[plt-scheme] HtDP newbie question, 12.4.2

You wrote: 
  "Where should this missing letter get added in the result of (insert-in-single-word s (rest word))? Correct! At the beginning." 
   
  It's not possible to add missing letters...they are missing!  
   
  What I need is a prefix function.  For the word (list 'A 'T 'W), I need:
  cycle 1: (empty)
  cycle 2: (list 'A)
  cycle 3: (list 'A 'T)
  cycle 4: (list 'A 'T 'W)
   
  Unfortunately, the recursive function call, "insert-in-single-word (rest word)," results in smaller and smaller words:
  cycle 1: 'T 'W
  cycle 2: 'W
  cycle 3: empty
   
  I just spent 2 hours trying to define an auxiliary function or add some extra "appends" in the main function, in order to insert a prefix in front of the X, but there is no way to add the missing letters.  They are gone forever.
  
Matthias Felleisen <matthias at ccs.neu.edu> wrote:
  
  On Mar 27, 2008, at 10:04 PM, Cooke Kelsey wrote:
    Hi, I think I see what you are driving at.  
... 
    the words are missing the letter A, the first of the current (****) word
... 
     I thought of your reminder to use helper functions, e.g. "add-prefix," but I keep going back to this register idea.  I don't know whether I am getting off track or what.


  What would "add-prefix" do? "Register" sounds like an old man's way of thinking about machines and lower-level representations of computations yet. But add-prefix seems definitely related to the "it's missing the letter ..." part. 
  

  Where should this missing letter get added in the result of (insert-in-single-word s (rest word))? Correct! At the beginning. 
  

  Keep going. -- Matthias
  




       
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