练习
- 部分用例的用法
% javac Testxxx.java
% java Testxxx [args]
- 以下出现
Integer.parseInt(args[x])的地方均可以替换为StdIn.readxxx()，不过这样变成了从标准输入中读取数据。题中要求从命令行接受数据，所以最好避免从标准输入中进行数据的获取。
1. Point2D 用例
public class TestPoint2D {
public static void main(String[] args) {
int N = Integer.parseInt(args[0]);
Point2D[] points = new Point2D[N];
for (int i = 0; i < N; i++) {
points[i] = new Point2D(Math.random(), Math.random());
points[i].draw();
}
if (N > 1) {
double min = points[0].distanceTo(points[1]);
for (int i = 0; i < N - 1; i++) {
for (int j = i + 1; j < N; j++) {
if (points[i].distanceTo(points[j]) < min) {
min = points[i].distanceTo(points[j]);
}
}
}
System.out.println("The min distance: " + min);
}
}
}
2. Interval1D 用例
public class TestInterval1D {
public static void main(String[] args) {
int N = Integer.parseInt(args[0]);
Interval1D[] intervals = new Interval1D[N];
for (int i = 0; i < N; i++) {
intervals[i] = new Interval1D(StdIn.readDouble(), StdIn.readDouble());
}
for (int i = 0; i < N - 1; i++) {
for (int j = i + 1; j < N; j++) {
if (intervals[i].intersects(intervals[j])) {
StdOut.println(intervals[i] + " -- " + intervals[j]);
}
}
}
}
}
3. Interval2D 用例
public class TestInterval2D {
private static int intersectCount = 0;
private static int containCount = 0;
private static double max(double a, double b) {
return Math.max(a, b);
}
private static double min(double a, double b) {
return Math.min(a, b);
}
public static void main(String[] args) {
int N = Integer.parseInt(args[0]);
double min = Double.parseDouble(args[1]);
double max = Double.parseDouble(args[2]);
// 设置绘图范围（可以保证所有矩形都绘制在窗格内）
StdDraw.setXscale(min, max);
StdDraw.setYscale(min, max);
// 供判断 contains 条件的点集合
Point2D[] leftTopPoints = new Point2D[N];
Point2D[] rightBottomPoints = new Point2D[N];
Interval2D[] interval2DS = new Interval2D[N];
for (int i = 0; i < N; i++) {
double x1 = StdRandom.uniform(min, max);
double x2 = StdRandom.uniform(min, max);
double y1 = StdRandom.uniform(min, max);
double y2 = StdRandom.uniform(min, max);
leftTopPoints[i] = new Point2D(min(x1, x2), min(y1, y2));
rightBottomPoints[i] = new Point2D(max(x1, x2), max(y1, y2));
interval2DS[i] = new Interval2D(new Interval1D(min(x1, x2), max(x1, x2)), new Interval1D(min(y1, y2), max(y1, y2)));
interval2DS[i].draw();
}
for (int i = 0; i < N - 1; i++) {
for (int j = 1; j < N; j++) {
if (interval2DS[i].intersects(interval2DS[j])) {
intersectCount++;
}
if (interval2DS[i].contains(leftTopPoints[j]) && interval2DS[i].contains(rightBottomPoints[j])) {
containCount++;
}
}
}
StdOut.println("Intersect count: " + intersectCount);
StdOut.println("Contain count: " + containCount);
}
}
4. 单纯的值传递
world
hello
5. String 对象不可变
所有的字符串方法都不会改变参数对象的值，而是返回一个新的 String 对象。
6. 回环变位
- 书中提示的方法
if ((s + s).indexOf(t) >= 0) {};
- 使用了字符串拼接的方法
if (s.concat(s).contains(t)) {};
7. 递归函数的返回值
返回一个倒序的字符串。
8. 数组引用
复制的是引用，不是数据。原始数据类型才会将数据复制下来，其余复制的都是对象的引用。
9. BinarySearch 中增加 Counter
我这里又定义了一个 Counter 类，以便于对记录值进行修改。
class Counter {
private int val = 0;
public int getVal() {
return val;
}
public void add() {
val++;
}
}
public class BinarySearch {
public static int rank(int key, int[] a, Counter counter) {
return rank(key, a, 0, a.length - 1, counter);
}
public static int rank(int key, int[] a, int lo, int hi, Counter counter) {
// 如果 key 存在于 a[] 中，它的索引不会小于 lo 且不会大于 hi
if (lo > hi) return -1;
int mid = lo + (hi - lo) / 2;
if (key < a[mid]) {
counter.add();
return rank(key, a, lo, mid - 1, counter);
} else if (key > a[mid]) {
counter.add();
return rank(key, a, mid + 1, hi, counter);
} else {
counter.add();
return mid;
}
}
public static void main(String[] args) {
Counter counter = new Counter();
int[] a = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
System.out.println(rank(6, a, counter));
System.out.println(counter.getVal());
}
}
10. 类 VisualCounter
- 实现：
public class VisualCounter {
private int val;
private int N;
private int max;
// 用来记录操作次数
private int counter = 0;
public VisualCounter(int N, int max) {
this.N = N;
this.max = max;
StdDraw.setXscale(0, N);
StdDraw.setYscale(0, max);
StdDraw.setPenColor(StdDraw.RED);
StdDraw.setPenRadius(.01);
}
public void add() {
if (val < max && counter < N) val++;
StdDraw.point(counter, val);
counter++;
}
public void reduce() {
if (counter < N) val--;
StdDraw.point(counter, val);
counter++;
}
}
- 用例：
public class TestVisualCounter {
public static void main(String[] args) {
VisualCounter counter = new VisualCounter(100, 100);
for (int i = 0; i < 50; i++) {
counter.add();
}
for (int i = 0; i < 30; i++) {
counter.reduce();
}
for (int i = 0; i < 20; i++) {
counter.add();
}
}
}
11. SmartDate 类
这里定义了 年-月-日-时-分-秒 这六个变量，设置了四个构造器，提供了对变量进行操作和得到当前星期的方法。因为代码比较易懂，所以这里就不会多作说明了。书上 表1.2.12 中 Date 类的另一种实现还挺有意思的，有兴趣的同学可以看一下，另外还可以阅读 edu.princeton.cs.algs4 包中 Date 类的 源码 。
当日期信息错误时，会报异常：
java.lang.IllegalArgumentException: Illegal date message.当时间信息错误时，会报异常：java.lang.IllegalArgumentException: Illegal time message.
