My BASIC interpreter written in BASIC
An interpreter for Tiny BASIC written in FreeBASIC
A few years ago I wrote an interpreter for Tiny BASIC, just to see if I knew how to do it. It’s written in a variant of BASIC called FreeBASIC, and it interprets a small subset of the BASIC language that dates back to 1975, Tiny BASIC. FreeBASIC is a modern BASIC that is an extended version of Microsoft’s QBasic.
The language I implemented only supports integers and 26 variables, A to Z. It has one array named “@”.
First is a short test program:
10 PRINT "Enter 10 numbers:"
20 FOR I = 1 TO 10
30 PRINT "Number ", I, "? ",
40 INPUT @(I)
50 NEXT I
100 N = 10
110 FOR I = 1 TO N-1
120 FOR J = 1 TO N-1
130 IF @(J) < @(J+1) THEN 170
140 X = @(J)
150 @(J) = @(J+1)
160 @(J+1) =X
170 NEXT J
180 NEXT I
200 FOR I = 1 TO 10
210 PRINT @(I),
220 NEXT I
225 PRINT
230 END
Here is the interpreter. Lines that start with a tic mark are comments. Most were put in for debugging purposes.:
'---script created on 11-08-2022 14:51:59 by
' LuckBasic
' This is my personal project to build a Basic interpreter.
' It's a high level project to prove to myself that i understand how to build an interpreter.
' I'm intentionaly using very simple algorithms and data structures. Readability outweighs performance.
'
' Gary Luckenbaugh - fall of '22
'print "start"
dim t as integer
'input t
dim shared buff as string
buff = ""
dim shared buff_after_parse_or as string
dim shared pc as integer ' Program counter for interpreter
const ops = 10
dim shared op(ops) as string
data "dummy", "+", "-", "*", "/", "(", ")", "=", "^", "@", ","
for t = 0 to ops
read op(t)
next t
'op(0) = "dummy"
'op(1) = "+"
'op(2) = "-"
'op(3) = "*"
'op(4) = "/"
'op(5) = "("
'op(6) = ")"
'op(7) = "="
'op(8) = "^"
'op(9) = "@"
op(10) = ","
' dim pTree as long
dim shared statement(5000) as string
dim shared at_array(5000) as integer
dim shared i as integer
dim shared vars(26) as string
declare function do_if(cmd as string) as integer
declare function next_pc(pc as integer) as integer
declare function do_goto(cmd as string) as integer
declare function do_gosub(cmd as string) as integer
declare function do_return() as integer
declare Function Eval_rpn(rpn as string) as string
declare function Expr_eval(e as string) as string
declare Function mypop() as string
declare Function rid_eol(inpval as string) as string
declare Function Get_variable(key as string) as string
declare function gettok() as string
declare function parse_factor() as string
declare function parse_exponent() as string
declare function parse_unary_minus() as string
declare function parse_term ()as string
declare function parse_expr () as string
declare function parse_condition () as string
declare function parse_and () as string
declare function parse_or () as string
declare sub edit_delete(linenum as integer)
declare sub edit_replace(linenum as integer, buff as string)
declare sub process_line(i as string)
declare sub process_tok(tok as string)
declare sub process_op(op as string)
declare sub process_num_or_var(num as string)
declare Sub mypush(tok as string)
declare sub Start_variable_store()
declare sub Set_variable(key as string, item as string)
declare sub Release_variable_store()
declare sub do_input_at_array(e as string)
declare Sub clearstack()
declare sub do_command(cmd as string)
declare Sub do_list()
declare sub do_print(cmd as string)
declare sub do_let(cmd as string)
declare sub do_new()
declare sub do_load(fname as string)
declare sub do_save(fname as string)
declare sub do_run()
declare sub do_input(cmd as string)
declare sub do_for(cmd as string)
declare sub do_next(cmd as string)
declare sub do_let_at_array(e as string)
declare sub Main()
Main()
function myucase(s as string) as string
