Java Quicksort算法无法正常工作

Question

I have two Arraylists, which should be sort descending due to the values in the second Arraylist(which is type double). Basically the values of the first Arraylist are not regarded, but the elements in the first Arraylist map to the ones in the second one, so both Arrays should get sortet due to the values in Arraylist2. (I hope this is more or less clear)

Problem:

Somehow when I print out the values of Arraylist2 (with the double values) after beeing sorted they are totally not sorted, but have a different sequence than before sorting. I also wrote some debug system.outs, which shows me the algoritm is running, but I have no idea, why it won`t sort correctly, I hope somebody can see where the problem is.

Code:

Call + Outputcode:

            String str = "";
        // DEBUG //
        for(int k = 0; k < test.size(); k++) {
            str += " " + test.get(k);   
        }
        System.out.println(str);
        System.out.println("------------------------------------------------------------------/r/n");
        sort(items, test, 0 ,(test.size() -1));
        String str2 = "";
        for(int k = 0; k < test.size(); k++) {
            str2 += " " + test.get(k);  
        }
        System.out.println(str2);
        // DEBUG //

Quicksort algoritm:

public void sort(ArrayList<Item> items, ArrayList<Double> nutzen, int l, int r) {
        if(l < r) {
            double pivot = nutzen.get(r);
            int p = partition(items, nutzen, l, r, pivot);
            sort(items, nutzen, l, p-1);
            sort(items, nutzen, p+1, r);
        }
    }

    private int partition(ArrayList<Item> items, ArrayList<Double> nutzen, int l, int r, double pivot) {
        int i = l;
        int j = r-1;
        do {
            do {
                i = i + 1;
            } while(i < r && nutzen.get(i) > pivot);
            do {
                j = j - 1;
            } while(j > l && nutzen.get(i) < l);
            if(i < j) {
                // Tausche Daten
                double tmp  = nutzen.get(i);
                Item tmp2 = items.get(i);
                nutzen.set(i, nutzen.get(j));
                nutzen.set(j, tmp);
                items.set(i, items.get(j));
                items.set(j, tmp2);
            }
        } while(i < j); 
        if (nutzen.get(i) > pivot) {
            double tmp  = nutzen.get(i);
            Item tmp2 = items.get(i);
            nutzen.set(i, nutzen.get(r));
            nutzen.set(r, tmp);
            items.set(i, items.get(r));
            items.set(r, tmp2);
        }
        return i;       
    }

Sample Output: (first is the unsorted Arraylist, second the sorted one)

 0.22540250447227192 0.5289855072463768 0.3245742092457421 0.35105028644175684 0.3773755656108597 0.2041172365666434 0.4091826437941473 0.33037437282902354 0.4383735705209657 0.28473648186173856 0.3422330097087379 0.25703446095478977 0.3373493975903614 0.2701873935264055 0.3221397891448653 0.34912891986062716 0.3018603018603019 0.31361550229474755 0.35785288270377735 0.27435456110154904 0.22781065088757396 0.27684563758389263 0.21881770349736923 0.4226451927769644 0.24628854206318995 0.3256219991270188 0.3844096293465801 0.3178254051228437 0.398093508851566 0.3544702638834187 0.20851528384279475 0.4041025641025641 0.21713772992373262 0.26014028667276606 0.3591307662981319 0.23153252480705622 0.27023319615912206 0.24333719582850522 0.29892573563755254 0.31568998109640833 0.27108784176847006 0.34125412541254124 0.279090113735783 0.3737704918032787 0.3326703132769766 0.22776967930029154 0.22143195827406353 0.27614293221229635 0.22866611433305717 0.533879374534624 0.28534031413612565 0.20782003213711836 0.21262837580829214 0.2137904468412943 0.2898398529797847 0.24622641509433962 0.3927108927108927 0.26053042121684866 0.3005334914048607 0.23183297180043383 0.24539571926331508 0.3479899497487437 0.4193054136874362 0.31155589123867067 0.31771595900439237 0.3897529734675206 0.3561643835616438 0.31221719457013575 0.477299880525687 0.2683881064162754 0.30484160191273163 0.20526154787396758 0.2362366474938373 0.3485633537447009 0.24390243902439024 0.2618308766485648 0.382782475019216 0.23864915572232645 0.466403162055336 0.31514030218933087 0.3074433656957929 0.3438485804416404 0.28774928774928776 0.29548816568047337 0.34277286135693213 0.5334967320261438 0.32756539235412474 0.2945334590009425 0.20973389355742297 0.25292242295430395 0.2336989640463132 0.328060522696011 0.4326647564469914 0.30530226274907124 0.3326499231163506 0.3077194219245682 0.2322235922729141 0.25569544364508395 0.3788049605411499 0.2476489028213166
------------------------------------------------------------------
 0.22540250447227192 0.5289855072463768 0.466403162055336 0.4383735705209657 0.3897529734675206 0.4193054136874362 0.4226451927769644 0.29548816568047337 0.3479899497487437 0.3438485804416404 0.477299880525687 0.3561643835616438 0.4041025641025641 0.3326703132769766 0.3844096293465801 0.4091826437941473 0.34277286135693213 0.398093508851566 0.3485633537447009 0.3927108927108927 0.4326647564469914 0.3005334914048607 0.28534031413612565 0.31155589123867067 0.31771595900439237 0.31221719457013575 0.30484160191273163 0.3074433656957929 0.31514030218933087 0.5334967320261438 0.32756539235412474 0.2898398529797847 0.25292242295430395 0.28774928774928776 0.533879374534624 0.2945334590009425 0.20973389355742297 0.27614293221229635 0.26053042121684866 0.2683881064162754 0.3737704918032787 0.279090113735783 0.34125412541254124 0.27108784176847006 0.31568998109640833 0.29892573563755254 0.2336989640463132 0.27023319615912206 0.328060522696011 0.3591307662981319 0.26014028667276606 0.30530226274907124 0.3544702638834187 0.3178254051228437 0.3256219991270188 0.3326499231163506 0.3077194219245682 0.27684563758389263 0.2322235922729141 0.27435456110154904 0.35785288270377735 0.31361550229474755 0.3018603018603019 0.34912891986062716 0.3221397891448653 0.2701873935264055 0.3373493975903614 0.25703446095478977 0.3422330097087379 0.28473648186173856 0.33037437282902354 0.25569544364508395 0.3773755656108597 0.35105028644175684 0.3245742092457421 0.2618308766485648 0.382782475019216 0.23864915572232645 0.24390243902439024 0.2362366474938373 0.20526154787396758 0.24539571926331508 0.23183297180043383 0.24622641509433962 0.2137904468412943 0.21262837580829214 0.20782003213711836 0.22866611433305717 0.22143195827406353 0.22776967930029154 0.24333719582850522 0.23153252480705622 0.21713772992373262 0.20851528384279475 0.24628854206318995 0.21881770349736923 0.22781065088757396 0.2041172365666434 0.3788049605411499 0.2476489028213166