PS: 没有注意到书中提供的 Date 的 API 😂，所以可能同书中的要求有所不同，变化不大，就不作修改了。
- 实现：
public class SmartDate {
private int year;
private int month;
private int date;
private int hrs;
private int min;
private int sec;
public SmartDate(int year, int month, int date) {
this(year, month, date, 0, 0, 0);
}
public SmartDate(int year, int month, int date, int hrs) {
this(year, month, date, hrs, 0, 0);
}
public SmartDate(int year, int month, int date, int hrs, int min) {
this(year, month, date, hrs, min, 0);
}
public SmartDate(int year, int month, int date, int hrs, int min, int sec) {
if (year >= 0) {
switch (month) {
default:
if (hrs >= 0 && hrs <= 23) {
if (min >= 0 && min <= 59) {
if (sec >= 0 && sec <= 59) {
this.hrs = hrs;
this.min = min;
this.sec = sec;
}
}
} else {
throw new IllegalArgumentException("Illegal time message.");
}
case 1:
case 3:
case 5:
case 7:
case 8:
case 10:
case 12:
if (date >= 1 && date <= 31) {
if (hrs >= 0 && hrs <= 23) {
this.year = year;
this.month = month;
this.date = date;
}
} else {
throw new IllegalArgumentException("Illegal date message.");
}
break;
case 4:
case 6:
case 9:
case 11:
if (date >= 1 && date <= 30) {
this.year = year;
this.month = month;
this.date = date;
} else {
throw new IllegalArgumentException("Illegal date message.");
}
break;
case 2:
if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0) {
if (date >= 1 && date <= 29) {
this.year = year;
this.month = month;
this.date = date;
} else {
throw new IllegalArgumentException("Illegal date message.");
}
} else {
if (date >= 1 && date <= 28) {
this.year = year;
this.month = month;
this.date = date;
} else {
throw new IllegalArgumentException("Illegal date message.");
}
}
}
} else {
throw new IllegalArgumentException("Illegal date message.");
}
}
public int getDayOfWeek() {
return (date + 2 * month + 3 * (month + 1) / 5 + year + year / 4 - year / 100 + year / 400) % 7;
}
public int getYear() {
return year;
}
public void setYear(int year) {
this.year = year;
}
public int getMonth() {
return month;
}
public void setMonth(int month) {
this.month = month;
}
public int getDate() {
return date;
}
public void setDate(int date) {
this.date = date;
}
public int getHrs() {
return hrs;
}
public void setHrs(int hrs) {
this.hrs = hrs;
}
public int getMin() {
return min;
}
public void setMin(int min) {
this.min = min;
}
public int getSec() {
return sec;
}
public void setSec(int sec) {
this.sec = sec;
}
}
- 测试用例：
public class TestSmartDate {
public static void main(String[] args) {
SmartDate date = new SmartDate(2019, 8, 27);
System.out.println(date.getDayOfWeek());
}
}
12. 方法 dayOfTheWeek()
上面已经有了 SmartDate 类的源码，这里只给出方法的实现。
public String dayOfTheWeek() {
int index = (date + 1 + 2 * month + 3 * (month + 1) / 5 + year + year / 4 - year / 100 + year / 400) % 7;
switch (index) {
case 0: return "Sunday";
case 1: return "Monday";
case 2: return "Tuesday";
case 3: return "Wednesday";
case 4: return "Thursday";
case 5: return "Friday";
case 6: return "Saturday";
default: return null;
}
}
13. Transaction 类型
参考了一下 edu.princeton.cs.algs4 包里的 Transaction 类，发现写这个类型是为了记录交易信息，类中包含了 人-时间-交易数目 。有兴趣的同学可以阅读 edu.princeton.cs.algs4 包中 Transaction 类的 源码 。
// 刚刚才写了 SmartDate 类，那我们就使用它来代替 Date 吧！
public class Transaction {
private final String who;
private final SmartDate date;
private final double amount;
public Transaction(String name, SmartDate date, double amount) {
who = name;
this.date = date;
this.amount = amount;
}
public String who() {
return who;
}
public SmartDate date() {
return date;
}
public double amount() {
return amount;
}
public String toString() {
return who() + "/" + date() + "/" + amount();
}
}
14. Transaction 类的 equals() 方法
这个方法是完全仿照书上 Date 类中的 equals() 方法写的。
public boolean equals(Object x) {
if (this == x) return true;
if (x == null) return false;
// 如果两者的类型不同则返回 false 值
if (this.getClass() != x.getClass()) return false;
// 类型转换
Transaction that = (Transaction) x;
if (this.who != that.who) return false;
if (this.date != that.date) return false;
if (this.amount != that.amount) return false;
return true;
}
