Myujth

So I help tutor an Algebra 2 class at my local high school, and the class is currently looking over matrices. Though not there, they will eventually get to multiplication of matrices. After taking Computer Science last year and learning Java, the teacher I help thought I should try to write a program to multiple matrices.

At the moment, I have up to defining the numbers for the first array that holds the information for the first matrix. However, I have a small issue. As represented by this picture:

the line asking for the index integers is being repeated after already recording the integers. I assume this is due to my layered for loops, but I can't be for certain. Usually new eyes see problems clearer. Any who could help me would be appreciated.

Code:

package matrixmultiplication;
import java.util.*;

public class MatrixMultiplication 

public static void main(String args) 
 System.out.println("What is the size of the first matrix?");
 int matrix1Rows = matrixRows();
 int matrix1Columns = matrixColumns();
 int matrix1 = new int[matrix1Rows * matrix1Columns];
 doubleSpace();
 System.out.println("What is the size of the second matrix?");
 int matrix2Rows = matrixRows();
 int matrix2Columns = matrixColumns();
 int matrix2 = new int[matrix2Rows * matrix2Columns];
 doubleSpace();
 if (matrix1Columns != matrix2Rows)
 System.out.println("These cannot be multiplied!");
 System.exit(0);
 else 
 matrix1Numbers(matrix1Rows, matrix1Columns);
 


public static int matrixRows()
 System.out.print("Rows:");
 Scanner rowSc = new Scanner(System.in);
 int rows = rowSc.nextInt();
 return rows;


public static int matrixColumns()
 System.out.print("Columns:");
 Scanner colSc = new Scanner(System.in);
 int cols = colSc.nextInt();
 return cols;


public static int matrix1Numbers(int rows, int cols)
 int numb = new int[rows * cols];
 for (int j = 0; j <= numb.length; j += rows)
 for (int i = 1; i <= cols; i++)
 for (int k = 1; k <= rows; k++)
 System.out.println("What is the value for index (" + k + "," + i + ")?");
 Scanner inp = new Scanner(System.in);
 if (j + k <= numb.length)
 numb[j + k - 1] = inp.nextInt();
 
 
 
 
 for (int i = 0; i < numb.length; i++)
 System.out.println(numb[i]);
 
 return numb;


public static void doubleSpace()
 System.out.println();
 System.out.println();

I use NetBeans 8.2 and the latest working version of Java for NetBeans.

Why is this line printing twice?

Because in your loop here:

for (int k = 1; k <= rows; k++)
 System.out.println("What is the value for index (" + k + "," + i + ")?");
 Scanner inp = new Scanner(System.in);
 if (j + k <= numb.length)
 numb[j + k - 1] = inp.nextInt();
 

You only ask for input if j + k <= numb.length but you loop while k <= rows, and print every iteration. There must a value for which you are still looping but j + k > numb.length. To fix this, change so that your loop with both of the conditions:

for(int k = 1; k <= rows && j + k <= numb.length; k++)

And then remove the if part, and ask for input on every iteration.

Also you are creating a new Scanner object inside of your loop, which will create a new Scanner on every iteration. Move the deceleration outside of the loop.

I'm not familiar with the matrixmultiplication package, so I may be rambling here.

for (int j = 0; j <= numb.length; j += rows){

I'm not entirely sure what the outer for loop your have is for, but this most outer for loop causes you to ask for the values of the indices cols times more than you want.
I feel that you originally wanted to use this outer for loop to iterate through each row, and wasn't planning on having the second for loop iterating through cols perhaps?

Also, Kevin Anderson mentions this above. You might avoid this problem altogether if you return a double int array as opposed to storing all values in the matrix in a single dimension. I personally feel it would make more sense.

Just nitpicking, but I wouldn't make a new scanner every time you want to use one in a different method. You could just make a field at the top of your class, instantiate it once in your main method, and then pass it in as a parameter to all methods using the scanner.