Question

How to find a largest sub-array

Asked by: banglapride

Hi I am trying write a function that will find the largest sub-array which also appears in reverse order somewhere in the array. Howevr I have to write the algorithm  first, I mean I have to find out the way how to solve this problem. If anybody can give me some idea or provide pseudocode, that would be great.

For example:

findMaxReverse ({4,1,2,3,0,7,3,2,1}) returns 3 (the largest sub-array is {1,2,3})
findMaxReverse ({8,6,1,9,8,1,6}) returns 2 (the largest sub-array is {6,1})

Thank you.

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Asked On
2007-12-10 at 18:19:13ID23014642
Tags

find

,

array

,

how

,

largest

Topics

C++ Programming Language

,

Java Programming Language

,

Algorithms

Participating Experts
5
Points
256
Comments
9

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Answers

 

by: UrosVidojevicPosted on 2007-12-10 at 18:22:36ID: 20446884

Is O(n^3) ok?

 

by: cmalakarPosted on 2007-12-10 at 20:14:32ID: 20447259

In java....

public class Test
{
        public static void main(String args[])
        {
          int values[] = {5,1,0,2,3,3,2,0,1};
          int result = 0; //Length of SubArray
          int indexOfSubArray = -1; //Finally Contains the index in the array, where the subArray starts
          
          if(values == null || values.length == 0)
            result = 0;
          else if(values.length == 1)
          {
            result = 1;
            indexOfSubArray = 0;
           
          }
          else
          {
            for(int i = 0; i < values.length-1; i++)
            {
              for( int j = i+1; j < values.length; j++)
              {
                if( values[i] == values[j])
                {
                  int subArrSize = getSubArrSize(values, i, j);
                  if(result < subArrSize)
                  {
                    result = subArrSize;
                    indexOfSubArray = i;
                  }
                  
                }
              }
            }
          }
          System.out.println("Length of Sub Array: " + result);
          System.out.println("Index Of Sub Array: " + indexOfSubArray);
        }
        
        public static int getSubArrSize(int values[], int i, int j)
        {
          int size = 0;
          while (i <= j)
          {
            if(values[i] == values[j])
                 size++;
            else
              return size;
            i++;
            j--;
          }
          return size;
        }
}
                                              
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by: cmalakarPosted on 2007-12-10 at 20:20:31ID: 20447279

There is a minor problem in the above code...

if the length of values array is > 1 and result is 0 finally, then the result should be assumed as 1.

 

by: yuy2002Posted on 2007-12-10 at 21:49:35ID: 20447548

Assuming that the subarray  and reversed subarray don't contain common elements.
like this:  { 1 2 3 2 1 }      the largest subarray is 1 2 ,  not 1 2 3.
pseudocode:
int *g_subArray=NULL; // subarray point
int g_subSize=0; // subarray size
void getSubArray( int array[], int size,int subSize = size/2)
{
       if(subSize ==1)
              return;
       if(compareArray(array,size,subSize))
       {
            return;
       }
       getSubArray(array , size ,--subSize );
}
 int* compareArray(int array[],int size, int subSize)
 {
     int i = 0;
     int j = 0;
     int *temparray;
     while( i<size-subSize-1) //loop: get subarray in sequence
     {
        temparray = reverseArray(array, subSize); //revertArray: reverse array
       
      if(findSubArray(array,temparray))//findSubArray: find subarray in array
      {
          g_subArray = array+4*j;
          g_subSize = subSize;;
          return g_subArray;
      }              
        i++;
     }
     return NULL;
 }
int* compareArray

 

by: ozoPosted on 2007-12-10 at 21:52:15ID: 20447554

 

by: ozoPosted on 2007-12-10 at 21:55:59ID: 20447565

 

by: itsmeandnobodyelsePosted on 2007-12-11 at 00:07:06ID: 20447896

0. Exit if the array is empty
1. Define variables for
       - first pos of longest subarray til now (p1lsa init with 0)
       - last pos of longest subarray til now (p2lsa init with 0)
       - first pos of current subarray (p1cur init with 0)
       - current pos in array (curpos == loop counter)
2. Iterate thru array in a loop beginning with 1, incrementing the loop counter
3. For each iteration
         - check whether the current digit is next in sequence to the previous digit
         - if no, check whether (curpos - 1 - p1cur) >  (p2lsa - p1lsa)
                 - if yes, assign p1cur to p1lsa and curpos-1 to p2lsa
                 - in any case assign curpos to p1cur
4. After loop, check whether (curpos -1 - p1cur) >  (p2lsa - p1lsa)  
          - if yes, assign p1cur to p1lsa and curpos-1 to p2lsa

The longest subarray is then determined by p1lsa and p2lsa

Regards, Alex

         

 

by: banglapridePosted on 2007-12-11 at 15:00:17ID: 20453714

thank s lot all of you, currently I am trying to write  this code based on the algorithm provided by itsmeandnobodyelse. Then I will compare all those. I really love to solve this kind of problem, I have another one almost same kind, I will post that one too. Its really great to compare other view and thoughts and compare with mine. I am a new member in this group and I hope I will enjoy this.

Thank you again.

20120131-EE-VQP-002

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