Expression evaluator

[na_th_an]

Well, I fixed things here and there and added support for parentheses. It wasn't that hard. I had problems with the part I had already coded 'cause it read the expressions from left to right... (silly me: recursion works upside down when it is NOT FINAL!!). I've written my own INSTR string which searches from END to BEGINNING.

Parentheses processing takes 22 lines

Code:

DECLARE FUNCTION instrRev% (st1$, st2$)
DECLARE FUNCTION eval% (cmd$)
CLS
PRINT eval%("3+(-3*4+(3-4*(4+3)))")
PRINT
PRINT 3 + (-3 * 4 + (3 - 4 * (4 + 3)))

FUNCTION eval% (cmd$)
   ' Preprocess parentheses **********************************************
   Init% = 1
   DO
      a% = INSTR(Init%, cmd$, "(")
      IF a% <> 0 THEN
         Init% = a% + 1: np% = 1: Ps% = Init%: Res$ = ""
         DO
            IF Ps% > LEN(cmd$) THEN ' Unbalanced Parentheses!
               Res$ = "0": Ps% = Init% + 1
               EXIT DO
            END IF
            m$ = MID$(cmd$, Ps%, 1)
            IF m$ = ")" AND np% = 1 THEN EXIT DO
            Res$ = Res$ + m$
            Ps% = Ps% + 1
            IF m$ = "(" THEN np% = np% + 1
            IF m$ = ")" THEN np% = np% - 1
         LOOP
         r% = eval%(Res$)
         cmd$ = LEFT$(cmd$, Init% - 2) + STR$(r%) + RIGHT$(cmd$, LEN(cmd$) - Ps%)
      END IF
   LOOP WHILE a% <> 0
   ' *********************************************************************
  
   cmd$ = LTRIM$(RTRIM$(cmd$))

   a% = instrRev%(cmd$, "+")
   b% = instrRev%(cmd$, "-")

   IF a% = 0 AND b% = 0 THEN
      a% = instrRev%(cmd$, "*")
      b% = instrRev%(cmd$, "/")
      IF a% = 0 AND b% = 0 THEN
         Res% = VAL(cmd$)
      ELSE
         IF a% > b% THEN
            Res% = eval%(LEFT$(cmd$, a% - 1)) * eval%(RIGHT$(cmd$, LEN(cmd$) - a%))
         ELSE
            Res% = eval%(LEFT$(cmd$, b% - 1)) / eval%(RIGHT$(cmd$, LEN(cmd$) - b%))
         END IF
      END IF
   ELSE
      ' Detect Unary "-" and remove:
      IF b% > 1 THEN
         lp$ = RTRIM$(LEFT$(cmd$, b% - 1))
         IF RIGHT$(lp$, 1) <= "0" OR RIGHT$(lp$, 1) >= "9" THEN b% = 0
      END IF
      IF a% > b% THEN
         Res% = eval%(LEFT$(cmd$, a% - 1)) + eval%(RIGHT$(cmd$, LEN(cmd$) - a%))
      ELSE
         Res% = eval%(LEFT$(cmd$, b% - 1)) - eval%(RIGHT$(cmd$, LEN(cmd$) - b%))
      END IF
   END IF
   eval% = Res%
END FUNCTION

FUNCTION instrRev% (st1$, st2$)
   Res% = 0
   FOR i% = LEN(st1$) TO 1 STEP -1
      IF MID$(st1$, i%, 1) = st2$ THEN Res% = i%: EXIT FOR
   NEXT i%
   instrRev% = Res%
END FUNCTION