expression evaluators

As you probably know JB lacks EVAL function

but the hardly stops anyone

Here three (old) attempts at evaluating math expression from string, two mine, one from old forum - original author is Balaji

If you have better one just add.

1. Really naïve attempt at evaluating.

Find innermost expression - evaluate - substitute back - continue until done.

At JB Files archive.

jbfilesarchive.com/phpBB3/viewtopic.php?f=5&t=1230

2. one with lot's of stacks - likely shunting yard related.

a program which parses and evaluates arithmetic expressions with An abstract-syntax tree (AST)

on Rosetta code

rosettacode.org/wiki/Arithmetic_evaluation#Liberty_BASIC

3. FreeCalc by Balaji
I think this one uses recursive descent approach
last version I have stored commented
'v.2.01 - modif by Stefan Pendl
'v.2.01 - modif by Stefan Pendl
   global inp$, pos, char$, error$, count, useDegrees
   dim var$(100), value(100)

   ' default for angular units is radians
   useDegrees = 0

   print "FreeCalc 2.0"
   print "Welcome. Enter 'help' to view FreeCalc Help. Enter 'exit' to close FreeCalc"
   print
   print "Trigonometric Functions use Radians!"
   print

[input]
   input inp$
   if inp$="" then [input]
   inp$=lower$(inp$)
   gosub [command]
   if command then [input]
   ans$=JBeval$()
   if error$<>"" then print:ans$="ERROR: ";error$
   print ans$
   if error$<>"" then print
   goto [input]


[command]
   command=1
   select case inp$
   case "exit"
       print "Thank You for using FreeCalc"
       END
   case "help"
       print "FreeCalc Help"
       print "    FreeCalc is an expression evaluator. It supports variables and can store values."
       print "Integers, floating point & scientific notation numbers are accepted.."
       print: print "Operators(In evaluation order):"
       print "-  -> Negative"
       print "() -> Parenthesis"
       print "*/ -> Multiplication, Division"
       print "+- -> Addition, Subtraction"
       print: print "Expressions can be entered directly. Assignment statements must start with ':'"
       print "Variables names must consist of one or more alphabets. No digits are allowed."
       print: print "Examples:"
       print "?12*15"
       print "?:side=5.1e2/2"
       print "?:area=pow(side,2)"
       print: print "Predefined functions:"
       print "pi(), sin(n), cos(n), tan(n), asin(n), acos(n), atan(n), log(n), exp(n), sqrt(n), pow(n, p)"
       print: print "Commands:"
       print "about   -> Displays information about FreeCalc"
       print "clear   -> Clears the screen"
       print "exit    -> Exit FreeCalc"
       print "help    -> FreeCalc help"
       print "var     -> Displays currently used variables and their values"
       print "degrees -> Values for trigonometric functions are degrees"
       print "radians -> Values for trigonometric functions are radians (the default)"
       print "Commands and variable names are not case sensitive": print

       case "clear"
       cls

       case "var"
       print "Total ";count;" variables stored in memory"
       for i=1 to count
           print var$(i);"=";value(i)
       next i

       case "degrees"
       print:print "Trigonometric Functions use Degrees!":print
       useDegrees = 1

       case "radians"
       print:print "Trigonometric Functions use Radians!":print
       useDegrees = 0

       case "about"
       print "FreeCalc"
       print "Version 2.0"
       print "Written by Balaji": print

       case else
       command=0
   end select
   return

function JBeval$()
   pos=0: error$=""
   call nextChar
   if char$=":" then
       JBeval$=assignment$()
   else
       JBeval$=str$(expression())
   end if
   if pos<=len(inp$) and error$="" then error$="Unknown Command: "; inp$
end function

function assignment$()
   call nextChar
   var$=getID$()
   if error$<>"" then exit function
   index=getIndex(var$)
   if match("=")=0 then exit function
   value=expression()
   value(index)=value
   assignment$=var$(index);"=";value(index)
end function

function expression()
   expression=addTerm()
   if error$<>"" then exit function
end function

