On 5/30/06, Thorkil Naur <[EMAIL PROTECTED]> wrote:
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
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