Java语言程序设计(基础篇)(原书第10版) 练习题答案 第3章

基础篇课后习题答案，做了大部分，虽然不一定是最佳代码，但是保证每个都能运行，如有更好的答案，欢迎讨论

3.1 解一元二次方程

double a, b, c, r1, r2;
Scanner scanner = new Scanner(System.in);
System.out.print("请输入a：");
a = scanner.nextDouble();
System.out.print("请输入b：");
b = scanner.nextDouble();
System.out.print("请输入c：");
c = scanner.nextDouble();

if (Math.pow(b, 2) - (4 * a * c) > 0) {
    r1 = (-b + Math.pow((Math.pow(b, 2) - (4 * a * c)), 0.5)) / 2 * a;
    r2 = (-b - Math.pow((Math.pow(b, 2) - (4 * a * c)), 0.5)) / 2 * a;
    System.out.print("方程式的两个根为：" + (double) Math.round(r1 * 1000) / 1000 + "和" +
            (double) Math.round(r2 * 1000) / 1000);
} else if (Math.pow(b, 2) - (4 * a * c) == 0) {
    r1 = (-b + Math.pow((Math.pow(b, 2) - (4 * a * c)), 0.5)) / 2 * a;
    System.out.print("方程式只有一个根：" + (double) Math.round(r1 * 1000) / 1000);
} else {
    System.out.print("该方程式无实数根");
}

3.3 解2乘2线性方程

double a, b, c, d, e, f, x, y;
Scanner scanner = new Scanner(System.in);
System.out.print("请输入a：");
a = scanner.nextDouble();
System.out.print("请输入b：");
b = scanner.nextDouble();
System.out.print("请输入c：");
c = scanner.nextDouble();
System.out.print("请输入d：");
d = scanner.nextDouble();
System.out.print("请输入e：");
e = scanner.nextDouble();
System.out.print("请输入f：");
f = scanner.nextDouble();


if ((a * d) - (b * c) == 0) {
    System.out.print("方程式无解");
} else {
    x = ((e * d) - (b * f)) / ((a * d) - (b * c));
    y = ((a * f) - (e * c)) / ((a * d) - (b * c));
    System.out.println("x 为：" + x);
    System.out.print("y 为：" + y);
}

3.4 随机月份

int month = (int) (Math.random() * 12) + 1;
System.out.println("随机数为：" + month);

switch (month) {
    case 1:
        System.out.print("该月份为一月");
        break;
    case 2:
        System.out.print("该月份为二月");
        break;
    case 3:
        System.out.print("该月份为三月");
        break;
    case 4:
        System.out.print("该月份为四月");
        break;
    case 5:
        System.out.print("该月份为五月");
        break;
    case 6:
        System.out.print("该月份为六月");
        break;
    case 7:
        System.out.print("该月份为七月");
        break;
    case 8:
        System.out.print("该月份为八月");
        break;
    case 9:
        System.out.print("该月份为九月");
        break;
    case 10:
        System.out.print("该月份为十月");
        break;
    case 11:
        System.out.print("该月份为十一月");
        break;
    case 12:
        System.out.print("该月份为十二月");
        break;
}

3.5 将来的日期

int day, after, newDay;
Scanner scanner = new Scanner(System.in);
System.out.print("今天是周几：");
day = scanner.nextInt();
System.out.print("请输入经过的天数：");
after = scanner.nextInt();

newDay = day + after;
newDay = newDay % 7;

switch (newDay) {
    case 0:
        System.out.print("周" + day + "经过" + after + "天是周日");
        break;
    case 1:
        System.out.print("周" + day + "经过" + after + "天是周一");
        break;
    case 2:
        System.out.print("周" + day + "经过" + after + "天是周二");
        break;
    case 3:
        System.out.print("周" + day + "经过" + after + "天是周三");
        break;
    case 4:
        System.out.print("周" + day + "经过" + after + "天是周四");
        break;
    case 5:
        System.out.print("周" + day + "经过" + after + "天是周五");
        break;
    case 6:
        System.out.print("周" + day + "经过" + after + "天是周六");
        break;
}

3.8 三个数排序

int a, b, c, tmp;
Scanner input = new Scanner(System.in);
System.out.print("请输入三个整数：");
a = input.nextInt();
b = input.nextInt();
c = input.nextInt();
if (a >= b) {
    tmp = a;
    a = b;
    b = tmp;
}
if (b >= c) {
    tmp = b;
    b = c;
    c = tmp;
}
if (a >= b) {
    tmp = a;
    a = b;
    b = tmp;
}
System.out.print("排列后的顺序为：" + a + "," + b + "," + c);

