Fig10_46.java

public class Fig10_46
{
    public static final long INFINITY = Long.MAX_VALUE;

    /**
     * Compute optimal ordering of matrix multiplication.
     * c contains the number of columns for each of the n matrices.
     * c[ 0 ] is the number of rows in matrix 1.
     * The minimum number of multiplications is left in m[ 1 ][ n ].
     * Actual ordering is computed via another procedure using lastChange.
     * m and lastChange are indexed starting at 1, instead of 0.
     * Note: Entries below main diagonals of m and lastChange
     * are meaningless and uninitialized.
     */
    public static void optMatrix( int [ ] c, long [ ][ ] m, int [ ][ ] lastChange ) 
    {
        int n = c.length - 1;

        for( int left = 1; left <= n; left++ )
            m[ left ][ left ] = 0;
        for( int k = 1; k < n; k++ )   // k is right - left
            for( int left = 1; left <= n - k; left++ )
            {
                // For each position
                int right = left + k;
                m[ left ][ right ] = INFINITY;
                for( int i = left; i < right; i++ )
                {
                    long thisCost = m[ left ][  i ] + m[ i + 1 ][ right ]
                         + c[ left - 1 ] * c[ i ] * c[ right ];
                    if( thisCost < m[ left ][ right ] )  // Update min
                    {
                        m[ left ][ right ] = thisCost;
                        lastChange[ left ][ right ] = i;
                    }
                }
            }
    }

    public static void main( String [ ] args )
    {
        int [ ] c = { 50, 10, 40, 30, 5 };
        long [ ][ ] m = new long [ 5 ][ 5 ];
        int lastChange[ ][ ] = new int [ 5 ][ 5 ];

        optMatrix( c, m, lastChange );
        for( int i = 1; i <= 4; i++ )
        {
            for( int j = 1; j <= 4; j++ )
                System.out.print( m[ i ][ j ] + "    " );
            System.out.println( );
        }
        for( int i = 1; i <= 4; i++ )
        {
            for( int j = 1; j <= 4; j++ )
                System.out.print( lastChange[ i ][ j ] + "    " );
            System.out.println( );
        }
    }
}