Hi,

Try to understand this:

type Matrix = [[Double]]

multSM :: Double -> Matrix -> Matrix
multSM d m = map (\ i -> map (\ j -> d*j ) i) m

Now try this for multSM (instead of the above definition)

multSM = map . map . (*)

Hope it helps,


On 2 February 2010 09:35, 조광래 <kwangraecho@gmail.com> wrote:

hi I have a question about haskell coding.

Matrices

One of many ways to define matrices in Haskell is the list of matrix rows, where a row is a list of double precision oating point numbers:

type Matrix=[[Double]] 

Using this type you should define the following functions (definition of dim is given as an inspirational example).

dim :: Matrix -> (Int,Int) --returns matrix dimension (number of rows and columns) 
dim m = if (not.isMatrix) m then error "Not a matrix" else (length m, length (head m)) 
 
isMatrix :: Matrix -> Bool --checks whether all rows have the same length 
isMatrix m =  and [length (head m) == length a | a <- m] 

I have done upto here and I am stuck on multSM.

multSM :: Double -> Matrix -> Matrix --multiplies a scalar and a matrix 

when I do:

multSM  d m = [[(b*a)| b<-[d], a<-(head m)]] or [map (*d) (head m)] 

...I get (using Hugs):

Main> multSM 3 [[2,3,4],[1,3,4],[6,3,0]] 
[[6.0,9.0,12.0]] 

So I solve 3 [2,3,4] but I do not know how to get the rest 3 [[1,3,4],[6,3,0].

Please help me on this problem.



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Ozgur Akgun