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# Multiplying Matrixes

## Problem

You need to multiply a pair of two-dimensional arrays, as is common in mathematical and engineering applications.

## Solution

Use the following code as a model.

## Discussion

It is straightforward to multiply an array of a numeric type. The code in Example 5-3 implements matrix multiplication.

Example 5-3. Matrix.java

```/**
* Multiply two matrices.
* Only defined for int: clone the code (or wait for Templates)
* for long, float, and double.
*/
public class Matrix {

/* Matrix-multiply two arrays together.
* The arrays MUST be rectangular.
* @author Tom Christiansen & Nathan Torkington, Perl Cookbook version.
*/
public static int[][] multiply(int[][] m1, int[][] m2) {
int m1rows = m1.length;
int m1cols = m1[0].length;
int m2rows = m2.length;
int m2cols = m2[0].length;
if (m1cols != m2rows)
throw new IllegalArgumentException(
int[][] result = new int[m1rows][m2cols];

// multiply
for (int i=0; i<m1rows; i++)
for (int j=0; j<m2cols; j++)
for (int k=0; k<m1cols; k++)
result[i][j] += m1[i][k] * m2[k][j];

return result;
}

public static void mprint(int[][] a) {
int rows = a.length;
int cols = a[0].length;
System.out.println("array["+rows+"]["+cols+"] = {");
for (int i=0; i<rows; i++) {
System.out.print("{");
for (int j=0; j<cols; j++)
System.out.print(" " + a[i][j] + ",");
System.out.println("},");
}
System.out.println(":;");
}
}```

Here is a program that uses the `Matrix` class to multiply two arrays of `int`s:

`// MatrixUse.java int x[][] = { { 3, 2, 3 }, { 5, 9, 8 }, }; ...`

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