Equation Calculator

CLS
ON ERROR GOTO ending
t1$ = "Equation Calculator 2.3.5"
t2$ = "By ePuter"
LOCATE 1, 40 - LEN(t1$) / 2: PRINT t1$
LOCATE 2, 40 - LEN(t2$) / 2: PRINT t2$
PRINT
PRINT " Welcome! The program allows you"
PRINT "to enter any equation containing"
PRINT "a parentheses ( or ), power as ^"
PRINT "*, /, +, -, ., and/or a negation"
PRINT "and will give you a step-by-step"
PRINT "solution according to the prior-"
PRINT "ity of the operations."
PRINT " The program will not understand"
PRINT "spaces or any other characters."
PRINT "Enjoy it!"
PRINT : PRINT "Example: 120*-(6+2/(4/2)^3)+60*(54-5)^(1/2)"
PRINT : PRINT
INPUT "Enter the equation: ", maineqzn$
PRINT
PRINT maineqzn$
power = 0
multiply = 1
divide = 2
add = 3
subtract = 4
DO
  p = 1
  DO
    c1 = INSTR(p, maineqzn$, "(")
    IF c1 = 0 THEN eqzn$ = maineqzn$: EXIT DO
    c2 = INSTR(c1 + 1, maineqzn$, "(")
    c3 = INSTR(maineqzn$, ")")
    IF c3 < c2 OR c2 = 0 THEN eqzn$ = MID$(maineqzn$, c1 + 1, c3 - c1 - 1): EXIT DO ELSE p = c1 + 1
  LOOP
  DO
    IF INSTR(eqzn$, "E") <> 0 THEN EXIT DO
    a = INSTR(eqzn$, "^"): opr = power
    IF a = 0 THEN a = INSTR(eqzn$, "*"): opr = multiply
    IF a = 0 THEN a = INSTR(eqzn$, "/"): opr = divide
    IF a = 0 THEN a = INSTR(eqzn$, "+"): opr = add
    IF a = 0 THEN a = INSTR(eqzn$, "-"): opr = subtract
    IF a = 1 THEN
      a = INSTR(a + 1, eqzn$, "-"): IF a = 0 THEN EXIT DO
    END IF
    IF a = 0 THEN EXIT DO
    i1 = a - 1
    DO
repeat1:
      i1 = i1 - 1
      IF i1 <= 0 THEN i1 = 0: GOTO found1
      b$ = MID$(eqzn$, i1, 1)
      SELECT CASE VAL(b$)
        CASE 1 TO 9: GOTO repeat1
        CASE 0:
          SELECT CASE b$
            CASE ".": GOTO repeat1
            CASE "0": GOTO repeat1
            CASE "-": i1 = i1 - 1: GOTO found1
            CASE ELSE: GOTO found1
          END SELECT
      END SELECT
    LOOP
found1:
    i1 = i1 + 1
    num1 = VAL(MID$(eqzn$, i1, a - i1))
    i2 = a + 1
    DO
repeat2:
      i2 = i2 + 1
      IF i2 >= LEN(eqzn$) THEN i2 = LEN(eqzn$) + 1: GOTO found2
      b$ = MID$(eqzn$, i2, 1)
      SELECT CASE VAL(b$)
        CASE 1 TO 9: GOTO repeat2
        CASE 0:
          SELECT CASE b$
            CASE ".": GOTO repeat2
            CASE "0": GOTO repeat2
            CASE ELSE: GOTO found2
          END SELECT
      END SELECT
    LOOP
found2:
    i2 = i2 - 1
    num2 = VAL(MID$(eqzn$, a + 1, i2 - a))
    SELECT CASE opr
      CASE power: num = num1 ^ num2
      CASE multiply: num = num1 * num2
      CASE divide:
        IF num2 = 0 THEN PRINT : PRINT "Warning: Division by zero.": END ELSE num = num1 / num2
      CASE add: num = num1 + num2
      CASE subtract: num = num1 - num2
    END SELECT
    IF num >= 0 THEN
      num$ = MID$(STR$(num), 2)
    ELSE
      num$ = STR$(num)
    END IF
    IF num1 < 0 THEN
      IF num >= 0 AND i1 > 1 THEN num$ = "+" + num$
    END IF
    eqzn$ = LEFT$(eqzn$, i1 - 1) + num$ + RIGHT$(eqzn$, LEN(eqzn$) - i2)
    IF c1 = 0 THEN
      PRINT eqzn$
    ELSE
      PRINT LEFT$(maineqzn$, c1) + eqzn$ + RIGHT$(maineqzn$, LEN(maineqzn$) - c3 + 1)
    END IF
    IF num < 0 THEN
      IF eqzn$ = STR$(num) THEN EXIT DO
    ELSE
      IF eqzn$ = MID$(STR$(num), 2) THEN EXIT DO
    END IF
  LOOP
  IF c1 <> 0 THEN
    maineqzn$ = LEFT$(maineqzn$, c1 - 1) + eqzn$ + RIGHT$(maineqzn$, LEN(maineqzn$) - c3)
    PRINT maineqzn$
  END IF
LOOP UNTIL c1 = 0
END
ending:
PRINT : PRINT "Warning: Syntax error or overflow.": END