For example, I have this string: "6119726089.12814713"
If I do (string->number "6119726089.12814713") - using the SISC implementation the result is 6.119726089128147e9 - and in Guile implementation is 6119726089.128147 but I would like an exact number, like: 611972608912814713/100000000 without loss precision.
I'd like a function like (string->exact) or something like this.
NOTE: please fix my non-native English and remove this message. Thanks.
Use (string->number "#e6119726089.12814713") to parse the number as exact. This works for at least Racket and Guile. It may not work correctly on other Scheme implementations, however; they are at liberty to parse as inexact first, then convert.
Here's a portable implementation of the string->exact function that the OP asked for. I've manually tested it with a range of inputs, but you should do your own testing to ensure it fits your needs:
(define (string->exact str)
(define zero (char->integer #\0))
(let loop ((result #f)
(factor 1)
(seen-dot? #f)
(digits (string->list str)))
(if (null? digits)
(and result (/ result factor))
(let ((cur (car digits))
(next (cdr digits)))
(cond ((and (not result) (not seen-dot?) (char=? cur #\-))
(loop result (- factor) seen-dot? next))
((and (not seen-dot?) (char=? cur #\.))
(loop result factor #t next))
((char<=? #\0 cur #\9)
(loop (+ (* (or result 0) 10) (- (char->integer cur) zero))
(if seen-dot? (* factor 10) factor)
seen-dot? next))
(else #f))))))