3.9 检查ISBN-10

int num, last;
String str, isbn;
Scanner input = new Scanner(System.in);
System.out.print("请输入九位数字，第一位以0开头：");
num = input.nextInt();

str = "0" + String.valueOf(num).toString();

last = ((Integer.valueOf(str.substring(0, 1)).intValue() * 1) +
        (Integer.valueOf(str.substring(1, 2)).intValue() * 2) +
        (Integer.valueOf(str.substring(2, 3)).intValue() * 3) +
        (Integer.valueOf(str.substring(3, 4)).intValue() * 4) +
        (Integer.valueOf(str.substring(4, 5)).intValue() * 5) +
        (Integer.valueOf(str.substring(5, 6)).intValue() * 6) +
        (Integer.valueOf(str.substring(6, 7)).intValue() * 7) +
        (Integer.valueOf(str.substring(7, 8)).intValue() * 8) +
        (Integer.valueOf(str.substring(8, 9)).intValue() * 9)) % 11;

if (last == 10) {
    isbn = str + "X";
} else {
    isbn = str + String.valueOf(last).toString();
}
System.out.print("ISBN为：" + isbn);

3.12 回文数字

Scanner input = new Scanner(System.in);
System.out.print("请输入三位数字：");
int num = input.nextInt();
if (num / 100 == num % 10) {
    System.out.print(num + "是回文数字");
} else {
    System.out.print(num + "不是回文数字");
}

3.15 彩票

int num = (int) (Math.random() * 1000);
String strNum = String.format("%03d", num);
System.out.println("随机数字已生成");

Scanner scanner = new Scanner(System.in);
System.out.print("请输入三位数：");
int usrNum = scanner.nextInt();
String strUsrNum = String.format("%03d", usrNum);

String num000 = strNum.substring(0, 1);
String num00 = strNum.substring(1, 2);
String num0 = strNum.substring(2, 3);

String usrNum000 = strUsrNum.substring(0, 1);
String usrNum00 = strUsrNum.substring(1, 2);
String usrNum0 = strUsrNum.substring(2, 3);

if (strNum.equals(strUsrNum)) {
    System.out.println("答案为：" + strNum);
    System.out.println("恭喜您，完全正确！奖金为10000美元");
} else if ((usrNum000.equals(num00) && usrNum00.equals(num0) && usrNum0.equals(num000)) ||
        (usrNum000.equals(num000) && usrNum00.equals(num0) && usrNum0.equals(num00)) ||
        (usrNum000.equals(num0) && usrNum00.equals(num00) && usrNum0.equals(num000)) ||
        (usrNum000.equals(num0) && usrNum00.equals(num000) && usrNum0.equals(num00)) ||
        (usrNum000.equals(num00) && usrNum00.equals(num000) && usrNum0.equals(num0))) {
    System.out.println("答案为：" + strNum);
    System.out.println("恭喜您，三个数字正确！奖金为3000美元");
} else if (usrNum000.equals(num000) ||
        usrNum000.equals(num00) ||
        usrNum000.equals(num0) ||
        usrNum00.equals(num000) ||
        usrNum00.equals(num00) ||
        usrNum00.equals(num0) ||
        usrNum0.equals(num000) ||
        usrNum0.equals(num00) ||
        usrNum0.equals(num0)) {
    System.out.println("答案为：" + strNum);
    System.out.println("恭喜您，包含正确数字！奖金为1000美元");
} else {
    System.out.println("答案为：" + strNum);
    System.out.println("很遗憾，没有回答正确");
}

3.17 猜拳

Scanner scanner = new Scanner(System.in);
System.out.print("石头(0)，剪刀(1)，布(2)，请出拳：");
int num = scanner.nextInt();
String[] name = {"石头", "剪刀", "布"};
System.out.println("你的出拳是......" + name[num]);

int sysNum = (int) Math.round(Math.random() * 2);
System.out.println("系统出拳是......" + name[sysNum]);

switch (num) {
    case 0:
        switch (sysNum) {
            case 0:
                System.out.print("平局!");
                break;
            case 1:
                System.out.print("你赢了!");
                break;
            case 2:
                System.out.print("你输了!");
                break;
        }
        break;
    case 1:
        switch (sysNum) {
            case 0:
                System.out.print("你输了!");
                break;
            case 1:
                System.out.print("平局!");
                break;
            case 2:
                System.out.print("你赢了!");
                break;
        }
        break;
    case 2:
        switch (sysNum) {
            case 0:
                System.out.print("你赢了!");
                break;
            case 1:
                System.out.print("你输了!");
                break;
            case 2:
                System.out.print("平局!");
                break;
        }
        break;
}

3.24 抽纸牌

Scanner scanner = new Scanner(System.in);
System.out.print("请用户抽取纸牌");
scanner.nextLine();

String[] num = {"Ace", "2", "3", "4", "5", "6", "7", "8", "9", "10", "Jack", "Queen", "King"};
String[] flower = {"黑桃", "红桃", "梅花", "方块"};

int numRandom = (int) Math.round(Math.random() * 13);
int flowerRandom = (int) Math.round(Math.random() * 4);

System.out.print("用户抽取的纸牌为：" + flower[flowerRandom] + num[numRandom]);