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Array Question - Strassens Method - Urgent

Posted on 2003-12-05
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Last Modified: 2010-03-31
Say Im implementing Strassens method for an array that is larger than 2x2(say 4x4 n=4). And say I calculated each sub array correctly (the 4 n/2 pieces) and I want to construct the new nxn matrix that holds the solution. Whats the easiest/most efficient way to reconstruct a larger array from the smaller pieces, just a for loop or is there something easier.  
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Question by:Akindo
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7 Comments
 

Author Comment

by:Akindo
ID: 9884020
Also, on the opp side of the question whats the easiest way to deconstruct the array in 4 n/2 pieces, basically the 4 quadrants...also a double for loop??
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Accepted Solution

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CEHJ earned 1000 total points
ID: 9884198
Looping, AFAIK, can't be avoided, but you can System.arraycopy where there's subarrays, so every single element need not be looped over.
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Author Comment

by:Akindo
ID: 9884446
how would it work for a two dimensional array, say I have

a=2x2
b=2x2
c=2x2
d=2x2

and I want a 4x4 array like
ab
cd
whats the most efficient way to do that?
 
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Expert Comment

by:objects
ID: 9884779
Are you actually using 2d array in your internal representation, and is it that required.
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Expert Comment

by:Tols
ID: 9885449
Maybe some tree structure coud simplify that;

class Matrix
{
     Matrix parent;
     int leafValue = null;
     Matrix[][] children = null; //could be ArrayList
     public Matrix(Matrix parent, int leafValue) //for leafs
     {
         this.parent = parent;
         this leafValue = leafValue;
     }
     public Matrix(Matrix parent) //for nodes
     {
         this.parent = parent;
     }
     public int getValue() //possible modification for other data
    {
         if (children == null) return leafValue;
         //calculate value of submatrix
         int val = children[0][0].getValue()+children[1][1].getValue() ....//recurrent calls
         return val;
    }

}



I hope it help ;-)
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Expert Comment

by:Tols
ID: 9885481
Above class can be modified fo keeping one data for each submatrixes - they shoud keep a range of data in global matrix...
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Expert Comment

by:objects
ID: 9885514
> and I want a 4x4 array like
> ab
> cd

int[][] result = new int[4][4];
for (int i=0; i<2; i++)
{
   System.arraycopy(a[i], 0, result[i], 0, 2);
   System.arraycopy(b[i], 0, result[i], 2, 2);
   System.arraycopy(c[i], 0, result[i+2], 0, 2);
   System.arraycopy(d[i], 0, result[i+2], 2, 2);
}
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