BASIC

BASIC (Beginner’s All-purpose Symbolic Instruction Code) was created at Dartmouth College in 1964 by John Kemeny and Thomas Kurtz, so that students who weren’t scientists could write programs. A version of it came built into most home computers of the late 1970s and 1980s, and for many people it was their first language. Here it runs in Bywater BASIC, an interpreter for the classic line-numbered style.

Every line has a number, and the program runs in number order. GOTO jumps to a line, and GOSUB jumps to a subroutine, a block of lines that ends in RETURN. Subroutines take no arguments, and every variable is global, so the programs here pass values by setting variables before each GOSUB. A name ending in $, such as L$, holds text. There’s no sort, no growable array, and no CSV reader, so the implementations write their own.

Quirks

• Every array needs a DIM with a fixed size before it’s used. A(3) = 1 on an undeclared A stops the program with “Assignment must be to variable”.
• \ isn’t integer division here: 7 \ 2 gives 3.5. INT(7 / 2) gives 3.
• Numbers are stored in double precision, but PRINT shows only about seven digits: SQR(2) prints as 1.4142136. So 0.1 + 0.2 prints 0.3 yet isn’t equal to 0.3. PRINT USING "#.######" shows six decimals.
• STR$ puts a space before every number, even a negative one: STR$(2) is " 2". There’s no LTRIM$, so the programs drop the space with MID$(STR$(N), 2).
• A FOR loop with nothing to count still runs once: FOR E = 1 TO 0 runs its body with E = 1. Where a range can be empty, the programs use WHILE instead.
• FOR and NEXT can’t share a line: FOR J = 1 TO 3 : NEXT J stops with “FOR without NEXT”.
• bwbasic can’t read command-line arguments, so the run script passes the data path in the DATA environment variable, read with ENVIRON$. OPEN "I", #1, ENVIRON$("DATA") fails to parse, so the programs copy it into F$ first.
• An IF inside another IF’s THEN ignores its own condition: IF A = 1 THEN IF B = 0 THEN PRINT "yes" ELSE PRINT "no" prints “yes” when B is 5. The programs join conditions with AND instead.
• An array element used inside a 2-D subscript can read the wrong cell: with FEAT(3) = 2, X(1, FEAT(3)) gave X(3, 2). Copying FEAT(3) into a plain variable first gives the right one.
• A misspelled statement isn’t an error. bwbasic stops at the line and hands it and every line after it to the shell, so PRNT "b" prints “PRNT: not found”.
• An error prints a message and drops to the bwBASIC: prompt, yet the interpreter still exits with status 0. The run script watches the output for ERROR or the prompt and fails the run instead.

git clone https://github.com/qnixon/dswithx.git
cd dswithx
python3 harness/run_tests.py --language basic