周报第二周

周报第二周

12.15(第八天)

矩阵相乘

1 三重循环是多数程序的极限

2 非法输入检查是程序正常运行的基本保障. 如果检查所有的非法输入, 会导致大量代码行, 这在商业代码中是必须的.

3.求解矩阵相乘常用两个办法，蛮力法和分治法，以下使用的主要是蛮力法解决低阶矩阵，分治法较高深，用来解决高阶矩阵，有兴趣可移步(48条消息) Java实现矩阵相乘问题_南 墙-CSDN博客_java矩阵乘法

package basic;
import java.util.Arrays;
public class MatrixMultiplication {
	public static void main(String args[]) {
		matrixMultiplicationTest();
	}
//	实现矩阵相乘的前提条件是前面矩阵的一行与后面矩阵的一列相等
	public static int[][] multiplication(int[][] paraFirstMatrix, int[][] paraSecondMatrix) {
		int m = paraFirstMatrix.length;
		int n = paraFirstMatrix[0].length;
		int p = paraSecondMatrix[0].length;

//   检查后面矩阵一列元素所含元素是否与前面矩阵的相等
		if (paraSecondMatrix.length != n) {
			System.out.println("The two matrices cannot be multiplied.");
			return null;
		}
//   结果组成的矩阵就是一个m*p阶方阵，在这里实现矩阵相乘
		int[][] resultMatrix = new int[m][p];
		for (int i = 0; i < m; i++) {
			for (int j = 0; j < p; j++) {
				for (int k = 0; k < n; k++) {
					resultMatrix[i][j] += paraFirstMatrix[i][k] * paraSecondMatrix[k][j];
				}//蛮力法解决解决低阶矩阵较轻松
			}
		}

		return resultMatrix;
	}
//  初始化前两个矩阵，并调用multiplication(int[][] paraFirstMatrix, int[][] paraSecondMatrix)方法
//	实现矩阵相乘
	public static void matrixMultiplicationTest() {
		int[][] tempFirstMatrix = new int[2][3];
		for (int i = 0; i < tempFirstMatrix.length; i++) {
			for (int j = 0; j < tempFirstMatrix[0].length; j++) {
				tempFirstMatrix[i][j] = i + j;
			}
		}
		System.out.println("The first matrix is: rn" + Arrays.deepToString(tempFirstMatrix));

		int[][] tempSecondMatrix = new int[3][2];
		for (int i = 0; i < tempSecondMatrix.length; i++) {
			for (int j = 0; j < tempSecondMatrix[0].length; j++) {
				tempSecondMatrix[i][j] = i * 10 + j;
			}
		}
		System.out.println("The second matrix is: rn" + Arrays.deepToString(tempSecondMatrix));

		int[][] tempThirdMatrix = multiplication(tempFirstMatrix, tempSecondMatrix);
		System.out.println("The third matrix is: rn" + Arrays.deepToString(tempThirdMatrix));
//		第一个矩阵乘以自己本身
		System.out.println("Trying to multiply the first matrix with itself.rn");
		tempThirdMatrix = multiplication(tempFirstMatrix, tempFirstMatrix);
		System.out.println("The result matrix is: rn" + Arrays.deepToString(tempThirdMatrix));
	}

}

运行结果

The first matrix is: 
[[0, 1, 2], [1, 2, 3]]
The second matrix is: 
[[0, 1], [10, 11], [20, 21]]
The third matrix is: 
[[50, 53], [80, 86]]
Trying to multiply the first matrix with itself.

The two matrices cannot be multiplied.
The result matrix is: 
null

今日单词

while语句

1 while 语句本质上比 for 更基础, 因此可以替代后者. 但 for 在很多时候更方便.

2 break 语句又出现了, 上次是在 switch 语句里. 都是表示跳出当前代码块.

这是使用while语句找出的不超过设置值的最大值的例子

package basic;
public class WhileStatement {
	public static void main(String args[]) {
		whileStatementTest();
	}
//  找出没有超过设置值的最大值
	public static void whileStatementTest() {
		int tempMax = 100;
		int tempValue = 0;
		int tempSum = 0;

//      方法一
		while (tempSum <= tempMax) {
			tempValue++;
			tempSum += tempValue;
			System.out.println("tempValue = " + tempValue + ", tempSum = " + tempSum);
		}
		tempSum -= tempValue;

		System.out.println("The sum not exceeding " + tempMax + " is: " + tempSum);

//		方法二
		System.out.println("rnAlternative approach.");
//		将值清零
		tempValue = 0;
		tempSum = 0;
		while (true) {
			tempValue++;
			tempSum += tempValue;
			System.out.println("tempValue = " + tempValue + ", tempSum = " + tempSum);

			if (tempMax < tempSum) {
				break;
			} 
		}
		tempSum -= tempValue;
		System.out.println("The sum not exceeding " + tempMax + " is: " + tempSum);
	}
}

运行结果

tempValue = 1, tempSum = 1
tempValue = 2, tempSum = 3
tempValue = 3, tempSum = 6
tempValue = 4, tempSum = 10
tempValue = 5, tempSum = 15
tempValue = 6, tempSum = 21
tempValue = 7, tempSum = 28
tempValue = 8, tempSum = 36
tempValue = 9, tempSum = 45
tempValue = 10, tempSum = 55
tempValue = 11, tempSum = 66
tempValue = 12, tempSum = 78
tempValue = 13, tempSum = 91
tempValue = 14, tempSum = 105
The sum not exceeding 100 is: 91

Alternative approach.
tempValue = 1, tempSum = 1
tempValue = 2, tempSum = 3
tempValue = 3, tempSum = 6
tempValue = 4, tempSum = 10
tempValue = 5, tempSum = 15
tempValue = 6, tempSum = 21
tempValue = 7, tempSum = 28
tempValue = 8, tempSum = 36
tempValue = 9, tempSum = 45
tempValue = 10, tempSum = 55
tempValue = 11, tempSum = 66
tempValue = 12, tempSum = 78
tempValue = 13, tempSum = 91
tempValue = 14, tempSum = 105
The sum not exceeding 100 is: 91