exercises and notes from "Gentle introduction"
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efim
365
basics.lisp
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365
basics.lisp
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(FIRST '(5 2 3))
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(SECOND '(5 2 3))
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(THIRD '(5 2 3))
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(REST '(5 2 3 5 1 6))
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(cons 1 '( 2 )) ; adds number to the list
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(cons 1 2) ; just a cons element
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(cons 'SINK '(OR SWIM)) ; adds symbol to the list
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(cons 'SINK 'SWIM) ; just a cons element with symbols
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(cons 'start nil) ; list is when last cons ends with `nil
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(cons 'start (cons 'end nil)) ; if we are to construct list from cons cells (and not from print notation)
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(cons 'nil 'nil) ; putting nil into CDR means having list with that item as CAR
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(cons '(PHONE HOME) nil)
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;; even though `nil is same as '() many properties of lists hold only for non-empty lists, i.e
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;; with at least one cons cell
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;; for example main relationship between CAR, CDR & CONS
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(equal
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'(1 2 3 4)
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(cons
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(car '(1 2 3 4))
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(cdr '(1 2 3 4))))
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;; doesn't work for () :
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(equal
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'()
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(cons
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(car '())
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(cdr '())))
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(cons
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(car '())
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(cdr '())) ; => (NIL)
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;;; exercises 3
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(defun half (x)
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(/ x 2))
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(defun cube (x)
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(* x x x))
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(defun pythag (x y)
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(sqrt (+ (* x x) (* y y))))
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;; variables /are named by/ symbols, they are *not* symbols
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;; funcitons are /aldo named by/ symbols
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;;; symbols and lists as data
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;; use ` - tilde to tell lisp interpreter
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;; to treat symbol not as a name for variable (and evaluate to its value)
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;; but to treat it as symbol (data)
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;;
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;; most symbols need to be quoted to be treated as symbols
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;; some exceptions are t and nil which are self evaluating
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(defun riddle (x y)
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(list `why `is `a x `like `a y))
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(riddle `raven `desk)
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;;; Evaluation Rule for quoted Quoted Objects:
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;;; quoted object evaluates to the object itself without the quote
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;; 3 ways of creating lists
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`(foo bar baz) ; everything inside list is not evaluated
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(list `foo `bar `baz (* 33 33)) ; allows for "computed elements"" "
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(cons `foo `(bar baz))
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;; 3.11
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(defun longer-than (l1 l2)
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(> (length l1) (length l2)))
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(defun call-up (caller callee)
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(list `hello callee `this `is caller `calling))
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(defun crank-call (caller callee)
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`(hello callee this is caller calling))
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;; 3.22 c
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(defun myfun (a b)
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(list (list a) b))
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;; 3.22 d
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(defun firstp (item list)
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(equal item (first list)))
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;; 3.22 e
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(defun mid-add1 (list)
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(cons
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(car list)
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(cons (+ 1 (cadr list))
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(cddr list))))
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;; 33.2 f
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(defun f-to-c (farenheit-temp)
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(/ (* 5 (- farenheit-temp 32)) 9))
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(symbol-name `equal)
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(symbol-function `equal)
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(symbol-name `lambda)
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(symbol-function `lambda)
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(eval (list `cons t nil))
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;;; equivalence of COND, IF, AND & OR - conditionals
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;; (and x y z w)
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;; (equal 1 1 1)
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(setq
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x 1
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y nil
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z 1
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w 1)
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(cond
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((not x) nil)
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((not y) nil)
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((not z) nil)
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((not w) nil)
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(t t))
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(defun boilingp (temp scale)
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(cond ((and (equal scale `celsius)
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(> temp 100)) t)
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((and (equal scale `farenheit)
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(> temp 212)) t)))
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(defun boilingp-2 (temp scale)
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(or
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(and (equal scale `celsius) (> temp 100))
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(and (equal scale `farenheit) (> temp 212))))
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(cond)
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;; 4.28
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(if (oddp 5) (evenp 7) `foo)
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(or (and (oddp 5) (evenp 7)) `foo)
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;; not sure what's up with that
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;; was that attempt to rewrite if into or&and?
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;; So, that written correctly! I get stops because of forced debugger!
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;; (step (eval (or (and (oddp 5) `(evenp 7)) ``foo)))
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(eval `(evenp 7))
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(eval ``foo)
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(step (if (oddp 5) `yes `no))
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(defun my-abs (x)
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(if (< x 0) (- x) x))
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(step (my-abs 15))
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;;; variables & side-effects
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(defun price-change (old new)
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(let* ((diff (- new old))
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(proportion (/ diff old))
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(percentage (* proportion 100.0)))
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(list `widgets `changed `by percentage `percent)))
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;;; 5.6 die exercise
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(defun throw-die ()
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"Return random value [1 6]."
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(+ 1 (random 6)))
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(defun throw-dice ()
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"Return list of 2 random die trows"
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(list (throw-die)
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(throw-die)))
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(defun snake-eyes-p (throw)
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"True if both dice in throw equal to 1."
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(and (equal 1 (first throw))
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(equal 1 (second throw))))
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(defun boxcars-p (throw)
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"True if both dice in throw equal to 6."
