Debugging Java Quicksort Code: Tips and Tricks for Optimal Results

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Discussion Overview

The discussion revolves around debugging a Java implementation of the quicksort algorithm, focusing on code issues and potential improvements. Participants share their observations and suggestions regarding the code structure, logic, and debugging techniques.

Discussion Character

  • Technical explanation
  • Debugging assistance
  • Exploratory

Main Points Raised

  • One participant notes that the variable randpiv should be a random number between start and finish, rather than between 0 and finish.
  • Another suggests adding conditional statements to identify problematic values during execution.
  • A participant points out that the code logic may lead to unreachable statements, specifically regarding the conditions for finish and start.
  • There is mention of using a source-level debugger to better understand the issues in the code.
  • One participant expresses a need to learn how to use Valgrind with Java, indicating prior experience with C++ and C.
  • Another participant highlights that certain statements in the code may never execute due to the flow of logic established by previous conditions.

Areas of Agreement / Disagreement

Participants express differing views on the correctness of the current implementation, with some identifying specific errors while others suggest debugging strategies. No consensus is reached on a definitive solution or correction.

Contextual Notes

There are indications of potential logical errors in the code related to variable conditions and flow control, which may affect the execution of certain statements. The discussion does not resolve these issues but highlights them for further examination.

perplexabot
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Hi all. I am currently working on quicksort and have written the following code and gives "almost" good results. I am not too fond of the data structure I have chosen but it is too late for me to change. Lesson learned. Can anyone help me debug this thing. I have been at it for a while. Anyway here is the code:

Code: 
        public static DLinkedList quickSort(DLinkedList unsortedList, int start, int finish){
122                 int randpiv;
123                 int offset = 0;
124         
125                 if(finish - start <= 0){
126                         return unsortedList;
127                 }
128                 if(finish == -1){
129                         randpiv = 0;
130                 }else{
131                         //fix here
132                         if(finish - start == 0){
133                                 randpiv = start;
134                         }else{  
135                                 randpiv =  start + myGen.nextInt(finish-start + 1);
136                         }       
137                 }       
138                 
139                 
140                 for(int i = start; i < randpiv; i++){
141                         if((long)unsortedList.distofElem(i) >= (long)unsortedList.distofElem(randpiv)){
142                                 System.out.println("want to insert: " + unsortedList.distofElem(i) + " AFTER " + u    nsortedList.distofElem(randpiv));
143                                 offset++;
144                                 System.out.println("distofElem(randpiv) = " + unsortedList.distofElem(randpiv));
145                                 DLNode moveMe = unsortedList.removeDist(unsortedList.distofElem(i));
146                                 System.out.println("distofElem(randpiv) = " + unsortedList.distofElem(randpiv-1));
147                                 unsortedList.insertAfter(moveMe, unsortedList.distofElem(randpiv-1));
148                         }       
149                 }       
150                 for(int i = randpiv + offset + 1; i <= finish; i++){
151                         if((long)unsortedList.distofElem(i) < (long)unsortedList.distofElem(randpiv)){
152                                 System.out.println("want to insert: " + unsortedList.distofElem(i) + " BEFORE " + unsortedList.distofElem(randpiv));
153                                 System.out.println("distofElem(randpiv) = " + unsortedList.distofElem(randpiv));
154                                 DLNode moveMe = unsortedList.removeDist(unsortedList.distofElem(i));
155                                 unsortedList.insertBefore(moveMe, unsortedList.distofElem(randpiv));
156                         }       
157                         randpiv++;
158                 }
159                 quickSort(unsortedList, start, randpiv - 1);
160                 quickSort(unsortedList, randpiv + 1, finish);
161                 return unsortedList;
162         }
(************************************************************************)
35         //insert before an index//
 36         public void insertBefore(DLNode putMe, long distance){
 37                 for(temp = myRoot; temp.getDistance() != distance; temp = temp.next){
 38                         System.out.println("[in list for] tempdist = " + temp.getDistance());
 39                 }
 40                 System.out.println("Inserting: " + putMe.getDistance() + " BEFORE: " + temp.getDistance());
 41                 if(temp == myRoot){
 42                         myRoot.prev = putMe;
 43                         putMe.next = myRoot;
 44                         putMe.prev = null;
 45                         myRoot = myRoot.prev;
 46                 }else{
 47                         temp.prev.next = putMe;
 48                         putMe.prev = temp.prev;
 49                         temp.prev = putMe;
 50                         putMe.next = temp;
 51                 }
 52                 listLength++;
 53         }
 54 
 55         //insert after an index//
 56         public void insertAfter(DLNode putMe, long distance){
 57                 for(temp = myRoot; temp.getDistance() != distance; temp = temp.next){
 58                         System.out.println(distance + " != " + temp.getDistance());
 59                         System.out.println("[in list for] tempdist = " + temp.getDistance());
 60                 }
 61                 if(temp == null){
 62                         System.out.println("NULLY!");
 63                 }
 64                 System.out.print("Inserting: " + putMe.getDistance() + " AFTER: ");
 65                 System.out.println(temp.getDistance());
 66                 if(temp == myTail){
 67                         myTail.next = putMe;
 68                         putMe.prev = myTail;
 69                         putMe.next = null;
 70                         myTail = myTail.next;
 71                 }else{
 72                         temp.next.prev = putMe;
 73                         putMe.next = temp.next;
 74                         temp.next = putMe;
 75                         putMe.prev = temp;
 76                 }
 77                 listLength++;
 78         }

