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

[Ozgur Akgun]

ok then,

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

On 29 April 2010 21:31, matthew coolbeth <mac01021 at engr.uconn.edu> wrote:

> 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 <ozgurakgun at gmail.com> 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
>>>
>>> _______________________________________________
>>> Beginners mailing list
>>> Beginners at haskell.org
>>> http://www.haskell.org/mailman/listinfo/beginners
>>>
>>
>>
>>
>> --
>> Ozgur Akgun
>>
>> _______________________________________________
>> Beginners mailing list
>> Beginners at haskell.org
>> http://www.haskell.org/mailman/listinfo/beginners
>>
>>
>
>
> --
> mac
>



-- 
Ozgur Akgun
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