function addTerm()
   ans=multTerm()
   do while instr("+-", char$)>0
       op$=char$
       call nextChar
       temp=multTerm()
       if error$<>"" then exit function
       if op$="+" then ans=ans+temp else ans=ans-temp
   loop
   addTerm=ans
end function

function multTerm()
   ans=factor()
   do while instr("*/", char$)>0
       op$=char$
       call nextChar
       temp=factor()
       if error$<>"" then exit function
       if op$="*" then
           ans=ans*temp
       else
           if temp=0 then
               error$="Division by zero"
               exit function
           end if
           ans=ans/temp
       end if
   loop
   multTerm=ans
end function

function factor()
   if isAlpha() then
       id$=getID$()
       if char$="(" then
           factor=getFunction(id$)
       else
           factor=getValue(id$)
       end if
       exit function
   end if
   select case char$
   case "("
       call nextChar
       factor=expression()
       if error$<>"" then exit function
       if match(")")=0 then exit function
   case "-"
       call nextChar
       factor=-1*factor()
   case else
       factor=getNum()
   end select
end function

function getValue(var$)
   i=getIndex(var$)
   getValue=value(i)
end function

function getIndex(var$)
   for i=1 to count
       if var$(i)=var$ then
           getIndex=i
           exit function
       end if
   next i
   count=count+1
   var$(count)=var$
   getIndex=count
end function

function getID$()
   do while isAlpha()
       getID$=getID$;char$
       call nextChar
   loop
   if getID$="" then error$="Expected variable"
end function

function getNum()
   do while isDigit()
       n$=n$;char$
       call nextChar
   loop
   if n$="" then error$="Expected Number"
   if char$="." then
       n$=n$;"."
       call nextChar
       do while isDigit()
           n$=n$;char$
           call nextChar
       loop
   end if
   if char$="e" then
       n$=n$;"e"
       call nextChar
       do while isDigit()
           n$=n$;char$
           call nextChar
       loop
   end if
   getNum=val(n$)
end function

function getFunction(fname$)
   if match("(")=0 then exit function
   if instr(" pi ", " ";fname$;" ") then call param0
   if instr(" sin cos tan asin acos atan sqrt exp log ", " ";fname$;" ") then call param1 n
   if instr(" sin cos tan ", " ";fname$;" ") and (useDegrees = 1) then n = asn(1) / 90 * n
   if instr(" pow ", " ";fname$;" ") then call param2 n1, n2
   if error$<>"" then exit function
   select case fname$
   case "pi"
       ans=asn(1)*2
   case "sin"
       ans=sin(n)
   case "cos"
       ans=cos(n)
   case "tan"
       ans=tan(n)
   case "asin"
       ans=asn(n)
   case "acos"
       ans=acs(n)
   case "atan"
       ans=atn(n)
   case "sqrt"
       ans=sqr(n)
   case "exp"
       ans=exp(n)
   case "log"
       ans=log(n)
case "pow"
       ans=n1 ^ n2
   case else
       error$="Unknown function: "; fname$
   end select
   if instr(" asin acos atan ", " ";fname$;" ") and (useDegrees = 1) then ans = 90 / asn(1) * ans
   getFunction=ans
end function

sub param0
   if match(")")=0 then exit sub
end sub

sub param1 BYREF num
   num=expression()
   if error$<>"" then exit sub
   if match(")")=0 then exit sub
end sub

sub param2 BYREF num1, BYREF num2
   num1=expression()
   if error$<>"" then exit sub
   if match(",")=0 then exit sub
   num2=expression()
   if error$<>"" then exit sub
   if match(")")=0 then exit sub
end sub

function isDigit()
   isDigit=(char$>="0" and char$<="9")
end function

function isAlpha()
   isAlpha=(char$>="a" and char$<="z")
end function

function match(c$)
   if char$=c$ then
       match=1
       call nextChar
   else
       error$="Expected ";c$
   end if
end function

sub nextChar
   pos=pos+1
   char$=mid$(inp$, pos, 1)
end sub