Temperature Conversion

This program converts temperatures between Fahrenheit, Celsius, and Kelvin. The user enters a numeric temperature and selects a unit (F, C, or K); an input validation loop at line 70 rejects anything other than these three letters before proceeding. Two subroutines handle the conversions: lines 200–220 convert Fahrenheit to Celsius using the standard (T−32)×5/9 formula, while lines 300–340 handle both Celsius-to-Fahrenheit and Kelvin-to-Fahrenheit by first normalising Kelvin to Celsius before applying the C×9/5+32 formula. After displaying the result, the program waits for any key press via INPUT A$ before clearing the screen and looping back to the start.

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Program Analysis

Program Structure

The program is organised into a main loop and two conversion subroutines:

1. Lines 10–150: Main loop — prompts for temperature and unit, dispatches to a subroutine, displays the result, waits for a keypress, then repeats.
2. Lines 200–220: Fahrenheit → Celsius subroutine.
3. Lines 300–340: Celsius or Kelvin → Fahrenheit subroutine.

Conversion Logic

Input unit	Subroutine	Formula	Output unit
F	200	(T − 32) × 5 / 9	C
C	300	C × 9/5 + 32	F
K	300	(T − 273) × 9/5 + 32	F

Note that Kelvin converts to Fahrenheit only; there is no direct K→C path presented to the user as a final result, though the intermediate Celsius value is computed in C at line 310.

Input Validation

Line 70 implements a validation loop by re-routing to line 60 (GOTO 60) if the entered string is not one of the three accepted unit codes. This is a common BASIC idiom that keeps the user in a tight loop until valid input is received, without any explicit error message being printed.

Key BASIC Idioms

• Subroutine dispatch via consecutive IF statements: Lines 90 and 100 each test Q$ independently and call the appropriate GOSUB. Because the two conditions are mutually exclusive (validated at line 70), only one subroutine fires per pass.
• Pause-via-INPUT: Line 130 uses INPUT A$ as a “press ENTER to continue” gate, a standard idiom where the variable A$ is never actually used.
• Shared subroutine for two input units: The Kelvin branch shares the Celsius→Fahrenheit subroutine (line 300) by adjusting the intermediate variable C at line 310, avoiding code duplication.

Notable Anomalies

• The Kelvin conversion uses 273 as the offset rather than the more precise 273.15. This introduces a small but non-trivial rounding error for scientific use.
• The program only converts in one direction per unit pair: F→C, C→F, and K→F. There is no C→K, K→C, or F→K path.
• Lines 350 and 360 (SAVE "1017%2" and RUN) appear to be a save-and-restart tail appended to the listing and are not part of the interactive program flow.