- Beginners - Haskell.org

sum problem
by Roelof Wobben 07 Feb '15

07 Feb '15

Hello, The exercises of CIS 194 are really hard. Now I have to sum all the numbers in a array with a catch If a number is greater then 10 it has to be dived in single numbers. so [ 1 , 12, 3] will be [ 1, 1,2,3] and then sum up. So far I have this : -- | sum all the digits of a array sumDigits :: [Integer] -> Integer sumDigits n | n == 0 = acc | n < 10 = acc + n | otherwise = sumDigits acc + n `mod` 10 + n `rem` 10 But now I see a message that acc is not known which is not wierd. But when I do sumDigits n acc I see a message that sumDigits has only 1 parameter where I give it to, When I only had to sum up i is easy , I could use map [+] xs Anyone who can give a me tip without giving the answer otherwise I do not learn anything. Roelof

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recursion problem.
by Roelof Wobben 07 Feb '15

07 Feb '15

Hello, I have figured out how I can make from 123 [1,2,3] I did it this way : -- | convert a number to a array in pieces where a negative number will be a empty array. toDigits :: Integer -> [Integer] toDigits n | n <= 0 = [] | otherwise = toDigits (n `div` 10) ++ [n `mod` 10] but now when I do toDigits 0 , I see [] as output where I was expected [0] But when I do toDigits 100 I see the right output [ 1,0,0] which surprises me because I tought that with the first 0 there will be a [] Or is the n the second time [0,0] and the thirth time [0] So it will be like this : toDigits 100 to Digits [1] ++ [ 0,0] toDigits [1] ++ [0] ++ [0] which leads to [1,0,0] Roelof

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Re: [Haskell-beginners] length problem
by Roelof Wobben 06 Feb '15

06 Feb '15

Oke, I have solved it already. The problem was that I did list.length but Haskell uses length list Still too much Ruby in my system :( Roelof Francesco Ariis schreef op 6-2-2015 om 17:50: > (off-list) please consider not using html in your mails, it's quite difficult > to read them for us who plaintext-friendly client > > On Fri, Feb 06, 2015 at 05:47:24PM +0100, Roelof Wobben wrote: >> <html> >> <head> >> >> <meta http-equiv="content-type" content="text/html; charset=windows-1252"> >> </head> >> <body bgcolor="#FFFFFF" text="#000000"> >> Hello, >> >> I have to double every second element from the right. >> >> So for a even length array that means : 1 , 3 , 5 and so on >> and for a non even lenght that means the 2,4 and so on. >> >> So I thought I could solve that on this way : >> >> -- | Doubles every second number from the right. >> doubleEveryOther :: [Integer] -> [Integer] >> doubleEveryOther [] = [] >> doubleEveryOther (x:[]) = [x] >> doubleEveryOther (x:(y:zs)) >> | ((x:(y:zs)).length) `mod` 2 /= 0 = [x] ++ (y * 2) : >> doubleEveryOther zs >> | otherwise = [x *2] ++ y : doubleEveryOther zs >> >> >> >> but this does not work because I see this error message : >> >> <div class="ide-error-span">src/Main.hs@14:8-14:16 </div> >> <div class="ide-error-msg">Couldn't match expected type ‘Int >> -> c0’ with actual type >> <div class="CodeMirror cm-s-default" style="font-size: 14px;">[> class="cm-variable-2">Integer]</div> >> > class="ide-error-collapse-btn"> …> inline;"> >> In the first argument of ‘(.)’, namely ‘(x : (y : zs))’ >> In the first argument of ‘mod’, namely ‘((x : (y : zs)) . >> length)’ >> In the first argument of ‘(/=)’, namely ‘((x : (y : zs)) . >> length) `mod` 2’ >> >> >> Can anyone give me a better way to check if I have a even or odd >> length array ? >> >> Roelof >> >> </div> >> >> </body> >> </html> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >

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length problem
by Roelof Wobben 06 Feb '15

06 Feb '15

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Couldn't match expected type ‘IO ()’ with actual type [Integer]
by Roelof Wobben 06 Feb '15