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(and (equal 6 (first throw))
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(equal 6 (second throw))))
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(defun instant-win-p (throw)
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"True if sum of dice in throw is 7 or 11"
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(let ((sum (+ (first throw) (secon throw)))
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(special-values (list 7 11)))
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nil))
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;; they meant type of word that is an article
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(setq articles `(the a an))
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(defun contains-article-p-1 (list)
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"Check whether article contains in the list."
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(intersection articles list))
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(setq test-sentence `(we will rock the boat))
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;; checks for each article
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(mapcar (lambda (article)
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(member article test-sentence)
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) articles)
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;; attempt to fold them into single boolean with or
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(apply `or (mapcar (lambda (article)
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(member article test-sentence)
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) articles))
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(step (apply `+ `(1 2 3)))
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(apply `equal `(2 3))
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(equal 2 3)
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(or nil nil t)
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(apply `or `( nil nil t))
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(step (apply `or (list nil nil nil)))
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(setq comparing-sentence `(large red shiny kube -vs- small shiny red four-sided pyramid))
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(list
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(length (intersection
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(cdr (member `-vs- comparing-sentence))
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(cdr (member `-vs- (reverse comparing-sentence)))))
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`common `features)
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;;; 6.9 programming with tables (as lists)
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(setq table1 `((object1 large green shiny cube)
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(object2 small wihte dull cube)
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(object3 large red shiny pyramid)
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(object3 green shiny sphere)))
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(setf quality-table
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`((green . color)
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(red . color)
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(white . color)
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(cube . shape)
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(pyramid . shape)
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(sphere . shape)
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(shiny . luster)
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(dull . luster)
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(large . size)
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(small . size)
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))
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(defun quality (x)
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"Get from hardcoded table."
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(cdr (assoc x quality-table))
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)
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(defun description (key)
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"Get description of object from hardcoded table."
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(cdr (assoc key table1)))
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(description `object2)
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;; notion of ~sublis~ function instead of mapcar for getting types of properties
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(defun differences (x y)
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"Get different properties from hardcoded table."
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(set-exclusive-or (description x)
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(description y)))
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(differences `object1 `object2)
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(remove-duplicates (sublis quality-table (differences `object1 `object2)))
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;;; 6.35 finite state automata
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(setq nerd-states `((sleeping . eating)
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(eating . waiting-for-a-computer)
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(waiting-for-a-computer . programming)
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(programming . debugging)
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(debugging . sleeping)))
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(defun nerdus (current-state)
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"Return next state for the nerdus in the provided CURRENT-STATE."
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(cdr (assoc current-state nerd-states)))
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(nerdus `programming)
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(defun sleepless-nerd (current-state)
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"Nerd that never sleeps."
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;; consequtive `sleep -> sleep` considered to be impossible
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(let ((possible-state (nerdus current-state)))
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(if (equal possible-state `sleeping)
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(nerdus `sleeping) possible-state)))
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(sleepless-nerd `sleeping)
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(sleepless-nerd `debugging)
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(nerdus `playing-guitar)
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(defun nerd-on-caffeine (current-state)
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"Advancing two states in one step."
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(nerdus (nerdus current-state)))
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(nerd-on-caffeine `sleeping)
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;; nested lists are trees and there are functions that can be used on them
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(subst `bb `aa `(In this list symbol aa would be substituted))
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;; args are NEW OLD TREE
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;; and whole tree structure is checked:
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(subst `bb `a `((a tree) In this list symbol (a hatter) aa would be substituted (a test)))
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;; sublis takes many substitutions, instead of first two args, the plist
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(defun royal-we (sentence)
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"Substitute all I to We."
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(subst `we `i sentence))
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;;; eq compares addresses, so lists would not be eq
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;;; equal compares elements of the lists
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;; symbols are singletons, so using ~eq~ is ok
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;; numbers might not be primitive in lisp implementations!
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;;
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;; eql - compares addresses in general case, for numbers: if of the same type compares values
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(eql 3 3)
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(eql 3 3.0)
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;; to test value of the numbers across types
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;; but works only with numbers, ok!
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(= 3 3.0)
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;;; Keyword arguments
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:keyword-symbol ; evaluates to self, cannot change value
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keyword-symbol ; different and not equal to actual keyword
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(symbolp :keyword)
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(keywordp :keyword)
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(keywordp `keyword)
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;;; 7.3 Applicative programming
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;; one of the styles, along with recursive and iterative
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(funcall #'cons 'a 'b)
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(setf fn #'cons)
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fn
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(type-of fn)
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(funcall fn 'c 'd)
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;; using mapcar
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(defun squaring (x)
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"Square input."
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(* x x))
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(squaring 9)
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(squaring '(1 2 2 3))
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(mapcar #'squaring '(1 2 2 3 9))
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(mapcar #'zerop '(20 340 0 -5 -6))
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(mapcar #'(lambda (x) `(hi there ,x))
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'(joe fred martha))
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;; 195
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(funcall #'(lambda (x)
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(or (equal x t)
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(equal x nil))) 1)
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(defun half (x)
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""
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(/ x 2.0))
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(defun average (x y)
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""
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(+ (half x) (half y)))
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(defun fact (n)
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""
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(cond ((zerop n) 1)
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(t (* n (fact (- n 1))))))
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(fact 4)
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