Question

LANGUAGE: SCHEME R5RS

Create a procedure called preceeding that will take a list of numbers as argument and determine the elements and indices of those elements that preceed a negative number in the given list. The returned information should be in the form of a pair of lists: ((values) . (indices)).
E.g.

(preceeding '(1 -2 3 4 -5 6 -7 -8)) → ((1 4 6 -7).(0 3 5 6)) ;note: drracket will display this as ((1 4 6 -7) 0 3 5 6)

Answer 1

Here is the function:

(define preceeding2 
  (lambda (lst indexes where)
    (if (< (length lst) 2)  (append ()  (cons indexes ()))
      (if (< (cadr lst) 0) 
        ;(append 
          (cons
            (car lst)
            (preceeding2 (cdr lst) (append indexes (cons where ())) (+ where 1) ) 
          ) 
          ;(append indexes (cons where ()) )
        ;)
       (preceeding2 (cdr lst) indexes (+ where 1))
      )
    )
  )
)

(define (preceeding lst)
   (preceeding2 lst '() 0)
  

)
(preceeding '(1 -2 3 4 -5 6 -7 -8))