[Haskell-cafe] Java or C to Haskell
I'm trying to write in Haskell a function that in Java would be something
like this:
char find_match (char[] l1, char[] l2, char e){
//l1 and l2 are not empty
int i = 0;
while (l2){
char aux = l2[i];
char[n] laux = l2;
while(laux){
int j = 0;
if(laux[j] = aux) laux[j] = e;
j++;
}
if compare (l1, laux) return aux;
else i++;
}
return '';
}
compare function just compares the two lists and return true if they are
equal, or false if they are not.
it is really a simple function, but I've been thinking about it a lot of
time and I can't get the goal. It works like this:
find_match 4*ha 4*5a 'h' returns '5' (5 matches with the h)
find_match 4*ns 4dhnn k returns '' (no match at all - lists
are different anyway)
I'm trying to use map for scanning the list l2, then using some function for
substitute the char, and finally something like zipWith x == y laux l1 for
comparing the lists, but I can't find a way for putting all this together!!
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Re: [Haskell-cafe] Java or C to Haskell
wow, the simpliest ever! Andrea Rossato wrote: On Wed, Sep 20, 2006 at 01:31:22AM -0700, Carajillu wrote: compare function just compares the two lists and return true if they are equal, or false if they are not. it is really a simple function, but I've been thinking about it a lot of time and I can't get the goal. I forgot, obviously, that lists are an instance of the Eq class... so, this is enough: comp l1 l2 = if l1 == l2 then True else False You never stop learning! andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Java-or-C-to-Haskell-tf2303820.html#a6404305 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Java or C to Haskell
That works good, but I have a problem with the return type, I forgot to
mention... can it be a [char]??
Donald Bruce Stewart wrote:
crespi.albert:
I'm trying to write in Haskell a function that in Java would be something
like this:
char find_match (char[] l1, char[] l2, char e){
//l1 and l2 are not empty
int i = 0;
while (l2){
char aux = l2[i];
char[n] laux = l2;
while(laux){
int j = 0;
if(laux[j] = aux) laux[j] = e;
j++;
}
if compare (l1, laux) return aux;
else i++;
}
return '';
}
Yikes!
compare function just compares the two lists and return true if they are
equal, or false if they are not.
it is really a simple function, but I've been thinking about it a lot of
time and I can't get the goal. It works like this:
find_match 4*ha 4*5a 'h' returns '5' (5 matches with the h)
find_match 4*ns 4dhnn k returns '' (no match at all - lists
are different anyway)
That's almost a spec there :)
How about:
import Data.List
findMatch s t c
| Just n - elemIndex c s = Just (t !! n)
| otherwise = Nothing
Using it in GHCi:
findMatch 4*ha 4*5a 'h'
Just '5'
findMatch 4*ns 4dhnn 'k'
Nothing
-- Don
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Re: [Haskell-cafe] Java or C to Haskell
Yes, they must be equal the whole way, I like this recursive solution :) Ketil Malde-3 wrote: Carajillu [EMAIL PROTECTED] writes: compare function just compares the two lists and return true if they are equal, or false if they are not. find_match 4*ha 4*5a 'h' returns '5' (5 matches with the h) find_match 4*ns 4dhnn k returns '' (no match at all - lists are different anyway) Must they be equal the whole way, or just up to the occurrence of the searched-for character? find_match (x:xs) (y:ys) c | x==c = Just y | x/=y = Nothing | True = find_match xs ys c find_match [] [] _ = Nothing Or, to check the whole list: find_match (x:xs) (y:ys) c | x==c xs == ys = Just y | x/=y = Nothing | True = find_match xs ys c find_match [] [] _ = Nothing -k -- If I haven't seen further, it is by standing in the footprints of giants ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Java-or-C-to-Haskell-tf2303820.html#a6404344 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Problems interpreting
Hi, I'm a student and I have to do a program with Haskell. This is the first
time I use this languaje, and I'm having problems with the indentation. I
want to check if this function is correct, but when I try to make the GHCi
interpret it, I get line 18:parse error (possibly incorrect indentation)
The function is:
-- Replaces a wildcard in a list with the list given as the third argument
substitute :: Eq a = a - [a] - [a] - [a]
substitute e l1 l2= [check_elem c | c - l1] where
check_elem::Eq a = a - [a]
check_elem x =
if x == e then return l2
-- Tries to match two lists. If they match, the result consists of the
sublist
-- bound to the wildcard in the pattern list.
