Custom Query (35 matches)

From SO: Hello! I'm a bit confused with select function in repa package:

select (\i -> True) (\i -> i) 10 gives the result

[0,1,2,3,4,5,6,7,8]

I thought i to be between 0 and 10 or 0 and 9. Why is it between 0 and 8?

Getting indices from [0..8] for an extend of 10 is confusing.

A few people have asked how to take sub-matrices. This is an index transformation that's easy to specify, but there should be a nice wrapper for it. We could make a shape polymorphic version using addIndex to add the starting offset.

let arr = fromList (Z :. (5 :: Int)) [1, 2, 3, 4, 5 :: Int] 
in  fromFunction (Z :. 3) (\(Z :. ix) -> arr ! (Z :. ix + 1))

Representing bitmaps as a 3D array isn't ideal because there's no way to perform a map over all the pixels.

With this representation:

Array DIM3 Word8

Suppose we want to convert the image to greyscale in a single operation. We'd really need a function like:

map1 :: (Array DIM1 a -> Array DIM1 a) -> Array DIM3 a -> Array DIM3 a

However, that doesn't work because we can't guarantee that the worker function always returns an array of the same length as the original. Because of this we can't guarantee that the result is rectangular, and Repa can only deal with rectangular arrays.

If instead we represented Bitmaps as:

Array DIM2 (Word8, Word8, Word8)

Then we could use the original map function and there'd be no problem.

Note that using triples is just as efficient as using a DIM3 array. Repa uses unboxed Data.Vectors to store the manifest array elements, and this library will unpack a vector of triples into three separate vectors holding each of the elements. There are no intermediate tuple constructors or pointers in the underlying representation.