Solution exercises from P01 to P07

p01.el

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+;; P01 (*) Find the last box of a list
+(defun my-last (lst)
+  (cond
+   ((eq lst '()) '())
+   ((eq (cdr lst) '()) lst)
+   (t
+    (my-last (cdr lst)))))
+
+(my-last '(a b c d)) ; '(d)

p02.el

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+;; P02 (*) Find the last but one box of a list
+(defun my-but-last (lst)
+  (cond
+   ((eq lst '()) '())
+   ((eq (cddr lst) '()) lst)
+   (t
+    (my-but-last (cdr lst)))))
+
+(my-but-last '(a b c d)) ; '(c d)

p03.el

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+;; P03 (*) Find the k'th element of a list
+(defun element-at (lst k)
+  (cond
+   ((eq lst '()) '())
+   ((eq k 1) (car lst))
+   (t
+    (element-at (cdr lst) (- k 1)))))
+
+(element-at '(a b c d) 3) ; c

p04.el

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+;; P04 (*) Find the number of element of a list
+(defun my-length (lst)
+  (defun my-length-rec (lst res)
+    (cond
+     ((eq lst '()) res)
+     (t
+      (my-length-rec (cdr lst) (+ 1 res)))))
+  (my-length-rec lst 0))
+
+(my-length '(a b c)) ; 3

p05.el

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+;; P05 (*) Reverse a list
+(defun my-reverse (lst)
+  (defun my-reverse-rec (lst res)
+    (cond
+     ((eq lst '()) res)
+     (t
+      (my-reverse-rec (cdr lst) (cons (car lst) res)))))
+  (my-reverse-rec lst '()))
+
+(my-reverse '(a b c)) ; '(c b a)

p06.el

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+;; P06 (*) Find out whether a list is a palindrome.
+(defun palindrome (lst)
+  (equal lst (reverse lst)))
+
+(palindrome '(x a m a x)) ; t
+(palindrome '(a n n a))   ; t
+(palindrome '(f a b i o)) ; nil

p07.el

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+;; P07 (**) Flatten a nested list structure.
+(defun my-flatten (l)
+  (cond
+   ((null l) nil)
+   ((atom l) (list l))
+   (t
+    (append (my-flatten (car l))
+	    (my-flatten (cdr l))))))
+
+(my-flatten '(a (b (c d) e))) ; '(a b c d e)