dim c(100) as string
dim flag as integer
dim i as integer
dim collect as string
flag = true
collect = ""
for i = 1 to len(s)
c(i) = mid(s, i, 1)
' Console.WriteLine("character and code = " + c(i) + " code = ", asc(c(i)))
if asc(c(i)) = 34 then ' toggle flag
' Console.writeline("Toggling flag at "+i)
if flag = true then
flag = false
else
flag = true
end if
end if
if flag = true then
c(i) = Ucase(c(i))
end if
' Console.WriteLine("character and code = " + c(i) + " code = ", asc(c(i)))
next i
for i = 1 to len(s)
collect = collect + c(i)
next i
myucase = collect
End function
Sub Main()
'input t
'print "I'm here"
'dim ug as integer
'input up
dim k as integer
for k = 1 to 5000
at_array(k) = 2*k ' test pattern
next k
for k = 1 to 5000
statement(k) = ""
next k
'op = new string(ops) { "dummy for 0" , "+", "-", "*", "/", "(", ")", "=", "^", "@", "," }
dim i as string
dim cont as integer
dim tok as string
' start_variable_store()
cont = true
do while cont = true
line input "basic> ", i
rid_eol(i)
i = myucase(i)
' print i
buff = i
tok = gettok()
if left(tok, 1) >= "0" and left(tok, 1) <= "9" then
process_line(i)
elseif tok <> "BYE" then
do_command(i)
elseif tok = "BYE" then
cont = false
end if
loop
' Release_variable_store()
end sub
sub edit_delete(linenum as integer)
statement(linenum) = ""
end sub
sub edit_replace(linenum as integer, buff as string)
statement(linenum) = buff
end sub
sub process_line(i as string)
dim line_tok as string
dim nexttok as string
buff = i
line_tok = gettok()
nexttok = gettok()
if nexttok = "~" then
edit_delete(val(line_tok))
else
edit_replace(val(line_tok), nexttok + buff)
' print "edit_replace(val(line_tok), nexttok + buff)", "next ",nexttok+buff
end if
end sub
sub do_command(cmd as string)
if cmd = "LIST" then
do_list()
elseif left(cmd, 5) = "PRINT" then
do_print(cmd)
elseif left(cmd, 3) = "LET" then
do_let(cmd)
buff = buff_after_parse_or ' look for extraneous stuff
if gettok() <> "~" then
print "warning: extra stuff after expression"
end if
elseif cmd = "NEW"then
do_new()
elseif left(cmd, 4) = "LOAD" then
do_load(trim(mid(cmd, 5)))
elseif left(cmd, 4) = "SAVE" then
do_save(trim(mid(cmd, 5)))
elseif cmd = "RUN" then
do_run()
else
do_let("LET "+cmd)
buff = buff_after_parse_or ' look for extraneous stuff
if gettok() <> "~" then
print "warning: extra stuff after expression"
end if
end if
end sub
sub do_load(fname as string)
dim ff as Ubyte
dim inbuff as string
dim myline as string
do_new()
ff = FreeFile
Open fname For Input As #ff
do while not eof(ff)
line input #ff, myline
process_line(myline)
loop
Close #ff
'For Each line As String In File.ReadAllLines(fname)
' process_line(line)
'next
end sub
sub do_save(fname as string)
dim ff as Ubyte
ff = FreeFile
Open fname For output As #ff
dim i as integer
for i = 1 to 5000
if statement(i) <> "" then
print #ff, str(i) + " " +statement(i)
end if
next i
Close #ff
end sub
sub do_new()
dim i as integer
for i = 1 to 5000
statement(i) = ""
next i
end sub
Sub do_list()
dim i as integer
for i = 1 to 5000
if statement(i) <> "" then
print str(i) + " " +statement(i)
end if
next i
End Sub
dim shared acvar(100) as string ' For loop variable
dim shared aistart(100) as integer ' start
dim shared ailast(100) as integer ' last
dim shared aistep(100) as integer ' step
dim shared ailine(100) as integer ' line number of For
dim shared for_stackp as integer ' For loop stack pointer
dim shared gosub_stack(100) as integer
dim shared gosub_stack_ptr as integer
sub do_run()
dim startpc as integer
dim stoprun as integer
dim cmd as string
dim target as integer
dim l as integer
startpc = 0
for_stackp = 0
gosub_stack_ptr = 0
stoprun = false
for l = 1 to 5000
if statement(l) <> "" then
startpc = l
exit for
end if
next l
if startpc = 0 then
print "No statements in buffer!"
