list - Haskell <<loop>> -


with getindex xs y want index of first sublist in xs length greater y.

the output is:

[[],[4],[4,3],[3,5,3],[3,5,5,6,1]] aufgabe6: <<loop>>

why getindex not work?

import data.list  -- die sortierfunktion -- mycompare b     | length < length b = lt     | otherwise = gt  sortlist :: [[a]] -> [[a]] sortlist x = sortby mycompare x  -- die indexfunktion -- getindex :: [[a]] -> int -> int getindex [] y = 0 getindex (x:xs) y     | length x <= y = 1 + getindex xs y     | otherwise = 0     (x:xs) = sortlist (x:xs)  main =     print (sortlist [[4],[3,5,3],[4,3],[3,5,5,6,1],[]])     print (getindex [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] 2)

getting terminate

the problem in case

getindex (x:xs) y     | length x <= y = 1 + getindex xs y     | otherwise = 0     (x:xs) = sortlist (x:xs)

you're confusing (x:xs) which. should instead do

getindex zs y     | length x <= y = 1 + getindex xs y     | otherwise = 0     (x:xs) = sortlist zs

giving

main> main [[],[4],[4,3],[3,5,3],[3,5,5,6,1]] 3 *main> getindex [[],[2],[4,5]] 1 2 *main> getindex [[],[2],[4,5]] 5 3

this gives number of first list of length @ least y in sorted list, answers question "how many lists of length @ y in original?"

how can find out other facts?

if want position original list, can tag entries position, using zip:

*main> zip [1..] [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] [(1,[4]),(2,[3,5,3]),(3,[4,3]),(4,[3,5,5,6,1]),(5,[])]

let's make utility function working those:

haslength likethis (_,xs) = likethis (length xs)

we can use this:

*main> haslength (==4) (1,[1,2,3,4]) true *main> filter (haslength (>=2)) (zip [1..] ["","yo","hi there","?"]) [(2,"yo"),(3,"hi there")]

which means it's easy write function gives index of first list of length longer y:

whichislongerthan xss y =      case filter (haslength (>y)) (zip [1..] xss) of          [] -> error "nothing long enough" -- change 0 or (length xss + 1) if prefer          (x:xs) -> fst x

this gives us

*main> whichislongerthan [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] 2 2 *main> whichislongerthan [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] 3 4 *main> whichislongerthan [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] 0 1

shorter?

but can similar tricks:

whichisshorterthan xss y =      case filter (haslength (<y)) (zip [1..] xss) of          [] -> error "nothing short enough" -- change 0 or (length xss + 1) if prefer          (x:xs) -> fst x

so get

*main> whichisshorterthan [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] 2 1 *main> whichisshorterthan [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] 1 5 *main> whichisshorterthan [[4],[3,5,3],[4,3],[3,5,5,6,1],[]] 0 *** exception: nothing short enough

generalised

let's pull out common theme there:

whichlength :: (int -> bool) -> [[a]] -> int whichlength likethis xss =      case filter (haslength likethis) (zip [1..] xss) of          [] -> error "nothing found" -- change 0 or (length xss + 1) if prefer          (x:xs) -> fst x

so can do

*main> whichlength (==5) [[4],[3,5,3],[4,3],[3,5,5,6,1],[]]  4 *main> whichlength (>2) [[4],[3,5,3],[4,3],[3,5,5,6,1],[]]  2

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