Re: [Haskell-cafe] hFileSize vs length
S. Alexander Jacobson wrote: I am using GHC 6.2 on windows and am finding that when I open a file and use hFileSize I get a different number than I get from reading in the file and calculating the length. I assume this is not a bug, but I don't know why its happening. Isn't that because of line end conversion? EOL on windows is \r\n (2 bytes), when read converted on the fly to \n (1 char). Try to open your file in binary mode. Also, why isn't there getFileSize function in System.Directory? System.Posix.Files has getFileStatus and fileSize. No idea if they work on windows. -Alex- __ S. Alexander Jacobson tel:917-770-6565 http://alexjacobson.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solution to Thompson's Exercise 4.4
Andy Georges wrote: Hi Kaoru, I have been working through the exercises in Thompson's The Craft of Functional Programming 2nd Ed book. I am looking for a solution web site for Thompson's book. Or maybe the people here can help. In exercise 4.4, I am asked to define a function howManyOfFourEqual :: Int - Int - Int - Int - Int which returns the number of integers that are equal to each other. For example, howManyOfFourEqual 1 1 1 1 = 4 howManyOfFourEqual 1 2 3 1 = 2 howManyOfFourEqual 1 2 3 4 = 0 A solution which is applicable to any number of arguments is this: when this example occurs in the text the new Haskell coder has not been introduced to most of what you suggest. Some of the exercises I felt were meant to be a little painful so that when you were introduced to a new concept a chapter or two later you would think oh, that would have made X.Y so much easier. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solution to Thompson's Exercise 4.4
Kaoru Hosokawa ([EMAIL PROTECTED]) wrote: I hope to find a better solution. I googled but couldn't find the answer. Here is what I have. I do not have working Haskell interpreter at the moment (being on amd64), but this is what I have in my archive: weakAscendingOrder :: Int - Int - Int - Bool weakAscendingOrder a b c | (a b) (b == c) || (a == b) (b c)= True | otherwise = False howManyEqual :: Int - Int - Int - Int howManyEqual a b c | (a == b ) (b == c) = 3 | weakAscendingOrder a b c = 2 | (a /= b) (b /= c) = 0 isEqual :: Int - Int - Int - Int - Bool isEqual x a b c | (x == a) = True | (x == b) = True | (x == c) = True | otherwise = False howManyOfFourEqual :: Int - Int - Int - Int - Int howManyOfFourEqual a b c d | isEqual a b c d = howManyEqual b c d + 1 | otherwise = howManyEqual b c d Pls. test it ;) Sincerely, Gour -- Registered Linux User | #278493 GPG Public Key | 8C44EDCD ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parity of the number of inversions of a permutation
On Fri, 11 Mar 2005, William Lee Irwin III wrote: On Wed, Mar 09, 2005 at 12:42:09PM +0100, Henning Thielemann wrote: I'm searching for a function which sorts the numbers and determines the parity of the number of inversions. I assume that there are elegant and fast algorithms for this problem (n * log n time steps), e.g. a merge sort algorithm. A brute force solution with quadratic time consumption is countInversions :: (Ord a) = [a] - Int countInversions = sum . map (\(x:xs) - length (filter (x) xs)) . init . tails That's not a permutation, that's a cycle. Permutations are sets of disjoint cycles (which commute). ??? A permutation is a bijective function (here on a finite set), isn't it? Ok, the list representation is not immediately a permutation. But why a cycle? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solution to Thompson's Exercise 4.4
I had a go with things along this theme and came up with a couple of options, with different type signatures. I use some functions from the Data.List library. If we know that, as with Ints, we are dealing with list members that are instances of Ord, we can do: howManyEqual :: (Eq a, Ord a) = [a] - Int howManyEqual = maximum . (0 :) . map length . group . sort Otherwise, we end up less efficient, with: howManyEqual :: Eq a = [a] - Int howManyEqual = countEach 0 where countEach best [] = best countEach best list@(x:_) = let (xs, others) = partition (== x) list in countEach (max (length xs) best) others -- Mark ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Solution to Thompson's Exercise 4.4
Hi all, when this example occurs in the text the new Haskell coder has not been introduced to most of what you suggest. I didn't realise that. All apologies. mvg, Andy ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] RE: Newbern's Example 7
Jeff,
Thank you for your reply.
After fn is lifted, where is bind used in allCombinations?
The bind happens inside the liftM2 function, which can be defined as
(taken from http://www.haskell.org/onlinereport/monad.html):
liftM2 f = \a b - do { a' - a; b' - b; return (f a' b') }
Using the above, I think that allCombinations can be rewritten:
allCombinations fn (l:ls) =
foldl (\a b - do { a' - a; b' - b; return (fn a' b') }) l ls
I'm not sure how allCombinations (+) [[1,2][2,3]] gets to the final
result, since the above would seem to yield:
allCombinations (+) [1,2]:[[2,3]] =
foldl (\a b - do { a' - a; b' - b; return (fn a' b') }) [1,2] [[2,3]]
allCombinations (+) [1,2]:[[2,3]] =
return ((+) [1,2] [2,3])
You mentioned on your site that the operation of the List monad is key to
comprehending this function, but I still wasn't able to grok it. Any
pointers would be appreciated.
- Alson
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[Haskell-cafe] Re: Solution to Thompson's Exercise 4.4
Thanks to all the replies! It seems that there are ways to solve the exercise if I use constructs that are found in later chapters of the book. Sean could be right in that some of the exercises are meant to be difficult to solve and they prepare you for later chapters. Tried Gour's suggestion, but didn't work for howManyOfFourEqual 4 4 2 2 = 3 Anyway the approach was similar to mine, and if I use isEqual I could get a simpler solution. Thanks again. -- Kaoru Hosokawa [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Solution to Thompson's Exercise 4.4
Kaoru Hosokawa wrote: Thanks to all the replies! It seems that there are ways to solve the exercise if I use constructs that are found in later chapters of the book. Sean could be right in that some of the exercises are meant to be difficult to solve and they prepare you for later chapters. As a programmer, I was unhappy with my answers to some of the earlier exercises because they were clearly inefficient and/or inelegant. This kept me reading the book, knowing the answers would present themselves. On more than one occasion I would fret for a few hours over an exercise and then a day or two later learn something new two chapters later which made the exercise much, much easier. Sometimes trivial. As an aside, I kept all of the exercises in revision control. So I can look back at what I first wrote and my later changes. A habit I plan to keep as I move on to other programming texts and languages. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Solution to Thompson's Exercise 4.4
Date: Sat, 12 Mar 2005 23:39:21 -0800 From: Sean Perry [EMAIL PROTECTED] Cc: Haskell-Cafe@haskell.org As an aside, I kept all of the exercises in revision control. So I can look back at what I first wrote and my later changes. A habit I plan to keep as I move on to other programming texts and languages. That's a nice approach. But I can't resist asking: once you've learned Haskell, what is there left to move on to? ;-) Mike ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