here is the input i feed it:
29403 97291 7211 487075 10000000 9902 89600 1620 22500 180000 152 10 14700 1026046 6646 19769 17708 10201 16250 533026 1424561 880025 12930 91210 25004 580800 926718

Here is the output i get:
10
152
1620
6646
7211
9902
10201
12930
14700
16250
17708
19769
22500
25004
29403
89600
97291
180000
487075
880025
533026
1424561
91210
10000000
1026046
580800
926718

which is close, but not quite.

Thanks for reading.
 
Last edited:
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Shouldn't randpiv be a random number between start and finish, not 0 and finish?

There may be more errors - I didn't look beyond that one.
 


You might want to add some if statements to compare for the problem values like

Code: 
    if ( ... == 91210)
        print(" ");  // or any statement you can set a breakpoint on

Then use a source level debugger to see what's going on.
 


AlephZero said:
Shouldn't randpiv be a random number between start and finish, not 0 and finish?

There may be more errors - I didn't look beyond that one.
YES! thank you so much. You found an error! I will change my OP code accordingly with some other adjustments.

rcgldr said:
You might want to add some if statements to compare for the problem values like

Code: 
    if ( ... == 91210)
        print(" ");  // or any statement you can set a breakpoint on

Then use a source level debugger to see what's going on.
I have already done if's and print pairs. I need to learn how to use valgrind with java. I used to know how to use it with C++ and C

Thank you guys.
 
Hi I have updated my code but still not working properly. If anyone can help that would be great.
 
perplexabot said:
Code: 
125                 if(finish - start <= 0){
126                         return unsortedList;
127                 }
128                 if(finish == -1){
129                         randpiv = 0;

Do you realize that the only time this last statement will execute is if finish==-1 and start<-2?

Code: 
130                 }else{
131                         //fix here
132                         if(finish - start == 0){
133                                 randpiv = start;

Do you realize that this last statement will never get executed? You have already returned from the method when finish - start==0 thanks to your first if statement.

Code: 
134                         }else{  
135                                 randpiv =  start + myGen.nextInt(finish-start + 1);

what does myGen.nextInt(finish-start + 1) do?

Code: 
140                 for(int i = start; i < randpiv; i++){
141                         if((long)unsortedList.distofElem(i) >= (long)unsortedList.distofElem(randpiv)){
142                                 System.out.println("want to insert: " + unsortedList.distofElem(i) + " AFTER " + u    nsortedList.distofElem(randpiv));
143                                 offset++;
144                                 System.out.println("distofElem(randpiv) = " + unsortedList.distofElem(randpiv));
145                                 DLNode moveMe = unsortedList.removeDist(unsortedList.distofElem(i));
146                                 System.out.println("distofElem(randpiv) = " + unsortedList.distofElem(randpiv-1));
147                                 unsortedList.insertAfter(moveMe, unsortedList.distofElem(randpiv-1));
148                         }       
149                 }

It woud be helpful if you posted your complete code (for all the custom classes you are using) since it is not at all obvious what some of these method calls do or what a DLNode is.

Code: 
150                 for(int i = randpiv + offset + 1; i <= finish; i++){
151                         if((long)unsortedList.distofElem(i) < (long)unsortedList.distofElem(randpiv)){
152                                 System.out.println("want to insert: " + unsortedList.distofElem(i) + " BEFORE " + unsortedList.distofElem(randpiv));
153                                 System.out.println("distofElem(randpiv) = " + unsortedList.distofElem(randpiv));
154                                 DLNode moveMe = unsortedList.removeDist(unsortedList.distofElem(i));
155                                 unsortedList.insertBefore(moveMe, unsortedList.distofElem(randpiv));
156                         }       
157                         randpiv++;
158                 }

Why are you incrementing your pivot (randpiv) inside this for loop?

Code: 
159                 quickSort(unsortedList, start, randpiv - 1);
160                 quickSort(unsortedList, randpiv + 1, finish);

Are you planning and doing something with the result of these recursive calls to quicksort, or are you just doing extra processing for kicks? You haven't assigned the return parameter to anyting for these calls.