Java学习-Day5

while 语句

一、while 和 for

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.

for 循环方便是因为控制循环的语句在开始就已经声明，而 while 循环需要在循环内部更新数据或使用 if-else 语句进行判断后使用 break 跳出当前循环体.

break 语句又出现了, 上次是在 switch 语句里. 都是表示跳出当前循环体. 当然 break 对 for 循环也同样适用.

二、示例

示例描述

给定一个上界，从1开始求和，取和小于上界的最大值.

输入

一个正整数表示的上界

输出

小于上界和的最大值

具体代码

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package test;

/**
 * This is the ninth code. Names and comments should follow my style strictly.
 * 
 * @author Shihuai Wen wshysxcc@outlook.com.
 */
public class WhileStatement {

	/**
	 *********************
	 * The entrance of the program.
	 * 
	 * @param args Not used now.
	 *********************
	 */
	public static void main(String args[]) {
		whileStatementTest();
	}// Of main

	/**
	 *********************
	 * The sum not exceeding a given value.
	 *********************
	 */
	public static void whileStatementTest() {
		int tempMax = 100;
		int tempValue = 0;
		int tempSum = 0;

		// Approach 1.
		while (tempSum <= tempMax) {
			tempValue++;
			tempSum += tempValue;
			System.out.println("tempValue = " + tempValue + ", tempSum = " + tempSum);
		} // Of while
		tempSum -= tempValue;

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

		// Approach 2.
		System.out.println("\r\nAlternative approach.");
		tempValue = 0;
		tempSum = 0;
		while (true) {
			tempValue++;
			tempSum += tempValue;
			System.out.println("tempValue = " + tempValue + ", tempSum = " + tempSum);

			if (tempMax < tempSum) {
				break;
			} // Of if
		} // Of while
		tempSum -= tempValue;

		System.out.println("The sum not exceeding " + tempMax + " is: " + tempSum);
	}// Of whileStatementTest
}// Of class WhileStatement

运行截图

综合任务 1

一、任务描述

找出成绩最好、最差的同学。但有挂科的同学不参加评比.

二、任务要求

数据要求

进行学生成绩的随机生成, 区间为 [50, 100].

数据存放

学生的成绩存放于一个矩阵，其中行表示学生，列表示科目。如：第 0 行表示第 0 个学生的数学、语文、英语成绩。如下所示. 

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    数学 语文 英语 ...
    0    1   2
0   79   83  91
1  ...   ... ...
2  ...   ... ...
3  ...   ... ...
...

三、实现

输入

使用随机生成的在区间 [50, 100]的整数填充的矩阵

输出

矩阵各行和的值最小和最大的行编号（从0开始）

具体代码

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package test;

import java.util.Arrays;
import java.util.Random;

/**
 * This is the tenth code, also the first task.
 * 
 * @author Shihuai Wen wshysxcc@outlook.com.
 */
public class Task1 {

	/**
	 *********************
	 * The entrance of the program.
	 * 
	 * @param args Not used now.
	 *********************
	 */
	public static void main(String args[]) {
		task1();
	}// Of main

	/**
	 *********************
	 * Method unit test.
	 *********************
	 */
	public static void task1() {
		// Step 1. Generate the data with n students and m courses.
		// Set these values by yourself.
		int n = 10;
		int m = 3;
		int lowerBound = 50;
		int upperBound = 100;
		int threshold = 60;

		// Here we have to use an object to generate random numbers.
		Random tempRandom = new Random();
		int[][] data = new int[n][m];
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < m; j++) {
				data[i][j] = lowerBound + tempRandom.nextInt(upperBound - lowerBound);
			} // Of for j
		} // Of for i

		System.out.println("The data is:\r\n" + Arrays.deepToString(data));

		// Step 2. Compute the total score of each student.
		int[] totalScores = new int[n];
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < m; j++) {
				if (data[i][j] < threshold) {
					totalScores[i] = 0;
					break;
				} // Of if

				totalScores[i] += data[i][j];
			} // Of for j
		} // Of for i

		System.out.println("The total scores are:\r\n" + Arrays.toString(totalScores));

		// Step 3. Find the best and worst student.
		// Typical initialization for index: invalid value.
		int tempBestIndex = -1;
		int tempWorstIndex = -1;
		// Typical initialization for best and worst values.
		// They must be replaced by valid values.
		int tempBestScore = 0;
		int tempWorstScore = m * upperBound + 1;
		for (int i = 0; i < n; i++) {
			// Do not consider failed students.
			if (totalScores[i] == 0) {
				continue;
			} // Of if

			if (tempBestScore < totalScores[i]) {
				tempBestScore = totalScores[i];
				tempBestIndex = i;
			} // Of if

			// Attention: This if statement cannot be combined with the last one
			// using "else if", because a student can be both the best and the worst. 
			if (tempWorstScore > totalScores[i]) {
				tempWorstScore = totalScores[i];
				tempWorstIndex = i;
			} // Of if
		} // Of for i

		// Step 4. Output the student number and score.
		if (tempBestIndex == -1) {
			System.out.println("Cannot find best student. All students have failed.");
		} else {
			System.out.println("The best student is No." + tempBestIndex + " with scores: "
					+ Arrays.toString(data[tempBestIndex]));
		} // Of if

		if (tempWorstIndex == -1) {
			System.out.println("Cannot find worst student. All students have failed.");
		} else {
			System.out.println("The worst student is No." + tempWorstIndex + " with scores: "
					+ Arrays.toString(data[tempWorstIndex]));
		} // Of if
	}// Of task1

}// Of class Task1

运行截图

常见问题

实际代码中，for 和 if 是最常见的, switch 和 while 使用少得多.

使用了 continue, 它是指继续跳过本次循环后面的代码，直接进入下一次循环. 而 break 是跳出整个循环体.

if-else 应该保持对同一个表达式或变量的值进行判断。当有多变量的时候尽量多写几个if-else 保持代码可读性，毕竟人脑能考虑到的情况是有局限性的。

总结

结合了之前所有的知识点。相比起枯燥乏味的示例，实际解决一个问题更加能够提升自己的能力，尤其是在代码出现问题时候进行的Debug操作。

最后要相信人脑的局限性，要像计算机一样思考才能写出更好的代码。