[repost as original seems to have gone to email; my newsreader has
somehow acquired a 'Reply' button where 'Followup' normally goes.]
On 10/03/2018 14:23, Ben Bacarisse wrote:
Ben Bacarisse <ben.use...@bsb.me.uk> writes:
Off topic: I knocked up this Haskell version as a proof-of-concept:
import Data.List
pn n l = pn' n (map (:[]) l)
where pn' n lists | n == 1 = lists
| otherwise = diag (pn' (n-1) lists) lists
diag l1 l2 = zipWith (++) (concat (inits l1))
(concat (map reverse (inits l2)))
Notes:
map (:[]) l turns [1, 2, 3, ...] into [[1], [2], [3], ...]
inits gives the list of initial segments of l. I.e. (inits "abc") is
["", "a", "ab", "abc"].
concat joins a list of lists into one list.
zipWith (++) l1 l2 makes a list by pair-wise appending the elements of
l1 and l2.
What's the output? (And what's the input; how do you invoke pn, if
that's how it's done?)
--
Bartc
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