exit sub
end if
pc = startpc
do while not stoprun and pc <> 0
'print "top of run loop, pc = ";pc
cmd = statement(pc)
' print "cmd is ";cmd;" at ";pc
if left(cmd, 3) = "LET" then
do_let(cmd)
buff = buff_after_parse_or ' look for extraneous stuff
if gettok() <> "~" then
print "warning: extra stuff after LET expression on line " + str(pc)
end if
elseif left(cmd, 5) = "PRINT" then
do_print(cmd)
elseif left(cmd, 4) = "GOTO" then
pc = do_goto(cmd) - 1
elseif left(cmd, 5) = "GOSUB" then
pc = do_gosub(cmd) -1
elseif left(cmd, 7) = "RETURN" then
pc = do_return()
elseif cmd = "END" or cmd = "STOP" then
stoprun = true
elseif left(cmd, 2) = "IF" then
target = do_if(cmd)
if target <> 0 then
pc = target - 1
end if
elseif left(cmd, 5) = "INPUT" then
do_input(cmd)
Elseif left(cmd, 3) = "FOR" then
do_for(cmd)
elseif left(cmd, 4) = "NEXT" then
do_next(cmd)
else
cmd = "LET " + cmd
do_let(cmd)
end if
' print "pc is before next check is ";pc; " cmd is ";cmd
if left(cmd, 4) <> "NEXT" then
'PRINT "WHAT THE HELL IS cmd when <> NEXT";cmd
pc = next_pc(pc)
end if
'print "pc after NEXT check is ";pc
loop
end sub
sub do_print(cmd as string)
' print "do_print ";cmd
dim tok as string
dim comma_last as integer
comma_last = false
buff = mid(cmd, 6)
tok = gettok()
if tok = "~" then
print ""
Exit sub
end if
do while tok<> "~"
comma_last = false
if lefT(tok, 1) = chr(34) then
tok = mid(tok, 2, len(tok) - 2)
print tok + " ";
tok = gettok()
if tok = "," then
tok = gettok()
comma_last = true
end if
else
print Expr_eval(tok + buff) + " ";
buff = buff_after_parse_or: 'print "buff_after_parse_or", buff_after_parse_or
tok = gettok()
if tok = "," then
tok = gettok()
comma_last = true
end if
end if
loop
if not comma_last then
print ""
end if
end sub
function do_if(cmd as string) as integer
Dim b as integer
dim linenum as integer
b = val(Expr_eval(mid(cmd, 3)))
buff = buff_after_parse_or ' look for extraneous stuff
if gettok() <> "THEN" then
print "warning: THEN not found right after expression end on line "; pc
end if
i = instr(1, cmd, "THEN")
if i <> 0 then
linenum = val(mid(cmd, i + 4))
else
print "malformed command is :" + cmd + ": missing THEN"
linenum = 0
end if
if b then
do_if = linenum
else
do_if = 0
end if
end function
sub do_for(cmd as string)
cmd = mid(cmd, 4)
buff = cmd
dim cvar as string
cvar = gettok()
if gettok() <> "=" then
print "Missing = in FOR statment on line "+str(pc)
exit sub
end if
dim istart as integer
dim ilast as integer
dim istep as integer
istart = Val(Expr_eval(buff))
buff = buff_after_parse_or
if gettok() <> "TO" then
print "Missing TO in FOR on line "+str(pc)
exit sub
end if
ilast = Val(Expr_eval(buff))
buff = buff_after_parse_or
istep = 1
if gettok() = "STEP" then
istep = Val(Expr_eval(buff))
end if
buff = buff_after_parse_or
if gettok() <> "~" then
print "Extraneous stuff after STEP value on line " +str(pc)
end if
for_stackp = for_stackp + 1 ' Put For info on stack
acvar(for_stackp) = cvar: ' print "acvar is ";cvar
aistart(for_stackp) = istart: 'print "istart is ";istart
ailast(for_stackp) = ilast: 'print "ilast is "; ilast
aistep(for_stackp) = istep: 'print "istep is "; istep
ailine(for_stackp) = pc: 'print "pc is ";pc
' input i
Set_variable(cvar, str(istart))
end sub
sub do_next(cmd as string)
if for_stackp >= 1 then
' Console.WriteLine("NEXT "+acvar(for_stackp))
else
print "stack underflow NEXT without FOR on line "+str(pc)
pc = next_pc(pc)
exit sub
end if
buff = mid(cmd, 5)
dim controlvariable as string
controlvariable = gettok()
if controlvariable <> acvar(for_stackp) then
print "mismatched next on line "+str(pc)
end if
dim cvar as string
