问题
I am having trouble implementing a function that given something like this:
'((+(d + e) + f) + (a + (a + c))
Returns this:
'(d + (e + (f + (a + (a + c)))))
Now the catch is that I have to use associativity to get the answer so I only want to use this property:
'((a + b) + c) = '(a + (b + c))
To get the correct output. I know how to implement the function for this case:
'((a + b) + c) -> '(a + (b + c))
However I cannot seem to figure out how to implement it for the first case above (or any other case larger than three items). I am not looking for answers only some guidance. If you would like to see code snippets let me know and I can post some. Also, I created a function that stripped the '+ from the list to make it easier to process.
I think this defines the grammar:
var ::= a | b | c | d | e | f | g
fpip ::= var | (fpip + fpip)
Where a valid fpip can be:
fpip = '((a + b) + c)
or
fpip = '((a + b) + (c + d))
and at it's most atomic level:
fpip = '(a + b)
edit: yes the initial '+ should be erased. Regardless of how many plus signs there are in the initial input there should only be (amount-of-vars - 1) '+ in the output. For example:
'(+ + + + (a + b) + + c) -> '(a + (b + c)
回答1:
I think these are the cases to consider. Let's call the the recursive rewriter REWRITE.
var -> var
(var + var) -> (var + var)
(var + (fip1 + fpip2)) -> (var + (REWRITE (fip1 + fpip2))
((fip1 + fpip2) + var) -> (REWRITE (fip1 + (fip2 + var))
((fip1 + fpip2) + (fip3 + fpip4)) -> (REWRITE (fip1 + (fip2 + (fip3 + fip4))))
来源:https://stackoverflow.com/questions/49864511/reordering-parentheses-using-associative-property-in-racket