## Wednesday, 26 August 2009

may be thought of as functions whose domains are isomorphic to contiguous subsets of the integers.

Arrays in Java:

An array object contains a number of variables. The number of variables may be zero, in which case the array is said to be empty. The variables contained in an array have no names; instead they are referenced by array access expressions that use nonnegative integer index values. These variables are called the components of the array. If an array has n components, we say n is the length of the array; the components of the array are referenced using integer indices from 0 to n - 1, inclusive.

This probably sums up the difference between the languages very well. Haskell says it in one sentence, whereas Java waffles a little bit more!

In Haskell arrays are constructed with the `array` constructor. The first argument specifies a pair of bounds so we can create an array mapping the integers 3, 4 and 5 to 10 times their value.

`  *Main> array (3,5) [ (i,i*10) | i <- [3..5]]  array (3,5) [(3,30),(4,40),(5,50)] `

Arrays can also be multidimensional in which case more bounds need to be provided.

`  *Main> (array ((0,0),(1,1)) [ ((i,j),i*2+j) | i <- [0..1], j <- [0..1]])   array ((0,0),(1,1)) [((0,0),0),((0,1),1),((1,0),2),((1,1),3)]`

I struggled to get the PPM package in Hackage working last time because I didn't understand arrays in the slightest. Now that I have a little understanding (only a little!) I can actually visualize the ray tracing code...

One thing I'm still not understanding is how I should format Haskell code. Here's the code to render the image and save it as a PPM.

`image :: [Sphere] -> Point -> Int -> Int -> Array (Int,Int) Int image world eye width height =     array       ((0,0),(width,height))       [((i,j),truncate (255 * (value (colorAt world eye (fromIntegral i) (fromIntegral j))))) |         i <- [0..width], j<- [0..height]]imageWord16 :: Array (Int,Int) Int -> Array (Int,Int) Word16imageWord16 image = fmap (fromIntegral :: Int -> Word16) imagesaveImage :: String -> [Sphere] -> Point -> Int -> Int -> IO ()saveImage filename world eye width height = arrayToFile filename (imageWord16 (image world eye width height))`

Note the horrible usage of `fromIntegral` and `truncate` to convert an integer to a float and back again. I think this is because I should have been more general in my types on the Point data type and specified it as a number rather than a float.

The type of `saveImage` looks a little funny but this is because it returns an IO action rather than a value. This value is a monad, but again I'll ignore this and hope repeatedly using them will lead to understanding! For now, I just grok that its type indicates it does something rather than returns something. The finished code weighs in at about 100 lines, which is more or less exactly the same as Clojure, but with the advantage of static typing. I found static typing to be in equal measure incredibly useful and incredibly frustrating! Hopefully it'll lean towards useful as I understand things a bit more.