dim start as integer
dim last as integer
dim istep as integer
dim iline as integer
start = aistart(for_stackp): 'print "next start ";start
last = ailast(for_stackp): 'print "next last ";last
istep = aistep(for_stackp): 'print "next istep ";istep
iline = ailine(for_stackp): 'print "next iline ";iline
cvar = acvar(for_stackp)
dim v as integer
v = Val(Get_variable(cvar))
v = v + istep
'print "v is ";v
if istep >= 0 then
if v > last then
'print "******* v > last so now pc is ";pc
pc = next_pc(pc)
'print "******** and now pc is ";pc
for_stackp = for_stackp - 1
else
pc = next_pc(iline)
end if
else
if v < last then
pc = next_pc(pc)
else
pc = next_pc(iline)
end if
end if
'print "next pc is ";pc
Set_variable(cvar, str(v)): 'print "set ";cvar;" to ";str(v)
dim mwait as integer
' input mwait
end sub
function next_pc(pc as integer) as integer
dim i as integer
for i = pc + 1 to 5000
if statement(i) <> "" then
next_pc = i
exit function
end if
next i
next_pc = 0
end Function
function do_goto(cmd as string) as integer
do_goto = val(mid(cmd, 5))
end function
function do_gosub(cmd as string) as integer
do_gosub = val(mid(cmd, 6))
gosub_stack_ptr = gosub_stack_ptr + 1
gosub_stack(gosub_stack_ptr)= pc
end function
function do_return() as integer
if gosub_stack_ptr = 0 then
print "return without matching GOSUB at line ";pc
return 0
end if
do_return = gosub_stack(gosub_stack_ptr)
gosub_stack_ptr = gosub_stack_ptr - 1
end function
sub do_let(cmd as string)
dim cvar as string
dim firstchar as string
buff = mid(cmd, 4)
cvar = gettok()
if cvar = "@" then
do_let_at_array(buff)
exit sub
end if
firstchar = left(cvar, 1)
if not (firstchar >= "A" and firstchar <= "Z") then
print "Variable name must follow LET"
exit sub
elseif gettok() <> "=" then
print "Missing '=' in LET " + cmd
exit sub
end if
Set_variable(cvar, Expr_eval(buff))
end sub
sub do_let_at_array(e as string)
dim leftval as integer
dim rightval as integer
dim rpn as string
buff = e
if gettok() <> "(" then
print "an open left paren ( must follow @ in let"
exit sub
end if
rpn = parse_or()
if gettok() <> ")" then
print "missing right paren ) in LET @"
exit sub
end if
buff_after_parse_or = buff
leftval = Val(Eval_rpn(rpn))
buff = buff_after_parse_or
If gettok() <> "=" then
print "missing = in LET @"
exit sub
end if
rightval = Val(Eval_rpn(parse_or()))
if not (leftval >= 1 and leftval <= 5000) then
print "subscript out of bounds on left side of assignment is "+str(leftval)
exit sub
end if
at_array(leftval) = rightval
end sub
sub do_input(cmd as string)
dim cvar as string
dim firstchar as string
buff = mid(cmd, 6)
cvar = gettok()
if cvar = "@" then
do_input_at_array(buff)
exit sub
end if
firstchar = left(cvar, 1)
if not (firstchar >= "A" and firstchar <= "Z") then
print"Variable name must follow INPUT"
exit sub
end if
dim user_input as string
input user_input
user_input = user_input
user_input = trim(user_input)
Set_variable(cvar, Expr_eval(user_input))
end sub
sub do_input_at_array(e as string)
dim user_input as string
dim subval as integer
dim inpval as integer
dim rpn as string
buff = e
if gettok() <> "(" then
print "an open left paren ( must follow @ in INPUT"
exit sub
end if
rpn = parse_or()
if gettok() <> ")" then
print "missing right paren ) in INPUT @"
exit sub
end if
subval = val(Eval_rpn(rpn))
line input user_input
inpval = Val(Expr_eval(user_input))
if not (subval >= 1 and subval <= 5000) then
' Console.WriteLine("subscript out of bounds on left side of INPUT @ is " + subval)
exit sub
end if
at_array(subval) = inpval
end sub
Function Eval_rpn(rpn as string) as string
buff = rpn