06 Feb '15

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Subject: Re: File I/O: reading from, writing to,
by Doug McIlroy 05 Feb '15

05 Feb '15

same file in one function One more safety feature in the "proper way": in most systems it will guarantee that a concurrent reader sees a coherent file image. ------------------------------------------------------------ > I strongly urge you to use the proper way with temp file and renaming, even making it a function if you need to repeat this several times The "proper way" is fraught with pitfalls: potentially changes in file ownership and permissions, loss of linkage when the file is known by multiple names, and even total failure if you don't have write permission on the directory.^C The proper way does have one safety advantage. The original file is highly likely to survive should the writing of the new contents stop prematurely for some external reason. Doug

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Re: [Haskell-beginners] File I/O: reading from, writing to,
by Doug McIlroy 05 Feb '15

05 Feb '15

same file in one function > I strongly urge you to use the proper way with temp file and renaming, even making it a function if you need to repeat this several times The "proper way" is fraught with pitfalls: potentially changes in file ownership and permissions, loss of linkage when the file is known by multiple names, and even total failure if you don't have The proper way does have one safety advantage. The original file is highly likely to survive should the writing of the new contents stop prematurely for some external reason. Doug

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File I/O: reading from, writing to, same file in one function
by Geoffrey Bays 05 Feb '15

05 Feb '15

Question: As I work my way through Learn You as Haskell, I have been writing small File IO programs. Unlike the example in that book, where the author reads from one file, writes altered data to a temp file and then deletes the original file and renames the temp file, I have been trying to read from a given file, alter the data and then overwrite the same file with the new data. Is this possible? In a main do block I have tried: contents <- readFile fileName -- do stuff to contents-- writeFile fileName result After using appendFile to add items to the file, when I call the function to read and write I get this error: openFile: resource busy, (file is locked) I have also tried using the file handles like so: handle <- openFile fileName ReadWriteMode contents <- hGetContents handle -- do stuff to contents -- hPutStr handle results hClose handle I have also tried opening with one handle to read, closing it, then using another handle for writing, but the error message is that the handle to the file is closed. I am thinking that the the author of LYAH maybe has good reasons to use a temp file, or is there another way that is not too difficult to grasp?? Many thanks, Geoffrey

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Re: [Haskell-beginners] [Haskell-cafe] ghc --make Vs runhaskell
by Brandon Allbery 04 Feb '15

04 Feb '15

On Wed, Feb 4, 2015 at 10:10 PM, Richard A. O'Keefe <ok(a)cs.otago.ac.nz> wrote: > Given things like sockets, STREAMS, and ptys, I am not > sure that it is _possible_ for a Unix system to determine whether two > streams > are or are not "related" reliably. > It's not. I was not implying that it was, or that it was even a good idea to try to do so. On the other hand, flushing *every* line-buffered FILE is rather overkill but less heinous than highly dubious and untestable assumptions about relationships. -- brandon s allbery kf8nh sine nomine associates allbery.b(a)gmail.com ballbery(a)sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net

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Re: [Haskell-beginners] [Haskell-cafe] ghc --make Vs runhaskell
by Madhu Babu 03 Feb '15

03 Feb '15

Thanks a lot. .this really helps.. On Feb 3, 2015 10:35 PM, "Tristan Seligmann" <mithrandi(a)mithrandi.net> wrote: > On 4 February 2015 at 05:23, Madhu Babu <madhub.bits(a)gmail.com> wrote: > > Basically, in the following code, we print a line first and the read the > > line from stdin. This behavior works perfectly fine when using > “runhaskell > > guess.hs”. But when i say “./guess”, i had to type in a number directly ( > > i.e. first prompt is skipped ). > > This is most likely due to stdio buffering; usually stdin/stdout are > line buffered, so data you write with putStr may not actually be > written until a newline is written. This post / thread details some > ways to handle this problem: > > https://www.haskell.org/pipermail/beginners/2010-March/003692.html > -- > mithrandi, i Ainil en-Balandor, a faer Ambar >

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