Thread: Adding two matrix together

Ted,

You should test to ensure the matrices a and b are the same size. You may only add matrices that are the same size. There's a length method of arrays but I think you'll need to check length of the outer array and each of the inner arrays. Once you take car of all that, move the return c line outside the second for loop. Inside both for loops you need this line.

c[i][j] = a[i][j] + b[i][j];

Along with checking your array szies you could size the c array appropriately instead of making it a static 3 x 3.

How do I test to see if the matrices are the same size.

Try this and see if it makes sense.

public class UtilityMatrix {
 
	public static int[][] addMatrix(int[][] a, int[][] b) {
		int[][] bad = {{0}}; // a dummy to send back if the matrices are not the same size
		int max_depth = 0; // stores the max length of the sub-arrays, in case a jagged array is passed
		// ensure arrays are the same size
		if (a.length != b.length) return bad; // empty set 
		for (int i = 0; i<a.length; i++){
			if (a[i].length != b[i].length) return bad; // empty set
			if (a[i].length > max_depth) max_depth = a.length;
		}
		// end tests
		int [][] c = new int [a.length][max_depth]; // now we know how big the return array should be.
		for ( int i = 0; i<a.length; i++){
			for( int j = 0; j<a[i].length; j++){
				c[i][j] = a[i][j]+b[i][j]; // here we add the corresponding elements
			}
		}
		return c; // send back the sum array
	} 
}

Do you know how I could write a matrix multiplication method

Remember that matrix multiplication requires that the number of rows in matrix A equal the number of columns in matrix B. The rows are the first dimension of the array and the columns are the second dimension.

Since jagged arrays just plain won't work for multiplication, you can bypass that test and just confirm that the sizes match

if (a.length != b[1].length) return bad; //sizes don't match

Next is the calculation. Say the first matrix is a[3][2] and the second is b[2][3]. The result would be matrix c[3][3].
c[1][1] = a[1][1] * b[1][1] + a[1][2] * b[2][1] + a[1][3] * b[3][1]
c[1][2] = a[2][1] * b[1][1] + a[2][2] * b[2][1] + a[2][3] * b[3][1]
...

Note what is repeated (the b matrix items) and what changes each time (the a matrix items) So the "a" matrix would be incremented in the inner loop and the "b" matrix would be incremented in the outer loop.

The tricky part is referencing your arrays correctly using your loop counters. Since you know the result matrix is 3x3, it will take two loops just to address each item in the c matrix and a third loop to increment the keys of the a and b arrays for each calculation of an element in array c.

See what you can do with that.

could you help me with writing the loops to do the calculations.