dim tok as string
clearstack()
tok = gettok()
if tok = "~" then
Eval_rpn = "0"
exit function
end if
do while tok<> "~"
process_tok(tok)
tok = gettok()
loop
dim r as string
r = mypop()
Eval_rpn = r
End Function
function Expr_eval(e as string) as string
clearstack()
buff = e
buff = parse_or()
dim tok as string
tok = gettok()
if tok = "~" then
Expr_eval = "0"
exit function
end if
do While tok<> "~"
process_tok(tok)
tok = gettok()
loop
dim r as integer
r = val(mypop())
Expr_eval = str(r)
End function
sub process_tok(tok as string)
' dim logical_op() as string = new string(8) { "dummy", "<", "<=", "=", ">", ">=", "<>", "AND", "OR" }
dim logical_op(9) as string = { "dummy", "<", "<=", "=", ">", ">=", "<>", "AND", "OR" }
dim i as integer
if tok = "UNM" then
mypush(str(-val(mypop())))
exit sub
end if
if tok = "GETAT" then
i = val(mypop())
if i < 1 or i > 5000 then
print "Subscript out of range 1..5000 for " + str(i)
mypush(str(0))
Exit Sub
end if
mypush(str(at_array(i)))
exit sub
end if
for i = 1 to ops
if op(i) = tok then
process_op(op(i))
exit sub
end if
next i
for i = 1 to 8
if logical_op(i) = tok then
process_op(logical_op(i))
exit sub
end if
next i
process_num_or_var(tok)
end sub
sub process_op(op as string)
dim leftval as integer
Dim rightval as integer
dim res as integer
rightval = Val(mypop())
leftval = Val(mypop())
If op = "+" then
res = leftval + rightval
elseif op = "-" then
res = leftval - rightval
elseif op = "*" then
res = leftval*rightval
elseif op = "/" then
res = leftval/rightval
elseif op = "^" then
res = leftval ^ rightval
elseif op = "<" then
res = leftval < rightval
elseif op = "<=" then
res = leftval <= rightval
elseif op = "=" then
' Console.WriteLine("= "+leftval+" "+rightval)
res = leftval = rightval
elseif op = ">" then
res = leftval > rightval
elseif op = ">=" then
res = leftval >= rightval
elseif op = "AND" then
' Console.WriteLine("and "+leftval+" "+rightval)
res = leftval and rightval
elseif op = "OR" then
res = leftval or rightval
else
print "Not an operator " + op
exit sub
End if
mypush(str(res))
end sub
sub process_num_or_var(num as string)
if left(num, 1) >= "0" and left(num,1) <= "9" then
mypush(num)
else
mypush(Get_variable(num))
end if
end sub
dim shared mystack(50) as string
dim shared msp as integer
Sub clearstack()
msp = 1
end sub
Sub mypush(tok as string)
' Console.WriteLine("push " + tok)
mystack(msp) = tok
msp = msp + 1
end sub
Function mypop() as string
msp = msp - 1
' Console.WriteLine("pop sp = "+sp + "tok = " + mystack(sp))
mypop = mystack(msp)
end function
' There must be an API for this, but for now this will work
' Found the API, but my function calls the API
Function rid_eol(inpval as string) as string
rid_eol = inpval
end function
sub Start_variable_store()
' pTree = Tree_New
End sub
sub Set_variable(key as string, item as string)
'print "Set *************** ";key;" to ";item
if len(key) <> 1 then
print "variable name " + key + " is too long"
exit sub
end if
dim nkey as integer
nkey = asc(key) - asc("A") + 1
vars(nkey) = item
'print "var ";nkey;" "; chr(nkey + asc("A") - 1); " in set_var";" is ";item
End sub
Function Get_variable(key as string) as string
' Get_variable = Tree_Get(pTree, key)
if len(key) <> 1 then
'print "variable name " + key + " is too long"
Get_variable = "dummy"
exit function
end if
dim nkey as integer
nkey = asc(key) - asc("A") + 1
Get_variable = vars(nkey)
'print "Get *************** ";nkey;" to ";vars(nkey); " nkey is ";nkey
end function
sub Release_variable_store()
' Tree_Free(pTree)
End sub
' here we go, the gettok function is the so-called lexical scanner.
