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Thread: Fibonacci with matrixes and recursive powers

  1. #1
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    Default Fibonacci with matrixes and recursive powers

    m[][]=AlgoritmiMatrici.identita(2);-->it generates the 2x2 identity matrix
    AlgoritmiMatrici.prodotto(m,m);-->is the result of the multiplication between m and m

    This code should calculate the umteenth number of Fabionacci series, but it alwyas returns 1.

    Can anyone help me?

    public static int matrice1(int n)
      {
      	int m[][]=AlgoritmiMatrici.identita(2);
     
      	potenzamatrice(m, n-1);
     
      	return m[0][0];
      }
      public static void potenzamatrice(int[][] m, int n)
      {
        	if(n>1)
    	{
    		potenzamatrice(m, n/2);
    		m=AlgoritmiMatrici.prodotto(m,m);
    	}	
    	if(n%2==1)
    	{
    		int a[][]={{1,1},{1,0}};
    		m=AlgoritmiMatrici.prodotto(m,a);
    	}
     }


  2. #2
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    Default Re: Fibonacci with matrixes and recursive powers

    Hey Enrico,

    Can you please post what your prodotto method does? Seems like your multi dimensional integer array is getting filled after the call to prodotto method i.e AlgoritmiMatrici.prodotto(m,m);.

    From the code, seems like what you are returning is the 1st element of your 1st array [return m[0][0];]. So may be there is a possibility that your prodotto method is populating the values in such a way that it always happens to be ''1" at the first element.

    I hope you are getting me.

    Goldest
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    Default Re: Fibonacci with matrixes and recursive powers

    Here you are.
    It returns a matrix which is the multiplication between two matrixes

    public static int [][] prodotto(int t1[][],int t2[][]) {
    	//it is possible only if t1[0].length = t2.length
    	int n = t1.length;
    	int m = t2[0].length;
    	int prod[][] = new int [n][m];
    	for (int i=0 ; i<n; i++)
    		for (int k=0; k<m ; k++){
    			int s=0;			
    			for (int r=0;r<t1[0].length;r++)
    				s=s+t1[i][r]*t2[r][k];
    			prod[i][k]=s;
     
    		}//end for
     
    	 return prod;
    	}//fine prodotto
    Last edited by Enrico123; December 27th, 2010 at 01:52 PM.

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    Default Re: Fibonacci with matrixes and recursive powers

    The array passed to one of the functions is being recreated and the caller expects the change to take effect on the passed object, which does not happen in java. Take the following as an example:
    	public static void main(String[] args){
    		int[] m = new int[10];
    		for ( int i = 0; i < 10; i++ ){
    			m[i] = -1;
    		}
     
    		test(m);
    		System.out.println(m[0]);
    	}
     
    	public static void test(int[] m){
    		int v[] = new int[10];
    		for ( int i = 0; i < m.length; i++ ){
    			v[i] = 1;
    		}
    		m = v;
    	}
    Will print -1

  5. #5
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    Default Re: Fibonacci with matrixes and recursive powers

    Ok.
    I tried to return the value of the matrix after the product, but the result is always the same...

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