Re: [Haskell-cafe] Splitting a list (Modified by Doaitse Swierstra)
I have the impression that my posting did not make it, so I am trying again: How about: splitList weight l = munch l weight where munch [] _ = [[]] munch p@(x:xs) w = if w >= x then let (r:rr) = munch xs (w-x) in (x:r):rr else []:munch p weight main = print (splitList 18 [1,5,3,17,8,9]) Doaitse Swierstra PS: note that your problem is a bit underspecified: -- what to return for an empty list -- what to do if a number is larger than the weight On 2004 apr 21, at 15:42, Steve Schafer wrote: I have a list of integers, e.g.: [1,5,3,17,8,9] I want to split it into a pair of lists, with the criterion being that the sum of the elements in the first list is as large as possible, but not exceeding a threshold value. For example, if the threshold is 10, the result should be: ([1,5,3],[17,8,9]) and then I want to recursively apply this process to the remainder of the list, with the end result being a list of lists of integers. Using the same list along with a threshold of 18, I would get: [[1,5,3],[17],[8,9]] I have devised a means of doing this: 1) Create an auxiliary list of integers, where the n'th element is equal to the sum of the first n elements of the original list. 2) Zip the auxiliary list with the original list. 3) Use span to break the list in two according to the threshold. 4) Unzip the two resulting lists and discard the auxiliary portions. 5) Repeat from step 1, operating on the tail of the list, until there's nothing left. Here's the code that implements this: runningSum :: (Ord a, Num a) => [a] -> [a] runningSum []= [] runningSum (i:[])= i : [] runningSum (i:j:js) = i : runningSum (i+j : js) zipWithSum :: (Ord a, Num a) => [a] -> [(a,a)] zipWithSum xs= zip (runningSum xs) xs threshold:: (Ord a, Num a) => [a] -> a -> ([(a,a)],[(a,a)]) threshold xs t = let test x = (t >= (fst x)) in span test (zipWithSum xs) splitFirst :: (Ord a, Num a) => [a] -> a -> ([a],[a]) splitFirst xs t = let (ys,zs) = threshold xs t in (snd (unzip ys), snd (unzip zs)) splitAll :: (Ord a, Num a) => [a] -> a -> [[a]] splitAll [] _= [] splitAll xs t= let (ys, zs) = splitFirst xs t in ys : (splitAll zs t) (One thing that's missing from this code is a check to verify that no single element in the list is greater than the threshold, which should raise an error, rather than get stuck in an infinite loop.) The algorithm as implemented works fine, but it seems overly complicated and not very elegant. I get the feeling that I'm missing some obvious simplification, but I can't find it. Any ideas? Thanks, -Steve Schafer ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Syntax for modifying nested product types
I have data objects where each component is a labelled field through which
I access or modify it. I have a hierarchy of these - many of the fields
are themselves such data objects, so I may need to apply a few selector
functions to get down to what I want (call these "deep").
For fairly flat things, the updates using field labels are okay. However,
when I have a function that needs to tweak various bits of various "deep"
fields, based on various conditions, then it gets really ugly, not least
because of the conditions: foo { bar = baz } becomes,
let old = bar foo in foo { bar = if cond then f old else old }
or whatever. Do I need such a let for each component and sub-component of
the object that has a child that may be modified, or will
foo { bar = if cond then f (bar foo) else bar foo }
be suitably optimised? (The if's inside the {} because there may be
multiple fields (or children thereof) being modified based on different
conditions.)