function gettok() as string
dim collect as string
dim b as string
dim b1 as string
dim i as integer
do while left(buff, 1) = " "
buff = mid(buff, 2)
loop
if buff = "" then
' console.writeline("~")
gettok = "~"
exit function
end if
collect = ""
b = left(buff,1)
b1 = mid(buff, 2, 1)
if b = "<" and b1 = "=" then
buff = mid(buff, 3)
' console.writeline(b + b1)
gettok = b + b1
exit function
end if
if b = "<" and b1 = ">" then
buff = mid(buff, 3)
' console.writeline(b + b1)
gettok = b + b1
exit function
end if
if b = "<" then
buff = mid(buff, 2)
' console.writeline(b)
gettok = b
exit function
end if
if b = ">" and b1 = "=" then
buff = mid(buff, 3)
' console.writeline(b + b1)
gettok = b + b1
exit function
end if
if b = ">" then
buff = mid(buff, 2)
' console.writeline(b)
gettok = b
exit function
end if
for i = 1 to ops
' console.writeline("checking for :" + b + ": against :" + op(i)+":")
if b = op(i) then
' console.writeline("found " + b)
' console.writeline(b)
gettok = b
buff = mid(buff, 2)
exit function
end if
next i
' console.writeline(b + " is not an op")
if b >="A" and b <= "Z" then
do while b >= "A" and b <= "Z" or b >= "0" and b <= "9"
collect = collect + b
buff = mid(buff, 2)
b = left(buff, 1)
loop
' console.writeline(collect)
gettok = collect
exit function
end if
'print "processing "; "b is ";b; " buff is ";buff;" collect is ";collect
if b >= "0" and b <= "9" then
do while b >= "0" and b <= "9"
collect = collect + b
buff = mid(buff, 2)
b = left(buff, 1)
loop
'print "numeric token ";collect
' console.writeline(collect)
gettok = collect
' Console.WriteLine("number token is " + collect)
exit function
end if
if asc(b) = 34 then ' quote mark
collect = b
buff = mid(buff, 2)
b = left(buff, 1)
do while Asc(b) <> 34
collect = collect + b
buff = mid(buff, 2)
b = left(buff, 1)
loop
' console.writeline(collect + chr(34))
gettok = collect + chr(34)
buff = mid(buff, 2)
' Console.WriteLine("text token is " + collect + chr(0x22))
exit function
end if
gettok = "~": rem end of file token
end function
' The remainder of the program constitutes a recursive descent parser. Each function can be
' viewed as a production in a context free grammar. A CFG is a grammar where you can predict what
' is coming based on looking at the next symbol. High precedence operators appear first in the function
' list. as you read down the code, you'll see lower and lower precedence operators.
' A factor is a high precedence bundle of tokens. An identier, a number, a parenthesized expression,
' or a unary minus followed by an expression. you will see tok, and rpn in all the following functions.
' each function returns the rpn of the converted expression fragment.
