;;;
;;; CS 200
;;; 
;;; listprocs.ss
;;;
;;; This file contains many of the list procedures we have defined in class.
;;;

(require-library "trace.ss")
                 
(define (insertlg f lst start)
  (if (null? lst) 
      start
      (f (car lst) (insertlg f (cdr lst) start))))

;;; Evaluates to the list parameter with exactly one instance of el removed.
(define (delete lst el)
  (if (null? lst) null ;; okay to not find it now: (error "Element not found!")
      (if (eqv? (car lst) el)
          (cdr lst)
          (cons (car lst) (delete (cdr lst) el)))))

(define (find-most cf lst)
  (insertlg
   (lambda (c1 c2)
     (if (cf c1 c2) c1 c2))
   lst
   (car lst)))

(define (revintsto n)
  (if (= n 0) null
      (cons n (revintsto (- n 1)))))

(define (intsto n)
  (reverse (revintsto n)))

(define (rand-int-list n)
  (if (= n 0) null
      (cons (random 100) (rand-int-list (- n 1)))))

(define (filter f lst)
  (insertlg
   (lambda (el rest) (if (f el) (cons el rest) rest))
   lst
   null))

;;;
;;; member? evaluates to true if thing is in list (could use memq for this)
;;;

(define (member? thing lst)
  (if (null? lst) #f
      (if (eq? thing (car lst)) #t
	  (member? thing (cdr lst)))))