Re: [Haskell-cafe] Newbie: Replacing substring?

23 Jul 2008

      Ronald,
Your algoritm is more simple and so it is better, I agree.
My algorithm is different and consists of two steps:
1) Split source string into a list of chunks not containing substring to be
replaced.
2) Concatenate chunks inserting new substring in between chuncks.

With this approach I get a 'bonus' function of spliting string into chunks
:)
*

Tue Jul 22 16:38:53 EDT 2008 **Ronald Guida* oddron at
gmail.comwrote:
*
*

If I want to replace a substring in a string, then I would search my
string left to right, looking for any occurrence of the substring.  If
I find such an occurrence, I would replace it and continue searching
from immediately after the replacement.  This algorithm can be
directly expressed in Haskell.  More efficient algorithms do exist.

replaceStr :: String -> String -> String -> String
replaceStr [] old new = []
replaceStr str old new = loop str
  where
    loop [] = []
    loop str =
      let (prefix, rest) = splitAt n str
      in
        if old == prefix                -- found an occurrence?
        then new ++ loop rest           -- yes: replace it
        else head str : loop (tail str) -- no: keep looking
    n = length old

On Tue, Jul 22, 2008 at 8:21 PM, Dmitri O.Kondratiev 
wrote:
...
Roberto thanks!
Shame on me, to post code without enough testing :(
Yet, thanks to your comments *I think* I have found the bugs you wrote
about and now my code works, please see corrected version below.
Extra substring at the end was a result of using foldr with initial element
of []. I fixed this with foldl and first chunk as its initial element.
Incomplete substitution in case of duplicate elements in the pattern was a
bug in my 'takeOut' function that I have also fixed.
Stiil the problem that I have not yet designed solution for is when a
substring to replace extends from the end of a string to the next string. In
other words - first part of substring ends the first string and second part
of substring starts the second string. My algorithm currently does not
account for such a case.
On the side: The more I use Haskell - the more I like it ! It helps me
think about the problem I solve much more clearly then when I use imperative
language.
Corrected code:
-- replace all occurances of "123" with "58" in a string:
test = replStr "abc123def123gh123ikl" "123" "58"
{--
In a string replace all occurances of an 'old' substring with a 'new'
substring
--}
replStr str old new = foldl ((\newSub before after -> before ++ newSub ++
after) new) firstChunk otherChunks
                      where chunks = splitStr str old
                            firstChunk = head chunks
                            otherChunks = tail chunks
{--
Split string into a list of chunks.
Chunks are substrings located in a string between 'sub' substrings
--}
splitStr str sub = mkChunkLst str sub []
    where
      -- mkChunkLst 'src string' 'substr-to-extract' 'list of chunks'
      -- makes list of chunks located between 'substr-to-extract' pieces in
src string
      mkChunkLst [] _ chunkLst = chunkLst
      mkChunkLst str sub chunkLst = mkChunkLst after sub (chunkLst ++
[chunk])
          where
            (chunk, _, after) = takeOut str sub [] []
{--
Take out substring from a string.
String is divided into:
"before substr" ++ "match" ++ "after substr"
where 'match' is substring to split out
--}
takeOut after [] before match = (before, match, after)
takeOut [] _ before match = (before, match, [])
takeOut (x:xs) (y:ys) before match
       | x == y = takeOut xs ys before (match ++ [x])
       | otherwise = takeOut xs (y:ys) (before ++ [x]) []
On Tue, Jul 22, 2008 at 7:39 PM, Roberto Zunino 
wrote:
...
Dmitri O.Kondratiev wrote:
...
I wrote my own version,  please criticize:
-- replace all occurances of "123" with "58" in a string:
test = replStr "abc123def123gh123ikl" "123" "58"
This is a tricky problem: first of all, you fail your own test! ;-)
*Main> test
"abc58def58gh58ikl58"
(Note the extra 58 at the end.)
Other common pitfalls:
*Main> replStr "abc1123def" "123" "58"
"abc1158def58"
(extra 1 ?)
*Main> replStr "abc12123def" "123" "58"
"abc121258def58"
(extra 12 ?)
A useful function from Data.List: stripPrefix
(Of course, there are more efficient string match algorithms)
Regards,
Zun.
--
Dmitri O. Kondratiev
dokondr@gmail.com
http://www.geocities.com/dkondr
-- 
Dmitri O. Kondratiev
dokondr@gmail.com
http://www.geocities.com/dkondr