[Help]Chapter 10 Practice 4

00Raiser · January 7th, 2010, 10:14 PM

I downloaded the code, but I am having trouble to understand how the inner function works even though the author stated it. Any one who understand this can give me suggestions?

Comment By the Author:

Quote:

Creating all permutations of the words in the phrase is the most difficult part of the exercise.
If you can crack this, then writing the file is trivial :-)
This method visualizes the permutations of the words as a graph with a dummy node at the top.
The number of levels below the dummy node corresponds to the number of words in the phrase and each path
from the dummy node down through the levels defines one permutation of the words in the phrase.
The argument to the method is the index of the word in the permutation to be assigned. When this method is called with an argument of 0,
it traverses each of the paths in the graph and thus creates all permutations. When a path has been traversed,
it is written to the file. The path variable records the 'length' of a path as it is traversed,
where the maximum length is the number of words in the phrase. When path reaches the maximum, a complete
path has been traversed so it is written to the file.

stalinvlad · October 25th, 2010, 12:40 PM

Its recursion which maybe throwing you off
If I replace the for loop with a while...

Code:

private static void permute(int n) {
    permutation[n] = ++path == 0 ? null: words[path-1];
    if(path == words.length) { 
       write();
    } else {
       i=0
      while( i<words.length) {
        if(permutation[i] == null) 
           permute(i);
         // You return HERE
         ++i;
      }
    }
      path--;
      permutation[n] = null;
  }

Where I comment YOU RETURN HERE is where, by golly, you return after each recursion (see the method calls itself)
Now when it does the value of i in the loop & the original value of n (the argument) will return to what ever was pushed on the stack at that call, and so the while loop will continue, ++i is it still less?
yes is it a null yes push it all on the stack and call myself again
or no
path --
make n null
return
(and if it was a recursive call that called it it will again jump back into the while loop)

Hope that helps

00Raiser · October 30th, 2010, 07:02 AM

Thank you for your help. I am really glad that there is someone as nice as you.

However...

Sorry to say that this was solved long ago. I couldn't find any help here so I figure that out myself after some hard logics. It was too long ago(as indicated by the date), because right now I am already an oracle certified J2EE developer.