DrScheme - Convert a string into a list of words

[Dallan]

Hello, I've been set the task to convert a string into a list of words within that string. As in, go through a string character by character, recording each character into a list, then when a space appears, break off that list and resume recording the next characters into a new list.
Eg
"Hello, world!" -> ("Hello" "world!")

I've been working on it for just over 2 days non-stop now, and everytime I try a solution, it just doesn't work. My main problem is trying to stop recording when I find a space, then start recording again. Since I've been working on it for so long, I have to much code to past all here, but here is what I think my most useful attempt is:

Note: I am trying to get this procedure to return a list of lists of characters, ie ( (#\I) (#\a #\m)). After I have this solved I can finish the rest by myself confidently.

(define [space? char]
; Check to see if a character is a space
  (equal? char #\space)
  )

(define [chars->list-of-chars char-list]
  
; If the list is empty, return a list with an empty list within it
  (if (empty? char-list)
      (list empty)
      
      ; otherwise if the first character in the list is a space
      (if (space? (first char-list))
          empty ; return an empty list
          
          ; else add the first character within a list, to the rest of the lists of characters (Recurse)
          (append (cons (first char-list) empty) (first-word (rest char-list))))
      )
  )

As you can see, when it finds a space, it returns empty (an empty list), which stops the recording of characters into the first list. As a result, if the first character in a string is a space, it returns an empty list, otherwise it will just return the first word.

This is the last section of an assignment that I have to do, I have the entire program working correctly, but without this piece, none of my hardwork will show =[

It's due in approx 24 hrs, so I would appreciate speedy replies

Thanks in advance

[Rashakil Fol]

What is this "first-word" function? You haven't said anything about that. It seems to me like you meant to be calling chars->list-of-chars for that...

What you need to do is the following strategy: Make a function that does the following:

" Hello, world foobar captcha"
->
("Hello" . " world foobar captcha")

only instead of strings, have them be lists of characters... and if the input string has no more words, have it return #f or '(), or whatever suits you.

That's pretty similar to what you have, only instead of looking for space-characters, use char-alphabetic? or perhaps that in combination with char-numeric? to recognize word characters. It looks like your words aren't supposed to include punctuation, after all.

Once you have a function that returns the first word and the rest of the words in a cons pair, it's fairly simple to go from that to returning a list of words.

[Dallan]

Sorry, forgot to change the procedure name when I recursed it :D

I think my main problem was that I was trying to remove the space chars from the list returned. Since I'm planning on returning a list of lists I can just apply the filter char-alphabetic? to each list afterwards. But my fundamental problem is much the same.

How can I tell it to put all the characters up to a space into one list, then continue, writing to a new list.

atm all I can return is
(first-word (string->list "I am a walrus"))
-> ((#\I) (#\space) (#\a) (#\m) (#\space) (#\a) (#\space) (#\w) (#\a) (#\l) (#\r) (#\u) (#\s) ())

All i need to change in this result is to have consecutive non-space characters to be within the same list (ie, change (#\a) (#\m) to (#\a #\m).

(define [space? char]
  (equal? char #\space)
  )

(define [first-word char-list]
  
  (if (empty? char-list)
      (list empty)
      
      ; If the character is alphabetic, record it
      (if (char-alphabetic? (first char-list)) 
          (cons (cons (first char-list) empty) (first-word (rest char-list)))
          
         ; If it's a space start recording the new list
          (if (space? (first char-list))
              (cons (list #\space) (first-word (rest char-list)))
              
          ; Otherwise just ignore it and move on to the next char
              (first-word (rest char-list)))
          )
      )
  )

Any ideas?

[Ancient Dragon]

what programming language is this? And why is it not posted in the appropriate board? I'd move it but don't know what language it is.

[Rashakil Fol]

I'd recommend forgetting everything and starting with this:

(define (parse-words chars current-word)
  ...)

In this case, 'chars' is your string and 'current-word' is the current word you're reading off the string.

For example, if you're converting the string "hello doctor foobar!", you'll end up recursing as in the following. (Note that where I draw strings, I really mean for there to be a list of chars. That would be a pain to write though.)

(parse-words "hello doctor foobar!" '())  ; "hello doctor foobar!" is really a list of chars
->
(parse-words "ello doctor foobar!" '(#\h))
->
(parse-words "llo doctor foobar!" '(#\e #\h))
(parse-words "lo doctor foobar!" '(#\l #\e #\h))
(parse-words "o doctor foobar!" '(#\l #\l #\e #\h))
(parse-words " doctor foobar!" '(#\o #\l #\l #\e #\h))
(cons (reverse '(#\o #\l #\l #\e #\h)) (parse-words (skip-leading-spaces " doctor foobar!") '()))

Now, this will have to be a helper-function. You'll want to have a function that converts the string to a list of chars and skips leading spaces before passing it to parse-words, and then recombines the words, represented as lists of chars, into string form.

And you'll need to write skip-leading-spaces. Maybe you should call it skip-leading-spaces-and-punctuation, or perhaps skip-leading-non-alphanumeric-chars. Or maybe 'drop-...' instead of 'skip-...'.

Oh, and this language is Scheme.

[Dallan]

I love you Rashakil :D

That fixed my main problem, Thanks so much!!!