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Posted on 2003-12-01
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Last Modified: 2008-02-01
Hello,

I'm trying to test the insertionSort, quickSort, and mergeSort but I'm getting compilation errors on parts of my program.  Below are the two methods (I think...) that are giving the errors:
-----------------------------------------------------------

private static void choosePivot(Comparable[] theArray, int first, int last) {

      int pivotIndex;
      int mid = (first + last)/2;
            if (theArray[first] <= theArray[mid]) {
                  if (theArray[mid] <= theArray[last]) {
                       pivotIndex = mid;
                  }
                  else {
                       pivotIndex = (theArray[first] <= theArray[last] ? last:first);
                  }
            }
            else if(theArray[last] <= theArray[mid]) {
                 pivotIndex = mid;
            }
            else {
                 pivotIndex = (theArray[first] <= theArray[last] ? first:last);
                 swap(theArray[first], theArray[pivotIndex]);
            }

}  // end choosePivot

//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//*//

 public void swap(int left, int right) {

         Comparable temp = theArray[left];
         theArray[left] = theArray[right];
         theArray[right] = temp;

 } // end swap()

-----------------------------------------------------

Can someone also explain what 'Comparable' is and what it does... because I get the following compilation errors:

operator <= cannot be applied to java.lang.Comparable,java.lang.Comparable  
                        - and -
swap(int,int) cannot be applied to (java.lang.Comparable,java.lang.Comparable)

----------------------------------------------------

Also, the compiler's complaing that it can't find 'theArray' in my swap method:

cannot resolve symbol: Comparable temp = theArray[left];
                                                                     ^

----------------------------------------------------

If you need the rest of my program, please let me know.  Thanks :)
0
Comment
Question by:luna621
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12 Comments
 
LVL 92

Expert Comment

by:objects
ID: 9856193
Comparable is an interface that defines how to compare objects.
It includes one method:
int compareTo(Object o)
See the javadoc for more details.

in your case to use it, instead of:
 if (theArray[first] <= theArray[mid]) {
you would do:
 if (theArray[first].compareTo(theArray[mid])<=0) {
0
 
LVL 92

Expert Comment

by:objects
ID: 9856202
> cannot resolve symbol: Comparable temp = theArray[left];

You need to pass theArray to your swap() method.

public void swap(int[] theArray, int left, int right) {
0
 

Author Comment

by:luna621
ID: 9856767
Yay!  It worked for the swap method, but how do I ".compareTo" for these?  Someone told me to write the ? : form, but I don't understand what that is... can you please explain?  
------------------------------------------------------------------------------------
            else {
            pivotIndex = (theArray[first] <= theArray[last] ? last:first);
            }
      }
      else if(theArray[last].compareTo(theArray[mid])<=0) {
            pivotIndex = mid;
      }
      else {
            pivotIndex = (theArray[first] <= theArray[last] ? first:last);
            swap(theArray[first], theArray[pivotIndex]);
      }

-----------------------------------------------------------------------------------

I also get this error:

swap(java.lang.Comparable[],int,int) cannot be applied to (java.lang.Comparable,java.lang.Comparable)

swap(theArray[first], theArray[pivotIndex]);

What does that mean?  Thank you!
0
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LVL 92

Expert Comment

by:objects
ID: 9856815
> pivotIndex = (theArray[first] <= theArray[last] ? last:first);

pivotIndex = (theArray[first].compareTo(theArray[last])<=0 ? last : first);

> swap(java.lang.Comparable[],int,int) cannot be applied to (java.lang.Comparable,java.lang.Comparable)

you need to pass the array as you have now added it as a param
you also should be passing the index not the array value

swap(theArray, first pivotIndex);
0
 

Author Comment

by:luna621
ID: 9864738
Yay!  It worked!  Okay, but now I'm testing the program.  Is there a way I can have random integers and Strings sorted through my program?  Is there a way to time how long it takes?

Here's my main method so far...
---------------------------------------

      public static void main(String args[]){

            long start = System.currentTimeMillis();
            Date startTime = new Date(start);

            System.out.println(startTime.toString());

//-------------------------------------------
// start testing... need to input random
// numbers and integers here.
//-------------------------------------------



            long stop = System.currentTimeMillis();
            Date stopTime = new Date(stop);

            System.out.println(stopTime.toString());


//-------------------------------------------
// find out how much time spent...
//-------------------------------------------

            Date totalTime = (stopTime - startTime);

      } // end main()

-------------------------------------------------

I seem to have a problem with this line:  Date totalTime = (stopTime - startTime);

It's giving me this error:  operator - cannot be applied to java.util.Date,java.util.Date

-------------------------------------------------

I believe this is the compacted version of something...

pivotIndex = (theArray[first] <= theArray[last] ? last:first);

Is there another way to write the above (without the question mark, etc.)?

-------------------------------------------------

Sorry for asking so many questions.  This is like a 3-in-1 question!  To be fair, I increased the points :)

-luna621 =^_^=

0
 
LVL 92

Accepted Solution

by:
objects earned 350 total points
ID: 9864764
> Date totalTime = (stopTime - startTime);

startTime and stopTime are longs, it should be:

long totalTime = (stopTime - startTime);

> pivotIndex = (theArray[first] <= theArray[last] ? last:first);
> Is there another way to write the above (without the question mark, etc.)?

Its called a ternary operator: <condition> ? <value> : <value>
It is equivalent to:

if (theArray[first] <= theArray[last])
{
   pivotIndex = last;
}
else
{
   pivotIndex = first;
}
0
 

Author Comment

by:luna621
ID: 9871890
Is there a way for me to have RANDOM integers read into the different sort methods?  Can I do the same for Strings?  Do I need to import anything (i.e. import java.util.*;)?
0
 
LVL 92

Expert Comment

by:objects
ID: 9871932
java.util.Random can be used for generating Random numbers.
0
 

Author Comment

by:luna621
ID: 9872031
I want to do something like this in my main to compare the execution times of each sorting method:

- Timings of insertionSort, quickSort, & mergeSort -

 Number of (random) elements: #####

insertionSort execution time in millisecs: #####
     quickSort execution time in millisecs: #####
   mergeSort execution time in millisecs: #####

----------------------------------

How would I make it sort the random ints & Strings?  My insertionSort method is:

public static void insertionSort(Comparable[] theArray, int n) {
...
}

Do I write it like this: insertionSort(???, Math.random());

I don't know what to put for the ??? part.  Because the method calls for a Comparable[] theArray... am I suppose to make an array with random ints?  How do I make that Comparable?
0
 
LVL 92

Expert Comment

by:objects
ID: 9872053
You create your array to sort and then fill it with random numbers using Random.

arr = new Comparable[10];
for (int i=0; i<arr.length; i++)
{
   arr[i] = new Integer(myrandom.nextInt());
}
insertionSort(arr, arr.length);
0
 

Author Comment

by:luna621
ID: 9872696
I think I got it!  Thank you very much!

-luna621 =^_^=
0
 
LVL 92

Expert Comment

by:objects
ID: 9872722
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