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

[Jean-Nicolas Jolivet]

Haha, sorry for the (rather long) description ;) The ending was really the important part :)

Basically I just wanted to explain the context of what I'm trying to do. Like I said, I already figured out a way to do all that... I'm just wondering if there is a better way to get to the final string result than building a list of Maybe Char and mapping it using mapMaybe...(considering that some characters of the initial string will NOT end up in the resulting string)... 

Every solution I found was iterating to a list of character one character at a time, and outputting a character as a result (whether it is by recursion, mapping, etc..), however, what I was trying to explain in my previous post was that; when I am processing the escape character, this character should not be added to the resulting string... This is where I decided to use the Maybe monad..I'm just wondering if there is a better way...! 


Jean-Nicolas Jolivet




On 2010-04-29, at 4:26 PM, 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 <jeannicolascocoa at gmail.com> 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
> 
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> 
> 
> -- 
> Ozgur Akgun

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