On Sun, 27 Mar 2005, Carter Tazio Schonwald wrote:
I've written a simple program to calculate determinants of matrices, and while it executes to completion it fails to yield a correct answer. (and yes, I know I'm using lists to represent matrices)
I would greatly appreciate any assistance in correcting this algorithm
--Carter Schonwald
import Prelude
first [] = [] first (a:b) = a
'head' on lists of lists without an error empty lists?
nth::Int->[a]->a nth _ [] = error "list is too small" nth 1 (a:b) = a nth n (a:b) = nth (n-1) b
(!!) ?
takeAfter::Int->[a]->[a] takeAfter _ [] = error "list too small" takeAfter 0 a= a takeAfter 1 (a:b) = b
Is this case necessary?
takeAfter n (a:b) = takeAfter ( n-1) b
Is it the same like 'drop'?
type Matrix = [[Rational]]
pad a = [x++x| x<- a]
maybe 'cycle' helps
time2 [] _ = [] time2 _ [] = [] time2 (a:b) (c:d) = (a * c):(time2 b d)
zipWith (*) ?
tupleProd (a,b) = a * b
uncurry (*) ... Here is another implementation of determinant: {- successively select elements from xs and remove one in each result list -} removeEach :: [a] -> [[a]] removeEach xs = zipWith (++) (List.inits xs) (tail (List.tails xs)) alternate :: (Num a) => [a] -> [a] alternate = zipWith id (cycle [id, negate]) det :: (Num a) => [[a]] -> a det [] = 1 det m = sum (alternate (zipWith (*) (map head m) (map det (removeEach (map tail m)))))