On 5/30/06, Thorkil Naur
Hello,
It seems that what you need is the technique of evaluating an expression in reverse polish notation by using a stack. This technique is well known in subjects related to compiler construction.
Basically, the expression (list of operands and operators) is processed sequentially from left to right. If an operand (in your case: A character c for which isNumber c is true) is encountered, it is pushed onto the stack. If an operator (in your case: A character c for which isLetter is true) is enountered, it pops its operands (one or more) from the top of the stack and pushes the result of applying the operator to the operands back onto the stack.
The following code sketch uses this idea:
... import Char ... isNumber = isDigit isLetter = isAlpha
cvBase stack c | isNumber c = Folha c:stack cvBase (tr:tl:stack) c | isLetter c = Node c tl tr : stack
cv s = let [t] = foldl cvBase [] s in t
Example using Hugs:
Main> cv "12H3V" Node 'V' (Node 'H' (Folha '1') (Folha '2')) (Folha '3') Main>
A bit OT perhaps, but I'd just like to point out this wonderful little code snippet from the Haskell wiki-page that I've always liked, in case nobody's seen it: calc :: String -> Float calc = foldl f [] . words where f (x:y:zs) "+" = y+x:zs f (x:y:zs) "-" = y-x:zs f (x:y:zs) "*" = y*x:zs f (x:y:zs) "/" = y/x:zs f xs y = read y : xs That's it. An RPN evaluator in a couple of lines. -- Sebastian Sylvan +46(0)736-818655 UIN: 44640862