On 26/04/12 19:06, Asranz wrote:
Oh sorry i mean i didt wit sums and multiplications but i dont get how
it can do it conbined in the same line
On 25 abr, 22:18, Asranz<alekx.on...@gmail.com> wrote:
Hi. im having a little problem i need to do a parser like to do
operations but i dont get how to do it very well
ex.
"1+2*3+4-5*6+7*8-9" = 431
i have to do it with sums but i dont get it how i can take all the
operations.
any advices?
Here it is:
------------------------------------------------------------------------
;(infix (1 + 4 * 5 - 10 / 3)) -> 5
(defmacro infix [expr]
`(loop [
ops# (filter #(not (number? %)) '~expr) ;operators
args# (filter #(number? %) '~expr) ;operands
]
(if (empty? ops#) (first args#)
(recur
(rest ops#)
(conj (rest (rest args#))
((eval (first ops#)) (first args#) (second args#)))))))
------------------------------------------------------------------------------
It is worth pointing out that this macro violates one of the
macro-writing 'rules'...that is, macros are not supposed to return
values but expressions. It would be nicer if instead of calculating the
actual result inside the macro, to simply transform the expression into
prefix form and let clojure calculate the result on that...it also would
be easier to write than this...
Jim
ps: the above macro does not handle parenthesis or operator
precedence....if you want to go the full way have a look at the incanter
source...hope that helps!
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