function parse_factor() as string
dim tok as string
dim rpn as string
' Console.WriteLine("Enter parse_factor with buff = '" + buff +"'")
tok = gettok()
if tok= "(" then
rpn = parse_or()
tok = gettok()
if tok <> ")" then
print "Missing ) before "+buff
buff = ""
parse_factor = "~"
exit function
end if
parse_factor = rpn
exit function
end if
if tok = "@" then
tok = gettok()
if tok <> "(" then
print "Error in let atsign"
buff = ""
parse_factor = "~"
exit function
end if
rpn = parse_or()
tok = gettok()
if tok <> ")" then
print "Missing ) before "+buff
buff = ""
parse_factor = "~"
exit function
end if
' Console.WriteLine("@ evaluated to "+rpn+"GETAT")
parse_factor = rpn + "GETAT "
Exit Function
end if
if tok = "-" then ' I think this is uneccesary after review
rpn = parse_or()
parse_factor = rpn + "UNM "
exit function
end if
' Console.WriteLine("token added to rpn is " + tok + " ")
rpn = tok + " "
parse_factor = rpn
' Console.WriteLine("Exit parse_factor with buff = '" + buff +"'")
end function
function parse_exponent() as string
dim tok as string
dim rpn as string
rpn = parse_factor()
tok = gettok()
do while tok = "^"
rpn = rpn + parse_exponent () + tok
tok = gettok()
loop
buff = tok + " " + buff
parse_exponent = rpn
end function
' parse a unary minus which has less precedence than exponentiation
' that's why unary - is checked here before exponential expressions above
'
' in the rpn we use a separate designation for unary minus (UNM)
' this makes it easy for the vm to understand the different
' interpretations
function parse_unary_minus() as string
dim tok as string
dim rpn as string
dim neg as integer
neg = false
tok = gettok()
do while tok = "-" or tok = "+"
if tok = "-" then
if neg = false then
neg = true
elseif neg = true then
neg = false
end if
end if
tok = gettok()
loop
buff = tok + " " + buff
rpn = parse_exponent()
if neg = true then
rpn = rpn + "UNM "
end if
parse_unary_minus = rpn
end function
function parse_term ()as string
dim tok as string
dim rpn as string
rpn = parse_unary_minus()
tok = gettok()
do while tok = "*" or tok = "/"
rpn = rpn + parse_unary_minus() + tok
tok = gettok()
loop
buff = tok + " " + buff
parse_term = rpn
end function
function parse_expr () as string
dim tok as string
dim rpn as string
rpn = parse_term()
tok = gettok()
do while tok = "+" or tok = "-"
rpn = rpn + parse_term() + tok
tok = gettok()
loop
buff = tok + " " + buff
parse_expr = rpn
end function
function parse_condition () as string
dim tok as string
dim rpn as string
rpn = parse_expr()
tok = gettok()
if left(tok, 1) = "=" or left(tok, 1) = ">" or left(tok, 1) = "<" then
rpn = rpn + parse_expr() + tok
tok = gettok()
end if
buff = tok + " " + buff
parse_condition = rpn
end function
function parse_and () as string
dim tok as string
dim rpn as string
rpn = parse_condition()
tok = gettok()
do while tok = "AND"
rpn = rpn + parse_condition() + tok + " "
tok = gettok()
loop
buff = tok + " " + buff
parse_and = rpn
end function
function parse_or () as string
dim tok as string
dim rpn as string
' Console.WriteLine("Entering parse_or with buff = " + buff)
rpn = parse_and()
tok = gettok()
do while tok = "OR"
rpn = rpn + parse_and() + tok + " "
tok = gettok()
loop
buff = tok + " " + buff
parse_or = rpn
buff_after_parse_or = buff
' Console.WriteLine("Leaving parse_or with buff = " + buff)
end function
That is a lovely trip down memory lane to see that BASIC program on the top. I have my old MC-10 in a corner, and your bubble sort program looks like something that would be right at home on it.
Now, the interpreter itself, that hardly looks like BASIC at all! :) Declared variables? Functions??
Anyway, kudos and congrats.
Oh, and as a random comment, I think it is most logical to consider unary - and exponentiation as being at the same precedence level. If you say - is lower precedence, then you cannot parse `4^-x`, because you can't use a lower-precederence operator inside a higher-precedence one without parentheses. It all works out if you say that they are the same precedence and will resolve ambiguities from right to left.
There are some great comments on [this Stack Exchange comment](https://math.stackexchange.com/a/3047801) that strangely only has two upvotes. It's a bit of syntax lore that seems to be poorly known. I've certainly messed it up myself.
The comment has some test cases that you could try out. For example,if the input is `4^-1^2`, what does your interpreter give back? You want it to go right to left, i.e. `4^(-(1^2))`, which gives back `0.25`. You don't want it to evaluate as `4^((-1)^2)` which would be `4`.