## martedì 29 dicembre 2009

Verlet integration algorithm written in 3 minutes

-- Verlet integration

projOneTwo ( a, b, c , d ,e ) = ( a, b)

nextStepInt (_ ,_ , 0 ,_ ,_ ) = []
nextStepInt ( xt ,vt , n , acc , h ) =
let
atph = acc . ( xt + ) $h vtph = vt + atph * h * 0.5 xtph = xt + h * vt + 0.5 * acc xt * h^2 in ( xtph, vtph , n-1 , acc , h ) : nextStepInt ( xtph, vtph , n-1 , acc , h ) timeInt ( xt ,vt , n , acc , h ) = map ( projOneTwo )$ nextStepInt ( xt ,vt , n , acc , h )

example ( spring ) :
timeInt (0 , 1 , 10000 , (\x -> 0.1 * ( - x ) ) , 0.2 )

## giovedì 24 dicembre 2009

### my delicious

now i'm in delicious: http://delicious.com/koteth

## martedì 8 dicembre 2009

Is only a fixed-point iteration over this function:

lgs x r = r * x * exp(- x )

### Dollar $operator in Haskell Do you know what is$ operator in Haskell?
$means simply , 'apply the left function at the right value'. f$ x := f x

it seems really trivial, isn't it ?
But, for example in this kind of situation, is really usefull:

zipWith ( $) ( cycle [ \x -> div (x + 1) 2 , \x -> div x 2 ] ) [1..] here you have a infinite list of function: a = cycle [ \x -> div (x + 1) 2 , \x -> div x 2 ] ( ie: [\x -> div (x + 1) 2 , \x -> div x 2 , \x -> div (x + 1) 2 , \x -> div x 2, ... ] ) and you want to apply every element of the list at the element at the same index in the second list: b = [1..] zipWith, for every index i takes the element a(i) of the left list and b(i) of the right list and execute what is requested inside the parentheses. In this situation is specified$ so:

a(i) \$ b(i ) := a(i) ( b(i) )

the result must be the following:

[ 1 ,1 , 2, 2 , 3 ,3 ... and so on.

well, do you want to know how to find 'prime numbers' in a quick and dirty