May 31, 2010

;;; SICP Section 3.1

;;; 3.1
(defun make-accumulator (n)
  (lambda (x)
    (incf n x)))

;;; 3.2
(defun make-monitored (f)
  (let ((count 0))
    (lambda (x)
      (cond ((eq x 'how-many-calls?) count)
     ((eq x 'reset-count) (setf count 0))
     (t (incf count)
        (funcall f x))))))

;;; 3.3 and 3.4 - updated for 3.7
(defun make-account (balance password)
  (let ((consecutive-attempts 0))
    (labels ((withdraw (amount)
        (if (>= balance amount)
     (setf balance (- balance amount))
     "Insufficient funds"))
      (deposit (amount)
        (setf balance (+ balance amount)))
      (dispatch (pass m)
   ;; Incorrect password?
   ((not (eq pass password))
    (lambda (x)
      (if (<= 7 (incf consecutive-attempts))
   "Calling cops"
   "Incorrect password")))
   ;; Withdrawing?
   ((eq m 'withdraw)
    (setf consecutive-attempts 0)
   ;; Depositing?
   ((eq m 'deposit)
    (setf consecutive-attempts 0)
   ;; Creating joint account?
   ((eq m 'joint)
    (lambda (new-pass)
      (lambda (pass m)
        (if (eq pass new-pass)
     (funcall #'dispatch password m)
     (funcall #'dispatch nil m)))))
   (t (error "Unknown request -- MAKE-ACCOUNT")))))

(defun withdraw (acc pwd amt)
  (funcall (funcall acc pwd 'withdraw) amt))

(defun deposit (acc pwd amt)
  (funcall (funcall acc pwd 'deposit) amt))

;;; 3.5
(defun random-in-range (low high)
  (let ((range (- high low)))
    (+ low (random range))))

(defun monte-carlo (trials experiment)
  (labels ((iter (trials-remaining trials-passed)
      (cond ((= trials-remaining 0)
      (float (/ trials-passed trials)))
     ((funcall experiment)
      (iter (- trials-remaining 1) (+ trials-passed 1)))
      (iter (- trials-remaining 1) trials-passed)))))
    (iter trials 0)))

(defun in-circle-p ()
  (let ((x (random-in-range 0 1.0))
 (y (random-in-range 0 1.0)))
    (<= (+ (expt (- x 0.5) 2)
    (expt (- y 0.5) 2))
 (expt 0.5 2))))

(defun estimate-integral (p x1 x2 y1 y2 trials)
  (let ((area-of-rect (* (- x2 x1)
    (- y2 y1)))
 (frac-inside (monte-carlo trials p)))
    (* frac-inside area-of-rect)))

;;; 3.6
(let ((st (make-random-state)))
  (defun rnd (sym)
    (cond ((eq sym 'generate)
    (lambda (x)
      (random x st)))
   ((eq sym 'reset)
    (lambda (x)
      (setf st x)))
   (t "Unknown symbol"))))

;;; 3.7
(defun make-joint (acc pass new-pass)
  (funcall (funcall acc pass 'joint) new-pass))

;;; 3.8
(let ((num 1))
  (defun f (n)
    (setf num (* num n))))

May 25, 2010

at Tuesday, May 25, 2010 Labels: Posted by Billy 0 comments

I recently updated to Ubuntu Lucid and immediately noticed UI performance degradation after using it for a short while. The sluggish performance affected window switching and scrolling in Firefox/Chrome/Evince etc. Nothing on my system indicated a memory leak or chewed-up processor.

I searched Ubuntu's bug tracking system and came across this post, which indicated the using the latest kernel version fix the problem. After following the steps here to update my kernel, my system runs as fast as ever. Hopefully other users experience my situation will stumble across this post and find it helpful.

May 22, 2010

at Saturday, May 22, 2010 Labels: Posted by Billy 0 comments

The Selfish Gene does a marvelous job explaining the core concepts of
evolution for the lay person. It begins with the predominate theory of
how life began in the primordial soup. It then explains how the first
replicators became the genes that make up our DNA and how the natural
environment shapes evolution by pruning the most adaptive and
advantageous of those genes.

Genes are responsible for manufacturing organisms to help them
replicate. They affect not only its own organism, but the surrounding
environment and other organisms. The book explains how the genes
affect the organisms behavior, and how the behavior of a population of
species follows an Evolutionary Stable Strategy (ESS). The book's
title refers to the concept that genes are really looking after
themselves first, but it also explains how altruistic behavior can
arise from the gene's selfishness. The author adamantly proposes that
we are the first species that can defy our genes and oppose the
ruthless nature of natural selection to make the world a better,
friendlier place.

It worries me that many people are ignorant of evolution. Never once
did my high school offer a class on the topic. In my opinion, everyone
should be familiar with the concepts because I believe it's important
to know how we came to be. I recommend this book to everybody.

May 8, 2010

at Saturday, May 08, 2010 Labels: , Posted by Billy 0 comments

Practical Common Lisp has turned into the de facto standard for beginners who want to learn Common Lisp, and deservingly so. The opening chapters delve right into the development style of Common Lisp and show how it differs from more mainstream languages. It covers all the basics and more, eventually leading into chapters explaining how to parse MP3 files and create and create an MP3 Shoutcast server from scratch.

Common Lisp is my favorite language to develop in, but most people I talk to dismiss it as being too old and irrelevant for today's computer applications. I highly disagree. The fact that the language is old is actually one of it's strengths: every design decision seems so well thought out. I've had numerous moments where I commented to myself "you know, it would be nice if this function could do this," and then discovered that it could after reading the documentation. And the development environment is the best out there. The Slime/Emacs/Lisp combination is way ahead of Visual Studio in terms of interactivity, customization, jumping through source files and debugging.

I recommend this book to all developers out there, as learning Common Lisp helped me in my day job as well. At least read through the chapter on creating a unit test framework to get a feel of what software development should be like.