Expr = {}
Expr.eval = 0
BinaryExpr = new Expr
BinaryExpr.eval = function()
if self.op == "+" then return self.lhs.eval + self.rhs.eval
if self.op == "-" then return self.lhs.eval - self.rhs.eval
if self.op == "*" then return self.lhs.eval * self.rhs.eval
if self.op == "/" then return self.lhs.eval / self.rhs.eval
end function
binop = function(lhs, op, rhs)
e = new BinaryExpr
e.lhs = lhs
e.op = op
e.rhs = rhs
return e
end function
parseAtom = function(inp)
tok = inp.pull
if tok >= "0" and tok <= "9" then
e = new Expr
e.eval = val(tok)
while inp and inp[0] >= "0" and inp[0] <= "9"
e.eval = e.eval * 10 + val(inp.pull)
end while
else if tok == "(" then
e = parseAddSub(inp)
inp.pull // swallow closing ")"
return e
else
print "Unexpected token: " + tok
exit
end if
return e
end function
parseMultDiv = function(inp)
next = @parseAtom
e = next(inp)
while inp and (inp[0] == "*" or inp[0] == "/")
e = binop(e, inp.pull, next(inp))
end while
return e
end function
parseAddSub = function(inp)
next = @parseMultDiv
e = next(inp)
while inp and (inp[0] == "+" or inp[0] == "-")
e = binop(e, inp.pull, next(inp))
end while
return e
end function
while true
s = input("Enter expression: ").replace(" ","")
if not s then break
inp = split(s, "")
ast = parseAddSub(inp)
print ast.eval
end while