-- Mark
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Re: [Haskell-cafe] Splitting a list
On 2004.04.22 15:02, I wrote: > splitAll :: (Real a) => a -> [a] -> [[a]] > splitAll = unfoldr . split > where split _ [] = Nothing > split n xs = let (ys,zs) = break ((> n) . snd) > (zip xs (scanl1 (+) xs)) > in Just (map fst ys, map fst zs) a slight improvement: splitAll :: (Real a) => a -> [a] -> [[a]] splitAll n = unfoldr split where split [] = Nothing split xs = let (ys,zs) = break ((> n) . snd) (zip xs (scanl1 (+) xs)) in Just (map fst ys, map fst zs) But in fact, I think you can do better still by not holding n constant but using a higher threshold on each split and not projecting out the values of the second component, thus only zipping the whole list once. --Joe Joseph H. Fasel, Ph.D. email: [EMAIL PROTECTED] Systems Planning and Analysis phone: +1 505 667 7158 University of Californiafax: +1 505 667 2960 Los Alamos National Laboratory post: D-2 MS F609; Los Alamos, NM 87545 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Splitting a list
This is a classic greedy algorithm, much like the text-wrapping problem. My main suggestion would be that you're not making use of some standard list functions that would simplify things. For example, your runningSum is just scanl1 (+) . Similarly, splitAll should use unfoldr. Another thing is that I would reverse the order of arguments of splitFirst and splitAll, since curried applications are probably more useful that way: splitAll :: (Real a) => a -> [a] -> [[a]] splitAll = unfoldr . split where split _ [] = Nothing split n xs = let (ys,zs) = break ((> n) . snd) (zip xs (scanl1 (+) xs)) in Just (map fst ys, map fst zs) Now, if you're concerned about all that zipping and projecting, you can instead define split via a straightforward recursion, or you could use a different kind of unfold that preserves the terminating value: unfoldrG :: (b -> Either (a,b) b) -> b -> ([a],b) unfoldrG f = unfold where unfold x = case f x of Right y -> ([],y) Left (a,y) -> let (bs,z) = unfold y in (a:bs,z) Here, you will define split by unfolding a pair consisting of a running sum and remaining list. Cheers, --Joe On 2004.04.21 07:42, Steve Schafer wrote: > I have a list of integers, e.g.: > > [1,5,3,17,8,9] > > I want to split it into a pair of lists, with the criterion being that > the sum of the elements in the first list is as large as possible, but > not exceeding a threshold value. For example, if the threshold is 10, > the result should be: > > ([1,5,3],[17,8,9]) > > and then I want to recursively apply this process to the remainder of > the list, with the end result being a list of lists of integers. Using > the same list along with a threshold of 18, I would get: > > [[1,5,3],[17],[8,9]] > > I have devised a means of doing this: > > 1) Create an auxiliary list of integers, where the n'th element is equal > to the sum of the first n elements of the original list. > > 2) Zip the auxiliary list with the original list. > > 3) Use span to break the list in two according to the threshold. > > 4) Unzip the two resulting lists and discard the auxiliary portions. > > 5) Repeat from step 1, operating on the tail of the list, until there's > nothing left. > > Here's the code that implements this: > > runningSum :: (Ord a, Num a) => [a] -> [a] > runningSum []= [] > runningSum (i:[])= i : [] > runningSum (i:j:js) = i : runningSum (i+j : js) > > zipWithSum :: (Ord a, Num a) => [a] -> [(a,a)] > zipWithSum xs= zip (runningSum xs) xs > > threshold:: (Ord a, Num a) => [a] -> a -> ([(a,a)],[(a,a)]) > threshold xs t = let test x = (t >= (fst x)) > in span test (zipWithSum xs) > > splitFirst :: (Ord a, Num a) => [a] -> a -> ([a],[a]) > splitFirst xs t = let (ys,zs) = threshold xs t > in (snd (unzip ys), snd (unzip zs)) > > splitAll :: (Ord a, Num a) => [a] -> a -> [[a]] > splitAll [] _= [] > splitAll xs t= let (ys, zs) = splitFirst xs t > in ys : (splitAll zs t) > > (One thing that's missing from this code is a check to verify that no > single element in the list is greater than the threshold, which should > raise an error, rather than get stuck in an infinite loop.) > > The algorithm as implemented works fine, but it seems overly complicated > and not very elegant. I get the feeling that I'm missing some obvious > simplification, but I can't find it. Any ideas? > > Thanks, > > -Steve Schafer > > ___ > Haskell-Cafe mailing list > [EMAIL PROTECTED] > http://www.haskell.org/mailman/listinfo/haskell-cafe > > Joseph H. Fasel, Ph.D. email: [EMAIL PROTECTED] Systems Planning and Analysis phone: +1 505 667 7158 University of Californiafax: +1 505 667 2960 Los Alamos National Laboratory post: D-2 MS F609; Los Alamos, NM 87545 ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Splitting a list
Steve Schafer writes (in the Haskell Cafe): I have a list of integers, e.g.: [1,5,3,17,8,9] I want to split it into a pair of lists, with the criterion being that the sum of the elements in the first list is as large as possible, but not exceeding a threshold value. For example, if the threshold is 10, the result should be: ([1,5,3],[17,8,9]) and then I want to recursively apply this process to the remainder of the list, with the end result being a list of lists of integers. Using the same list along with a threshold of 18, I would get: [[1,5,3],[17],[8,9]] I would get [[1,5,3], [17], [8,9], [], [], [] ..] Cheers, Ronny Wichers Schreur ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
