Expression evaluator

[Agamemnus]

in qb code, this is what one gets:

Code:

DECLARE FUNCTION reduce.string$ (string1$)
DEFINT A-Z
CLS
PRINT reduce.string$("5 + 5 * 13")

FUNCTION reduce.string$ (string1$)

string1$ = "(" + string1$ + ")"
DO
read.loc = 1
start.p = 0
op$ = ""
string2$ = ""
string3$ = ""
op.exists = 0
num1$ = ""
num2$ = ""
num3$ = ""
st$ = ""
i = 0
j = 0
string1a$ = ""
string1b$ = ""

'get the location of the innermost parentheses

DO
start.p = INSTR(read.loc, string1$, "(")
IF start.p = 0 THEN EXIT DO
read.loc = start.p + 1
LOOP

'find the end of the parentheses-enclosed statement.
end.loc = INSTR(read.loc, string1$, ")")

'removing the innermost ( ), save everything before as string2, everything after as string3, and extract the parentheses-enclosed statement as string1.

string2$ = MID$(string1$, 1, read.loc - 2)
IF LEN(string1$) - end.loc > 0 THEN string3$ = MID$(string1$, 1 + end.loc, LEN(string1$) - end.loc) ELSE string3$ = ""
string1$ = MID$(string1$, read.loc, end.loc - read.loc)

'find an operand, starting from the left and going in the order: /, *, -, +
DO
op.exists = INSTR(string1$, "/ "): IF op.exists THEN op$ = "/": GOTO ex1
op.exists = INSTR(string1$, "* "): IF op.exists THEN op$ = "*": GOTO ex1
op.exists = INSTR(string1$, "- "): IF op.exists THEN op$ = "-": GOTO ex1
op.exists = INSTR(string1$, "+ "): IF op.exists THEN op$ = "+" ELSE EXIT DO
ex1:
'operate on the two numbers left and right of the operand.
'PRINT string2$ + "(" + string1$ + ")" + string3$
num1$ = "": num2$ = ""
i = op.exists - 2
DO
st$ = MID$(string1$, i, 1)
IF INSTR("-0123456789/", st$) <> 0 THEN
num2$ = st$ + num2$
ELSE
EXIT DO
END IF
i = i - 1
IF i = 0 THEN EXIT DO
LOOP

j = i: i = op.exists + 2
DO
st$ = MID$(string1$, i, 1)
IF INSTR("-0123456789/", st$) <> 0 THEN
num1$ = num1$ + st$
ELSE
i = i - 1
EXIT DO
END IF
IF i = LEN(string1$) THEN EXIT DO
i = i + 1
LOOP

SELECT CASE op$
CASE "+": num3$ = LTRIM$(STR$(VAL(num1$) + VAL(num2$)))
CASE "-": num3$ = LTRIM$(STR$(VAL(num1$) - VAL(num2$)))
CASE "*": num3$ = LTRIM$(STR$(VAL(num1$) * VAL(num2$)))
CASE "*": num3$ = LTRIM$(STR$(VAL(num2$) / VAL(num1$)))
END SELECT

string1a$ = MID$(string1$, 1, j)
string1b$ = MID$(string1$, i + 1, LEN(string1$) - i)
string1$ = string1a$ + num3$ + string1b$

LOOP

'now set the value of the function
string1$ = string2$ + string1$ + string3$
IF INSTR(string1$, " ") = 0 THEN EXIT DO
LOOP
reduce.string$ = string1$
END FUNCTION