Merge Sort - 'almost' working

Hello everyone!
I have been learning how to program for a while and I thought it came time for algorithms. I bought Cormen's book and everything was going smoothly until it came to Merge Sort. I understand the theory but I am having trouble putting it into practise. I found online a few (working) examples (like this one: http://goo.gl/75R0z) and I tried to look up to them and write my own code (with slight changes) and so far so unsuccessful. The code compiles but it doesn't do it's thing. Can you give me a hand?
main function:
http://pastebin.com/y0HZ7Dkj
Here's the algorithm:
http://pastebin.com/U8XAKsiv

BR41N-FCK wrote:
If I had a project with a short deadline - sure, I'd use a pre-made (what else can we call it?) solution. But right now I am learning algorithms, so there's no point using ready ones.
The only reason for the sorting and linked-list exercises is to give students some experience with challenging, but solvable problems in programming. Don't get me wrong, there is great value in that. However, it would be better done in C or even Pascal. Computer Science teaching kind of went off the deep end in 1990 or and has only gotten worse.
Thank you for solving my problem. It was so simple... and maybe that's why I have overlooked it :-)
One of the skills a programmer must have is how to debug such problems. That doesn't always mean running in a debugger. I very rarely use a debugger. Most modern, UI or server work is far too complex for such things. Simple "printf" statements are usually all that is required.
Speaking of C++. Why would you call it awful? I mean it isn't the best for developing windowed apps (see: objective-c and c# on windows), but it is great for console ones and also it is good for education purposes thanks to its plain syntax (compared to c# and not to even mention the crazy syntax of objective-c).
Because I wasted so much of my programming career on it. I suppose it was a valuable lesson. I suspose if someone had told me it was junk in 1995 I probably would have ignored them then. Now I have a decent store of tacit wisdom that is hard to explain. I just "know" when someone does or doesn't know what they are talking about. Just because "everybody is doing it" doesn't mean everybody isn't wrong.
Objective-C syntax is actually quite simple. It is just the message passing through brackets. That is insignificant compared to cryptic C++11. You aren't even using C++ in this program. With the expection of iostream, you are only using C. I don't see any templates, no references, no const-correctness, no virtual functions, no inline code, no iterator traits, no lambdas, no locales, no iomanips. It's all junk that only makes life harder and life is short enough as it is.

Similar Messages

Merge sort not working

so i've been trying to write a merge sort algorithm for linked lists for an assignment and i'm so fing close, but it's really frustrating me because it isn't working correctly. i have a sort and a merge method and i'm doing this recursively. can someone point out what i'm doing wrong?? it prints the elements (i've been testing with random strings) and even changes their order, but no luck on them actually being sorted. help is massively appreciated.
sort method:
public static <T extends Comparable<T>> void sort(ADTListInterface<T> list){
     int s = list.size();
if (s<2)return;
MyLinkedList<T> listl = new MyLinkedList<T>();
MyLinkedList<T> listr = new MyLinkedList<T>();
int x=0;
for(; x<s/2; x++){
     listl.add(list.get(x));//add first half of list to new list
for(; x<s; x++){
     listr.add(list.get(x));//add rest of original list to new list
sort(listr);
sort(listl);
merge(listl, listr, list);
merge method:
public static <T extends Comparable<T>> void merge(ADTListInterface<T> listl,ADTListInterface<T>
     listr,ADTListInterface<T> list){
     while(!listl.isEmpty() && !listr.isEmpty()){
          if(listl.get(0).compareTo(listr.get(0))<= 0){//select smaller element and put back in original list
               list.remove(listl.get(0));
               list.add(listl.get(0));
               listl.remove(0);
          else{
               list.remove(listr.get(0));
               list.add(listr.get(0));
               listr.remove(0);
     //if anything is left in remaining lists put elements into original list
     while(!listl.isEmpty()){
               list.remove(listl.get(0));
          list.add(listl.get(0));
          listl.remove(0);
     while(!listr.isEmpty()){
               list.remove(listr.get(0));
          list.add(listr.get(0));
          listr.remove(0);
}

i've been testing with random stringsIt would be better to test on specific things, eg from simple to complex. For example, test it with:
-an empty list
-"a"
-"a","b"
-"b", "a"
-"c","b","a"
continue until your sorting fails, then find out why it fails there and not on the previous test.

Recursive merge sort, how does it work?

hey friends
I am stuck up with the code of recursive merge sort and am unable to understand as to how is the recursion working. Im posting the code for the merge sort and telling the details of where im facing the problem, if someone could explain me with an example as to where does the control go.
im not posting the working code but just the part where im facing confusion.
void mergesort(int[] a,int[] tmpa,int left,int right)
if(left<right)
int center=(left+right)/2;
mergesort(a,tmpa,left,center);//1
mergesort(a,tmpa,center+1,right);//2
merge(a,tmpa,left,center+1,right);
}plz explain as to how is the control being transferred in the two recursive calls labelled 1 and 2 in the above code. take for eg a series
4,2,5,7,1,8 to be sorted
thanks.

i am having trouble figuring out these two sttements
in the code
mergesort(a,tmpa,left,center);//1
mergesort(a,tmpa,center+1,right);//2
ie the recursive calls. when will the //2 be
executed? After //1.
. what will be the values of center+1,right
at which it starts executing ?Print them out or use a debugger to see for yourself. Better still, try to work through it by hand first and then use the print statements or debugger to see if you were right.
basically im having trouble with the recursive
calling. i have had read the tutorials but those are
for simple cases like factorial etc. if someone can
explain this in detailIt's the same priniciple. You start with the big piece, make a recursive call an a part of it, make a recursive call on a part of that, and so on. Ultimately, you get down to the smallest piece--the stopping condition--and you do the simple operation on that the results of that feed back up to the previous call, and its results feed back to the one before that, and so on until the second call's results feed back to where you called it on the whole thing.
It's really probably much easier to observe than to try to understand from an explanation though.

