exercises and notes from "Gentle introduction"

recursion.lisp

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+
+(null 'a)
+(null ())
+(null nil)
+
+(defun anyoddp (x)
+  (cond ((null x) nil)
+        ((odd (first x)) t)
+        (t (anyoddp (rest x)))))
+
+(trace anyoddp)
+(anyoddp (list 6 1))
+(untrace)
+(rest '(5 4 3 2 1))
+
+;;; my error was - function wasn't named odd, it was oddp
+;;; and in the debugger it looked like:
+;;; ("undefined function" 6)
+;;; and I thought that 6 was treated as undefined function in list (6 1) for some reason
+
+(defun laugh (n)
+  (defun laugh-inner (n acc)
+    (cond ((eq 0 n) acc)
+          (t (laugh-inner (- n 1) (cons 'HA acc )))))
+  (laugh-inner n ()))
+
+;; I tried to use append
+;; but how the hell do I add elements to list?
+;; do I just fucking manually create cons? ugh
+(cons 1 '(4 5 6))
+
+(laugh 4)
+
+(defun square-list (x)
+  (cond ((null x) ())
+        (t (cons (* (first x) (first x))
+                 (square-list (rest x))))))
+
+(square-list '(1 3 5 6))
+
+;;; count atoms.
+;;; recursion on nested lists - think of them as binary trees, with car and cdr, that's it
+
+(atom 1)
+
+(defun count-atoms (tree)
+  (cond
+    ((null tree) 0)                     ; without that ends of lists would be counted, but didn't include that due to misunderstanding
+    ((atom tree) 1)
+    (t (+ (count-atoms (car tree))
+          (count-atoms (cdr tree))))))
+
+(count-atoms '(1 (4 5) 2 ))
+
+(defun pairings (x y)
+  (cond ((or (null x) (null y)) ())
+        (t (cons (list (first x) (first y))
+                 (pairings (rest x) (rest y))))))
+
+(pairings '(1 2 3 7) '(a b c d))
+
+;;; looking into debugger
+(defun my-fact (n)
+  (cond ((zerop n)  (break "N is zero")
+        ;; 1
+         )
+        (t (* n (my-fact (- n 1))))))
+
+(my-fact 4)
+
+;;; So - debugger keys and functions
+;; C-j and C-k are for moving between frames
+;; for frame context commands R - return from frame with particular value (sly-db-return-from-frame)
+;; x - step over, s - step
+;; e - eval in frame, i - inspect in frame (show additional info on evaluated value)
+;; RET - toggle details for current frame
+;; v - show frames code
+;; d - pretty-print-eval-in-frame
+;; (searching all of functions by going into debugger bugger and looking at *helpful* (C-h C-f) "sly-db-"
+
+;; could also try to recompile thing, maybe r for restart, or R to return with value 1 from last frame and check how it all works
+
+;;; better way to define inner functions : LABELS
+;;; similar to LET form, binding names to arglists and body
+;;; they are "local functions"
+;;; can call each other, and reference parent variables
+
+(defun factor-tree (n)
+  (factors-help n 2))
+
+(defun factors-help (n p)
+  (cond
+    ((equal p n) n)
+    ((zerop (rem n p)) (list n p (factors-help (/ n p) p)))
+    (t (factors-help n (+ 1 p)))))
+
+(factor-tree 60)
+
+(trace factors-help)
+(untrace)                               ; and there's no need to try to do println, and less errors in that