How to convert String to Binary in Scheme

Question

So, I've ran into this problem where I've been trying to code a means to convert a binary number to a string in Scheme (R5RS), but I've been having some....problems. My tutor and I tried to solve it over the course of an hour, but we haven't been able to solve it.

In fact, as of the time I finished the last session, he left behind the following:

    (define binary->string (lambda (lst)
                         (define str (make-string 0)) 
                         (letrec ((recurse (lambda (l)
                                             ; turn into not null
                                             (if (null? l)
                                                 (lambda ()
                                                   (set! str (string-append str (number->string (car l))))
                                                   (recurse (cdr l)))
                                                 )
                                             )
                                           )
                                  )
                           (recurse lst)
                          )
                         str)
  )

(binary->string '(0 1 0 1 0 0 1 1))

See where it says "turn into not null"? Well, we've had some problems trying to convert the number into not null. I sure hope this helps. Thanks!

Answer 1

So you want this result?

> (binary->string '(0 1 0 1 0 0 1 1))
"01010011"

I see you've already used string-append and number->string, so you should also use apply and map:

(define (binary->string arg)
  (apply string-append
         (map number->string arg)))

> (binary->string '(0 1 0 1 0 0 1 1))
"01010011"

If you also want to convert it into decimal number, use string->number with radix 2:

> (string->number "01010011" 2)
83