Here is the list of lists: [[1,2,3],[1,2,3,4],[1,2,3]]
How can I increment each element of the second list by the length of the first list, and increment the third list by the length of the first list + second list? The first list should remain unchanged.
Intended output: [[1,2,3],[4,5,6,7],[8,9,10]]
Since the first list has length 3, the second list is generated by [1+3, 2+3, 3+3, 4+3].
Since the first list + second list combined have length 7, the third list is generated by [1+7, 2+7, 3+7].
Ideally it should work with any number of lists.
So far, I've had slight sucess using this:
scanl1 (\xs ys -> [y + length xs | y <- ys]) [[1,2,3],[1,2,3,4],[1,2,3]]
which outputs: [[1,2,3],[4,5,6,7],[5,6,7]]
There are many ways to solve this. A simple recursion is one approach:
lst :: [[Int]]
lst = [[1,2,3],[1,2,3,4],[1,2,3]]
incrLength :: [[Int]] -> Int -> [[Int]] -> [[Int]]
incrLength [] _ result = result
incrLength (x:xs) amount result =
incrLength xs (amount + length x) (result ++ [map (+amount) x])
(Edit: it is more efficient to use (:) in this function. See @amalloy comment below. The result then has to be reversed.
incrLength :: [[Int]] -> Int -> [[Int]] -> [[Int]]
incrLength [] _ result = reverse result
incrLength (x:xs) amount result =
incrLength xs (amount + length x) (map (+amount) x : result)
End Edit)
Another approach is to use scanl. We use length to get the length of the inner lists, then accumulate using scanl.
map length lst -- [3,4,3]
scanl (+) 0 $ map length lst -- [0,3,7,10]
init $ scanl (+) 0 $ map length lst -- [0,3,7]
Then we zip the lst and the accumulated value together, and map one over the other.
incrLength' :: [[Int]] -> [[Int]]
incrLength' lst =
[map (+ snd y) (fst y) | y <- zip lst addlst]
where
addlst =init $scanl (+) 0 $ map length lst
main = do
print $ incrLength lst 0 [] -- [[1,2,3],[4,5,6,7],[8,9,10]]