;;11/11/21

;;this 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 1 '(4 5 6))
(pick 4 '(4 5 6))

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


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

(set 'x 2)
x

(setq y 3)
y
x
(+ x y)
(* x y)

(setq z (* x y))
z


(car (list 3 4 'a 'b 6))


( cond
  ( (> x 5) (+ x 1))
  ( t (- x 1))
)

(if (not (or (> x 5) (< x 0))) (+ x 1) (- x 1))

(max 3 5)

(apply '+ '(3 4 5))

;;does not work. Invalid syntax. 3 4 5 need to be in a list --> '(3 4 5)
(apply '+ 3 4 5)

(setq op '*)

(apply op '(3 4 5))

(setq op2 'max)

(apply op2 '(3 6 9))


(numberp 4)

(numberp 'a)

(numberp '(3 4))

(numberp 100)

;;this function will return a list of the non-number members of the given list.
(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))
(no-num '(5 'a 'b 'flower 100 15 'car))