Answer 1

It has been a long time since I codified sorting algorithms (oh, the joys of complete APIs) but this strikes me as odd

       } while(j > l && nutzen.get(i) < l);

Wouldn't it better be

       } while(j > l && nutzen.get(i) < pivot);

Anyway, an advice. Instead of trying to sort 10 numbers and just reporting the exit, try with 3 or 4 and debug more seriously the inner workings of your code (at each step, which pivot is chosen, what are the resulting sublists, etc.).

Answer 2

I'm just starting out, so I haven't really looked at quicksort before, but I think I got it working. Just needed to tweak the 4 lines that I commented. Also, I changed l to left, r to right, i to a and j to z, to make it a little easier for me to keep track of. As I said, I'm still a noob, in my first year as a comp sci major, so I can't comment on how good the implementation is. It seemed like a good problem for me to work on and try to learn a little bit. Hope it helps you out some without being too "here's the answer"-ish.

The changes:

1. Offset the left and right counters further "outside" by 1 to account for them getting incremented and decremented before the comparison. Didn't seem to be parsing the entire range the way it was. Could have changed the inner do..while loops to while loops for same effect.
2. Changed l to pivot . You were comparing a value and an index. i was correct, so I figure that was just an oops.
3. Changed if (nutzen.get(a) > pivot) to if (nutzen.get(a) < pivot) . At this point I would need to spend more time figuring out the why, and I have other things that need doing. All I know is that it didn't work the other way, heh.

I didn't go crazy testing it out, but it does seem to work. I copied about 6 of your sample values and tried them out, and it worked. Good luck!

private static int partition(ArrayList<Double> items, ArrayList<Double> nutzen, int left, int right, double pivot) {
    int a = left-1; //offset to account for do..while incrementing prior to comparison
    int z = right; //offset to account for do..while decrementing prior to comparison
    do {
        do {
            a++;
        } while(a < right && nutzen.get(a) > pivot);

        do {
            z--;
        } while(z > left && nutzen.get(z) < pivot); //compare to pivot, not index

        if(a < z) {
            double tmp  = nutzen.get(a);
            Double tmp2 = items.get(a);
            nutzen.set(a, nutzen.get(z));
            nutzen.set(z, tmp);
            items.set(a, items.get(z));
            items.set(z, tmp2);
        }
    } while(a < z);

    if (nutzen.get(a) < pivot) {  //changed > to <
        double tmp  = nutzen.get(a);
        Double tmp2 = items.get(a);
        nutzen.set(a, nutzen.get(right));
        nutzen.set(right, tmp);
        items.set(a, items.get(right));
        items.set(right, tmp2);
    }
    return a;       
}

.22540250447227192 0.5289855072463768 0.3245742092457421 0.35105028644175684 0.3773755656108597 0.2041172365666434
------------------------------------------------------------------
0.5289855072463768 0.3773755656108597 0.35105028644175684 0.3245742092457421 0.22540250447227192 0.2041172365666434