Question

public class Homework2 {
    public static void main(String[] args){
        int num1 = (int) (Math.random()*(10-3+1)+3);
        int num2 = (int) (Math.random()*(10-3+1)+3);

        double[][] doubMatrix1 = new double[num1][num2];
        double[][] doubMatrix2 = new double[num1][num2];
        double[][] doubMatrix3 = new double[num1][num2];

        doubMatrix1 = getdoubMatrix(num1,num2);
        doubMatrix2 = getdoubMatrix(num1,num2);
        doubMatrix3 = addMatrices(doubMatrix1, doubMatrix2, num1, num2);        
        printDoubMatrix("First matrix", doubMatrix1);
        printDoubMatrix("Second matrix", doubMatrix2);
        printDoubMatrix("Result of adding", doubMatrix3);
        doubMatrix2 =transposeMatrix(num1,num2);



    }

    public static double[][] getdoubMatrix(int num1,int num2){

        double[][] tempArray = new double[num1][num2];
        for(int i = 0;i < tempArray.length;i++)
            for(int j = 0;j < tempArray[i].length;j++)
            {
            tempArray[i][j] = Math.random() * (100);                
            }
        return tempArray;
    }

    public static double[][] addMatrices(double[][] doubMatrix1, double[][] doubMatrix2,int num1,int num2)
    {

        double[][] tempArray = null;
        if(doubMatrix1.length == doubMatrix2.length)
            if(doubMatrix1[0].length == doubMatrix2[0].length)
            {
                 tempArray = new double[num1][num2];
                for(int i = 0; i< doubMatrix1.length;i++)
                      for(int j = 0; j< doubMatrix1[i].length;j++ )
                      {
                          tempArray[i][j] = doubMatrix1[i][j] + doubMatrix2[i][j];
                      }
            }
            else
            {
                return tempArray = new double[0][0];

        }

        return tempArray;
    }

    public static void printDoubMatrix(String text,double[][] doubMatrix1){

        System.out.println(text);
        for(int i = 0; i< doubMatrix1.length;i++)
              for(int j = 0; j< doubMatrix1[i].length;j++ )     
                  System.out.printf("%f\n", doubMatrix1[i][j]);                         
}

    public static double[][] transposeMatrix(int num1, int num2){
        double[][] tempArray = new double[num2][num1];
        for(int i = 0;i < tempArray.length;i++)
            for(int j = 0;j < tempArray[i].length;j++)
            {
                tempArray[i][j] = tempArray[j][i];
                System.out.printf("%f\n", tempArray[i][j]); 
            }    
        return tempArray;
    }

}

I have a problem when running this program, there was no error but when I run it it said the array index is out of bound, the problem is at the transpose method, can anyone tell me how to fix this problem?

Answer 1

The assignment in the for-loop of transpose method should be like: –

tempArray[i][j] = doubleMatrix2[j][i];

rather than: –

tempArray[i][j] = tempArray[j][i];

In the above code, you are assigning the value from a newly created array tempArray to itself only. It doesn’t make sense. It will not affect the array. Also it will throw an ArrayIndexOutOfBounds exception if row != col

You need to use the matrix you want to transpose.

Since you are invoking this method for doubleMatrix2

doubMatrix2 =transposeMatrix(num1,num2);

And your two matrix are like: –

tempArray[][] = new double[num2][num1];

doubleMatrix[][] = new double[num1][num2];

So it makes sense to assign doubleMatrix[j][i] to tempArray[i][j]. Because number of rows and columns are reversed in the two matrices.