Michael Michael - 7 months ago 58
Java Question

Java Bucket Sort on Strings

I can't figure out what would be the best way to use Bucket Sort to sort a list of strings that will always be the same length.

An algorithm would look like this:

For the last character position down to the first:
For each word in the list:
Place the word into the appropriate bucket by current character
For each of the 26 buckets(arraylists)
Copy every word back to the list


I'm writing in java and I'm using an arraylist for the main list that stores the unsorted strings. The strings will be five characters long each.

This is what I started. It just abrubdly stops within the second for loop because I don't know what to do next or if I did the first part right.

ArrayList<String> count = new ArrayList<String>(26);

for (int i = wordlen; i > 0; i--) {
for (int j = 0; i < myList.size(); i++)
myList.get(j).charAt(i)
}


Thanks in advance.

EDIT: This is what I have now. I know it doesn't work because if there were more than one strings that started with the same letter than it would blow up, but I think I'm more in the right direction. When I run it, even with words that I put it in to make sure there are no duplicates letters, it freaks out on the first set line:
count.set(myList.get(j).charAt(i), myList.get(j));
It's says "Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 5"

public void BucketSort(int wordlen) {

ArrayList<String> count = new ArrayList<String>(26);

//Make it so count has a size
for(int p = 0; p < 26; p++)
count.add(null);

for (int i = wordlen; i > 0; i--) { //for each letter
for (int j = 0; j < myList.size(); j++) //for each word
//Add the word to count based on the letter
count.add((int)myList.get(j).charAt(i) - 65, myList.get(j));
}
//Clear the main list so there aren't a bunch of unsorted words leftover
myList.clear();

//Add the words back in to the list based on their order in count
for (int m = 0; m < 26; m++)
myList.add(count.get(m));
}

Answer

This looks like homework to me, so I won't respond with a code solution.

But basically, the bit you're stuck on is setting up your buckets. Probably you want your buckets to be a Map<Character, List<String>> -- that is, you want to map each letter A - Z to a list of words that match that letter (for the position you're currently looking at). That list of words is your bucket.

Then, after you finish the inner loop you've got there, you do another loop through the contents of the map, going from A-Z (hint: for ( char ch = 'A'; ch <= 'Z'; ch++ )) and dumping the contents of the corresponding bucket back into your (emptied) list.