## Archive for the ‘Scheme’ Category

### Javascript and Lisp

As Douglas Crockford has said, javascript is Lisp is C's clothing. That is part of what make javascript fun to program, but I never realized how literally true that statement is until I found this fascinating article by Thomas Lord about GNU and Scheme, that claims:

I've read, since then, that up to around that point Brendan Eich had been working on a Scheme-based extension language for Netscape Navigator. Such was the power of the hegemony of the high level folks at Sun that the word came down on Mr. Eich: "Get it done. And make it look like Java." Staying true to his sense of Self, he quickly knocked out the first implementation of Mocha, later renamed Javascript.

Explains a lot. And shows what an under-appreciated genius Brendan Eich is.

### Learning Scheme: Euler Problem 3

Been a long time since I last posted; real life has a way of getting in the way. The past few months read like the back cover of A Series of Unfortunate Events: if you don't want to hear about a thwarted move, a house fire, 2 graduations, negotiations for a new job and a carrot salad with lots of garlic, then you should put this down and live someone else's life.

But I'm not complaining; everything looks like it's working out well.

Going back to Scheme, the next Project Euler problem is:

The prime factors of 13195 are 5, 7, 13 and 29. What is the largest prime factor of the number 600851475143 ?

And straightforwardly:

``````(define gpf ; greatest prime factor
(lambda (n try) ; n is the number to find the factor of; try is the lowest number to try
(cond
((> try (sqrt n)) n) ; no need to try a factor higher than the square root; if we get here, n is prime
((divides try n) (gpf (/ n try) try)) ; divide out the current trial factor and try again
(else (gpf n (+ try 1))))))
(define euler3
(lambda (n) (gpf n 2)))``````

`(euler3 600851475143)` gives us the answer 6857 pretty quickly.

But this is getting boring; I'm not using the interesting parts of Scheme. This sort of problem cries out for a prime number generator, and that cries out for `call-with-current-continuation`, and that cries out for some more playing and learning. I'll see what I can do.

### Learning Scheme, Euler problem 2

The second project Euler problem is:

Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be: 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ... Find the sum of all the even-valued terms in the sequence which do not exceed four million.

Continue reading ‘Learning Scheme, Euler problem 2’ »

### Learning Scheme

OK, so my new mini project is to learn Scheme. It can't be too hard, right? I mean Javascript is just Lisp in C's clothing, and I'm good at Javascript. I've installed PLT-Scheme and I figure the best way to learn it is solving Project Euler problems until I get bored.