where %8 is an inverse-video digit 8 in the filename — this acts as an auto-run flag so the program starts automatically when loaded.

Bugs and Anomalies

• Line 10 uses INPUT D where D is a numeric variable. Entering a non-numeric value (e.g. “12 JAN”) will cause an error. A string variable (D$) would be more appropriate for a date label.
• The first GOSUB 300 is called at line 18, before any day label is printed; the generated code is immediately consumed by the LPRINT "SUNDAY..." at line 100, so this is functionally correct but the placement is slightly misleading — the call logically belongs between lines 90 and 100.