Question

How can I create an array of length n, all zeroes, except for some index i being equal to 1.0?

For example, if my magic function is foo it would work as follows:

foo:: Int -> Int -> [Double]
> foo 3 0
[1.0, 0.0, 0.0]
> foo 2 1
[0.0, 1.0]
> foo 1 1
** Exception: index greater than length! 

Having a brain freeze…any help appreciated.

Answer 1

unitList :: Int -> Int -> [Double]
unitList len index
    | index < len  = replicate index 0 ++ 1 : replicate (len - 1 - index) 0
    | otherwise    = error "index out of range"

Note that that’s a list, not an array. Lists have O(i) indexing, arrays O(1), so one shouldn’t confuse the names of the datatypes.