;;pick function will return the nth element in the list l.
(defun pick (n l)
  ( cond
    ( (null l) l)
    ( (eq n 1) (car l))
    ( t ( pick (- n 1) (cdr l)))
  )
 )

;pick demo
(pick 1 '(4 5 6))
; 4
(pick 4 '(4 5 6))
;nil


;;leftmost function will return the leftmost atom in a given list l.
(defun leftmost (l)
  ( cond
    ( (null l) l)
    ( (atom (car l)) (car l))
    ( t (leftmost (car l)))
  )
)

;leftmost demo
(leftmost '(4 5 6))
; 4
(leftmost '( (4 5) 6 (3 4)))
; 4
(leftmost '( ( (2 3) 4) 5 (6 7)))
; 2
(leftmost '( ( (4) 5) 6 (3 4)))
; 4

;;car cdr car == cadar
;;cdr cdr car == cddar

(cdr (car (cdr '(9 (3 5) (7 5 (3 4))) ) ) )
; 5

(cdadr '(9 (3 5) (7 5 (3 4))))
; 5

(caadr '( (2 3) (1 2) (5 6 7)))
; 1

(car (car (cdr '((2 3) (1 2) (5 6 7)))))
; 1


;; declare x = 2
(set 'x 2)

x
; 2


;; set + quote so you don't need to put the quote before y
(setq y 3)
y
; 3

(+ x y)
; 5
(* x y)
; 6

;;you can find a car/cdr of a generated list
(car (list 3 4 'a 'b 6))
; 3

;;condition with manipulation
( cond
  ( (> x 5) (+ x 1))
  ( t (- x 1))
  )
; 1

;;conditions and or not 
(if (> x 5) (+ x 1) (- x 1))
; 1
(if (not (or (> x 5) (< x 0))) (+ x 1) (- x 1))
; 3
;same as before with cond


;; display maximum item of list
(max 3 5)
; 5

;; apply operator
(apply '+ '(3 4 5))
; 12


;;you can substitute ' with quote
(setq op '*)
; *

;;substitute our variable op for *
(apply op '(3 4 5))
; 60

;;same but with max
(setq op2 'max)
; max
(apply op2 '(3 4 5))
; 5
(apply op2 '(4 7 3))
; 7 

;; returns true if is a number
(numberp 4)
; t

(numberp 'a)


(numberp '(3 4))
; nil

(numberp 100)
; t


;;returns items in list that aren't numbers
(defun no-num (l)
  ( cond
    ( (null l) l)
    ( (numberp (car l)) (no-num (cdr l)))
    ( t (cons (car l) (no-num (cdr l))))
    )
  )


(no-num '(1 'a 'b 3 'flower))
; ('a 'b 'flower)

(no-num '(3 5 6))
; nil

x
; 2
y
; 3
(no-num '(a b c x y))
; (a b c x y) 
(no-num '(5 'a 'b 'flower 100 15 'car))
; ('a 'b 'flower 'car)