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Java Cookbook
book

Java Cookbook

by Ian F. Darwin
June 2001
Intermediate to advanced content levelIntermediate to advanced
888 pages
21h 1m
English
O'Reilly Media, Inc.
Content preview from Java Cookbook

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 ints:

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

ISBN: 0596001703Supplemental ContentCatalog PageErrata