(line 18) match :: Eq a = a - [a] - [a] - Maybe [a]
match _ _ _ = Nothing
{- TO BE WRITTEN -}
Thank you for your attention!
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Re: [Haskell-cafe] Problems interpreting
Wow! I'm starting to love this languaje, and the people who uses it!:) Andrea Rossato wrote: On Mon, Sep 18, 2006 at 12:54:34PM +0200, Albert Crespi wrote: Thank you very much for your reply! As I said, it is my first experience with Haskell, I have been programming in Java and C for some years, and I find this language very different from them. Anyway I'll try to fix the function with the information that you gave me. Thanks again! You're welcome. By the way, this is what the comments say you are trying to do: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a = a - [a] - [a] - [a] substitute e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else check_elem xs This is the result: *Main substitute 1 [1,2,3] [] [2,3] *Main substitute 1 [1,2,3] [7,8,9] [7,8,9,2,3] *Main Have fun with Haskell. Ciao Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6361815 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Finally I took Andrea's solution check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs I think it's easy to understand for me ( in my noob level), than the recursive one. I'm testing it and it's working really well. The other solutions are a little complicated for me, but I'm still trying to undestand them. Thanks! Andrea Rossato wrote: On Mon, Sep 18, 2006 at 12:25:21PM +0100, Neil Mitchell wrote: Why not: check_elem (x:xs) = if x == e then (l2 ++ xs) else x : check_elem xs Thanks Thank you! Lists are my personal nightmare...;-) Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6362822 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Not a good solution, it just substitutes the first occurrence of the item in the list. I'll try the others Carajillu wrote: Finally I took Andrea's solution check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs I think it's easy to understand for me ( in my noob level), than the recursive one. I'm testing it and it's working really well. The other solutions are a little complicated for me, but I'm still trying to undestand them. Thanks! Andrea Rossato wrote: On Mon, Sep 18, 2006 at 12:25:21PM +0100, Neil Mitchell wrote: Why not: check_elem (x:xs) = if x == e then (l2 ++ xs) else x : check_elem xs Thanks Thank you! Lists are my personal nightmare...;-) Andrea ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6362912 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Problems interpreting
Definitely I'll take this solution, I'm reading about Pointfree, I think it's not that dificult to understand. And moreover it's the simpliest way to write code. Jón Fairbairn-2 wrote: Andrea Rossato [EMAIL PROTECTED] writes: On Mon, Sep 18, 2006 at 04:16:55AM -0700, Carajillu wrote: Wow! I'm starting to love this languaje, and the people who uses it!:) You spoke too early. My code had a bug, a huge one... this is the right one: -- Replaces a wildcard in a list with the list given as the third argument substitute :: Eq a = a - [a] - [a] - [a] substitute e l1 l2= [c | c - check_elem l1] where check_elem [] = l1 check_elem (x:xs) = if x == e then (l2 ++ xs) else [x] ++ check_elem xs I think it's nicer to do it like this: substitute e l l' = concat (map subst_elem l) where subst_elem x | x == e = l' | otherwise = [x] since subst_elem has a more straightforward meaning than check_elem, and the concatenation is handled by a well known standard function. Also, it would usually be more useful to have the argument to replace /with/ before the argument to replace /in/, so that (substitute '*' wurble) is a function that replaces all the '*'s in it's argument with wurbles. And if you do that, you can write it like this: subst e l' = concat . map subst_elem where subst_elem x | x == e = l' | otherwise = [x] -- Jón Fairbairn [EMAIL PROTECTED] ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- View this message in context: http://www.nabble.com/Problems-interpreting-tf2290155.html#a6363827 Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
