Duncan Booth
2/25/2008 2:56:00 PM
MartinRinehart@gmail.com wrote:
> My parser has found an expression of the form CONSTANT_INTEGER
> OPERATOR CONSTANT_INTEGER. I want to fold this into a single
> CONSTANT_INTEGER.
>
> The OPERATOR token has an intValue attribute, '+' == 0, '-'== 1, etc.
> In C I'd put functions Add, Subtract, ... into an array and call
> ArithmeticFunctions[ intValue ] to perform the operation. Would I
> index into a list of functions in Python, or would IF/ELIF/ELIF ... be
> the way to go?
>
Well, you could add another attribute to the OPERATOR token:
token.operation = operator.add
or you could have an a list of operations keyed in token.intValue:
[ operator.add, operator.sub, ...]
or you could use a dictionary (that intValue attribute is just so C):
{ '+': operator.add, '-': operator.sub, ... }
I don't know the structure of your program, but I'd go for subclassing
your operator. So you would have classes OperatorAdd, OperatorSub, etc.
and then you can make the operation a method of the class. Seomthing
like:
class Token(object): ...
def foldConstants(self): pass
class Operator(Token): ...
def foldConstants(self):
self.left.foldConstants()
self.right.foldConstants()
if self.left.isConstant and self.right.isConstant:
self.value, self.isConstant = self.doFold()
class OperatorAdd(Operator): ...
def doFold(self):
return self.left.value + self.right.value, True