gistfile1.sls

#lang scheme

; http://pheattarchive.emporia.edu/courses/2002/cs220f02/anotherSchemeTtutorial/excercises.htm
; Exercise 1: 
; Write a recursive procedure (positions 1st e) which returns a list of numbers corresponding
; to the position  of every occurrence of element e in the list 1st.
; Assume the first element of the list is element 0. For example:
; (positions '(a b f b a f b b) 'b) ==> (1 3 6 7)
;
; hint: use a helper function

(define (positions lst e)
  (define (positions-helper xs i)
    (if (null? xs) '()
      (append
        (if (equal? (car xs) e)
          (list i)
          '())
        (positions-helper (cdr xs) (+ 1 i)))
     )
   )
  (positions-helper lst 0)
)

(positions '(a b f b a f b b) 'b)