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