Re: [Haskell-beginners] Re: Iterating through a list of char...

This works, except for the case where '=' appears more than twice consecutively. A string of multiple '=' should be treated as a single '='. On Thu, Apr 29, 2010 at 16:26, Ozgur Akgun wrote:

I won't attempt writing a general-case function now. If I understood your (rather long) description correctly, you want to - subtract 42 from the ascii value of the char, except when it is preceeded by '=', in which case you subtract 106 instead.

foo :: [Char] -> [Char] foo ('=':x:xs) = chr (ord x - 106) : foo xs foo (x:xs) = chr (ord x - 42) : foo xs foo _ = []

Hope I understood the problem correctly. Best,

On 29 April 2010 20:37, Jean-Nicolas Jolivet wrote:

...

First I would like to thank everyone for the very interesting replies and suggestions I got so far!...

I tried to implement (and at the very least understand) most of them!...

To add to the context here, what I am trying to do is:

-apply a "transformation" to a character (in my case, subtracting 42 to its ASCII value, which I obtain with chr(ord(c) - 42) -if the character is preceded by a specific character (that would be, an escape character, in this case '=') then subtract 106 to its value instead of 42... -if the character is the escape character itself, '=', then skip it altogether (keeping in mind that the next character needs to be escaped)...

I managed to do it, however I'm not totally satisfied in the way I did it... the problem was that... as I just explained, in some cases, the character that is being processed has to be "skipped" (and by that I mean, not added to the resulting string). This happens when the processed character IS the escape character...

What I did was to build a List of Maybe Char.... my function does the proper operation on the character and returns a "Just Char" when the character is processed, or Nothing when it is the escaped character... so basically I would end up with something like: [Just 'f', Just 'o', Just 'o', Nothing]... I am mapping this using mapMaybe to end up with a proper String...

Would there be any more efficient way of doing this? Considering that the escape character should NOT be added to the resulting string, is there any way I can avoid using the Maybe monad?

Once again, thanks everyone for all the suggestions!

Jean-Nicolas Jolivet

On 2010-04-28, at 10:56 AM, Jean-Nicolas Jolivet wrote:

...

Hi there!

I'm trying to iterate through each character of a string (that part I can do!) however, I need to apply a transformation to each character...based on the previous character in the string! This is the part I have no clue how to do!

I'm totally new to Haskell so I'm pretty sure I'm missing something obvious... I tried with list comprehensions...map... etc... but I can't figure out how I can access the previous character in my string in each "iteration".... to use simple pseudo code, what i need to do is:

while i < my_string length: if my_string[i-1] == some_char: do something with my_string[i] else do something else with my_string[i]

I'm using imperative programming here obviously since it's what I am familiar with...but any help as to how I could "translate" this to functional programming would be really appreciated!

Jean-Nicolas Jolivet

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- Ozgur Akgun

_______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

-- mac