External Multi Way Merge Sort in Java!

Hi guys,
some month ago I've sent [this post|http://forums.sun.com/thread.jspa?forumID=31&threadID=5310310] to the Java Programming Forum to notify the availability of an External Multi Way Merge Sort written in Java, tuned and tested over GB of text data and under LGPL license, that can help you in dealing with huge logs or CVS files.
I want here just to notify you about this implementation.
You can run the sorting algo via command line or you can call from any Java class instantiating the ExternalSort class, setting its properties, and calling run().
Searching in the forum I saw many people that ask for an help in sorting huge textual files that doesn't fit in memory. My post solve this problem, but I think that very few people are aware of it :( ....
Hope to help who need this feature. See you!

Nice work this SmallText,
did not try it, but like the idea
I have been doing something similar in c (long, long time ago) and what made our compression really tight for typical csv files was decision to build one huffman coder for each field. The trick was to include field separator symbol into static huffman coder for every field, that provides you with End Of Field signal where to switch to next decoder.
for ExternalSort, I would suggest to make it multithreaded. Having so many many cores and low prices of ram today, it makes a lot sense. Easy way to do it is to have IO_Thread(s) and Worker_Threads (the first ones do read and write and second ones are for sorting).... this is powerfull as you utilize many cores and can easily extend some processing (like compression, token frequency counting or whatever...) via callbacks in each phase (chunking/sorting/merging)
again, congrats, really nice work

A problem on merge sort

This merge sort test can not run as expected.
Can any one check it for me? Thank you
Source code attached
//MergeSort.java
//application to show the mergesort
public class MergeSort{
     public static void main(String [] args){
          if(args.length == 0){
               System.err.println("Format: java mergeSort [number1] [number2]...");
               return;
          int [] sortList = new int[args.length];
          try{
               for(int i = 0 ; i < args.length ; i ++){
                    sortList[i] = Integer.parseInt(args);
          catch(NumberFormatException ex){
               System.err.println("Input format error, only integer numbers before error been sorted");
          mergeSort(sortList, 0, sortList.length-1);
          System.err.print("Sorted list : ");
          for(int i = 0 ; i < sortList.length ; i ++)
               System.err.print(sortList[i] + " ");
     public static void mergeSort(int [] sortList, int first, int last){
          if(first < last){
               int mid = (first + last) / 2;
               mergeSort(sortList, first, mid);
               mergeSort(sortList, mid + 1, last);
               merge(sortList, first, mid, last);
     private static void merge(int [] sortList, int first, int mid, int last){
          int indexLeft = first;
          int indexRight = mid + 1;
          int indexTemp = 0;
          boolean isEmpty = false;
          int [] tempList = new int[sortList.length];
          while(!isEmpty){
               if(sortList[indexLeft] < sortList[indexRight]){
                    tempList[indexTemp] = sortList[indexLeft];
                    indexLeft++;
               else{
                    tempList[indexTemp] = sortList[indexRight];
                    indexRight++;
               indexTemp ++ ;
               if(indexLeft > mid||indexRight > last)
                    isEmpty = true;
          while(indexLeft <= mid){
               tempList[indexTemp] = sortList[indexLeft];
               indexTemp++;
               indexLeft++;
          while(indexRight <= last){
               tempList[indexTemp] = sortList[indexRight];
               indexTemp++;
               indexRight++;
          for(int i = first ; i <= last ; i++)
               sortList[i] = tempList[i];

Use code tags.
Does this look like a debug for free forum to you?
What isn't working? A compile error? I see a syntax error in your code.
What is the input, what is the output?

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}Edited by: geforce2000 on Dec 13, 2009 3:37 AM
Edited by: geforce2000 on Dec 13, 2009 3:38 AM

BeForthrightWhenCrossPostingToOtherSites
Here's one: [custom-renderer-almost-works|http://www.java-forums.org/awt-swing/23831-custom-renderer-almost-works.html]
Any others?
Next issue: TellTheDetails
You may understand fully just what is not working here, but we don't since you tell us that it doesn't work, but never really tell us how.

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public class ArraySort {
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Edited by: 799505 on Oct 1, 2010 6:34 PM
Edited by: 799505 on Oct 1, 2010 7:15 PM
Edited by: 799505 on Oct 1, 2010 7:39 PM

799505 wrote:
I am trying to implement merge sort in an iterative manner. I keep receiving an index out of bounds error, but I cannot tell what is causing it. Can anyone help point out where this is happening?Your computer already did point that out. You got a stack trace after that error message, right? The stack trace tells you what line of code threw the exception. Without that information it is extremely difficult to find that out, but with that information it's trivial. So look at the stack trace. If you can't figure out what it's telling you then post it here.

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