[Haskell-cafe] Re: please help me to find errors from my first app
thanks. I know how to do it. I should treat it as a stream. -- Thanks Regards Changying Li ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: please help me
On Sun, 7 Apr 2002, Eric wrote: | Hi there, I am a beginner a haskell,and I have some | difficulty in dealing with an assignment that is to | translate a string representation of a list of | appointments into a list of appointments. For example: [requirements snipped] | How can I finish the requirement make use of library | functions such as words, unwords, lines and break. Hey Leo, Well, I'm just learning haskell too, but I decided to give this a shot. I think I solved the main problem, but I did hit a few snags of my own so I can't be sure. Here goes: Basically, I defined appointment as a single type... But I couldn't figure out how to print that type. So I can't really test what I've done, other than I know it compiles. :) Can someone show me how to fill in show down here? ) module Main where data Appt = Appt (Bool, Int, Int, String) instance Show Appt where show x = ??? I then used liness and words to break the multi-line string into a list of lists of words: strToApps :: String - [Appt] strToApps x = map lineToApp (lines x) lineToApp x = wordsToApp (words x) Since the starting ! was optional, the structure of the list could go in two directions here: wordsToApp :: [String] - Appt wordsToApp ws | head ws == ! = mkAppt True (tail ws) | otherwise = mkAppt False (ws) Now it's just a matter of parsing the rest of the line. For simplicity's sake, I took the liberty of assuming you always used the same number of digits for the hours, so I didn't have to search for the -: mkAppt :: Bool - [String] - Appt mkAppt isImp (w:ws) = Appt (isImp, start, done, note) where (hs, hd) = splitAt 3 w -- assumes zero-padded (eg 01-03) start = atoi hs done = atoi hd note = foldr1 concat ws concat a b = a ++ ++ b That atoi function came from working through the exercises in Rex Pages online book ( http://www.cs.ou.edu/cs1323h/textbook/haskell.shtml ) horner str = foldr1 op (reverse str) where op d s = d + (10 * s) atoi str = horner [digitToInt d | d - str] Then I defined a main function: -- I wonder what a refec is. :) main = do print $ strToApps ! 10-11 lecture\n12-13 lunch at the refec :( ... And that's it! I think it would work if I could figure out how to print Appt objects (or whatever you call them in haskell). Meanwhile, I get this because of the way I defined show: Main main [???,???] Cheers, - Michal http://www.sabren.net/ [EMAIL PROTECTED] Give your ideas the perfect home: http://www.cornerhost.com/ cvs - weblogs - php - linux shell - perl/python/cgi - java ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Please help me
At 2001-02-08 13:49, FAIZAN RAZA wrote: write a Haskell function cartesianProduct which when given three lists (to represent three sets) of integers returns a list of lists of ordered triples. That's easy. Just define 'product' as a function that finds the cartesian product of any number of lists, and then once you've done that you can apply it to make the special case of three items like this: cartesianProduct a b c = product [a,b,c] At least, that's how I would do it. -- Ashley Yakeley, Seattle WA ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Please help me
At 2001-02-08 02:04, Ashley Yakeley wrote: That's easy. Just define 'product' as a function that finds the cartesian product of any number of lists, and then once you've done that you can apply it to make the special case of three items like this: cartesianProduct a b c = product [a,b,c] At least, that's how I would do it. eesh, 'product' is something else in the Prelude. Better call it 'cartprod' or something. -- Ashley Yakeley, Seattle WA ___ Haskell-Cafe mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell-cafe
RE: Please help me
Faizan,
A clue is to use list comprehensions (which are very like ZF set notation)
First think how you would define a cartesian product in set notation
X x Y x Z = {(x,y,z) | ...}
and then think how this is written in list comprehension notation
Chris
-Original Message-
From: FAIZAN RAZA [mailto:[EMAIL PROTECTED]]
Sent: 08 February 2001 13:49
To: [EMAIL PROTECTED]
Subject: Please help me
Hello
Please help me to solve this questions
Question
Cartesian Product of three sets, written as X x Y x Z is
defined as the set
of all ordered triples such that the first element is a
member of X, the
second is member of Y, and the thrid member of set Z. write a Haskell
function cartesianProduct which when given three lists (to
represent three
sets) of integers returns a list of lists of ordered triples.
For examples, cartesianProduct [1,3][2,4][5,6] returns
[[1,2,5],[1,2,6],[1,4,5],[1,4,6],[3,2,5],[3,2,6],[3,4,5],[3,4,6]]
Please send me reply as soon as possible
Ok
I wish you all the best of luck
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