This implementation is very limited by BASIC, especially QuickBasic. The memory space cannot be resized (not Turing complete). It is initially set to 20000, but can be changed before running the program. Input will not accept return characters at all. A buffer is programmed in for input, as well.
The program first asks for a filename for BF source code. If no filename is given (return hit without typing anything), the program accepts source from std in. To end your std input, enter a blank line (completely blank, no whitespace). Source files don't need a blank line since BASIC has EOF support. The program then cleans up the code similarly to the Java implementation and reports an uneven bracket error if it exists. Input works with a programmed buffer. It takes in an entire line if the buffer is empty, or it takes the first character from the buffer if it's not (just like the way you'd expect a buffer to work). Looping works the same as in the Java implementation, as well.
CLS memsize = 20000 instChars$ = "+-<>.," 'valid characters ptr = 0 'memory pointer INPUT "Filename (blank to use std in)...? ", filename$ IF filename$ = "" THEN DO LINE INPUT line$ source$ = source$ + line$ LOOP WHILE line$ <> "" ELSE OPEN filename$ FOR INPUT AS #1 DO LINE INPUT #1, line$ source$ = source$ + line$ LOOP UNTIL EOF(1) END IF CLOSE 'close the file if it's open 'let's clean the code up bktCnt = 0 FOR i = 1 TO LEN(source$) char$ = MID$(source$, i, 1) in = 0 'check to see if this is a valid instruction character IF instr(instChars$,char$) THEN code$ = code$ + char$ 'count brackets IF char$ = "[" THEN bktCnt = bktCnt + 1 IF char$ = "]" THEN bktCnt = bktCnt - 1 END IF NEXT i IF bktCnt THEN 'mismatched brackets PRINT "Uneven brackets" END END IF DIM SHARED memory(0 TO memsize) AS INTEGER 'fixed-size memory space :( inLine$ = "" 'input buffer FOR i = 1 TO LEN(code$) 'loop through the code DIM inChar AS STRING * 1 'single character for input instruction$ = MID$(code$, i, 1) 'get the instruction we're on SELECT CASE instruction$ CASE "+" memory(ptr) = memory(ptr) + 1 CASE "-" memory(ptr) = memory(ptr) - 1 CASE "." PRINT CHR$(memory(ptr)); CASE "," IF inLine$ = "" THEN LINE INPUT inLine$ 'buffer input inChar = LEFT$(inLine$, 1) 'take the first char off the buffer inLine$ = MID$(inLine$, 2) 'delete it from the buffer memory(ptr) = ASC(inChar) 'use it CASE ">" ptr = ptr + 1 IF ptr > 20000 THEN PRINT "Memory pointer out of range" END END IF CASE "<" ptr = ptr - 1 IF ptr < 0 THEN PRINT "Memory pointer out of range" END END IF CASE "[" IF memory(ptr) = 0 THEN bktCnt = 1'count the bracket we're on i = i + 1'move the code pointer to the next char WHILE bktCnt <> 0 'count nested loops till we find the matching one IF MID$(code$, i, 1) = "]" THEN bktCnt = bktCnt - 1 IF MID$(code$, i, 1) = "[" THEN bktCnt = bktCnt + 1 i = i + 1 'search forward WEND END IF CASE "]" IF memory(ptr) <> 0 THEN bktCnt = -1'count the bracket we're on i = i - 1'move the code pointer back a char WHILE bktCnt <> 0 'count nested loops till we fine the matching one IF MID$(code$, i, 1) = "]" THEN bktCnt = bktCnt - 1 IF MID$(code$, i, 1) = "[" THEN bktCnt = bktCnt + 1 i = i - 1 'search backwards WEND END IF END SELECT NEXT i