课后习题

3.1 SolveEquations.java

import java.util.Scanner;
public class SolveEquations {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter a,b,c: ");
        double a = input.nextDouble();
        double b = input.nextDouble();
        double c = input.nextDouble();
        //1.判别式，b^2 - 4ac > 0 ?
        double Discriminant =  b * b - 4 * a *c;
        //2.定义好一元二次方程的两个根
        double r1 = ((-b + Math.pow(Discriminant,0.5)) / (2 * a));
        double r2 = ((-b - Math.pow(Discriminant,0.5)) / (2 * a));
        //根据判断条件，分别输出相对应的内容
        if (Discriminant > 0){
            System.out.print("The equation has two roots " + r1 +" and " + r2);
        }
        else if(Discriminant == 0){
            System.out.println("The equation has one root " + r1);
        }
        else {
            System.out.println("The euqation has no real roots");
        }
    }
}

3.2 AddThreeNumbers.java

import java.util.Scanner;
public class AddThreeNumbers {
    public static void main(String[] args) {
        int number1 = (int) (System.currentTimeMillis() % 10);
        int number2 = (int) (System.currentTimeMillis() / 10 % 10);
        int number3 = (int) (System.currentTimeMillis() / 10 / 10 % 10);
        //Create a Scanner
        Scanner input = new Scanner(System.in);
        System.out.print(
                "What is " + number1 + " + " + number2 + " + " + number3 + " ? ");
        int answer = input.nextInt();
        System.out.println(number1 + " + " + number2 + " + " + number3 + " = " + answer + " is " +
                (number1 + number2 + number3 == answer));
    }
}

3.3 TwoTwoLinearEquation.java

import java.util.Scanner;
public class TwoTwoLinearEquation {
    public static void main(String[] args) {
        double a, b, c, d, e, f, x, y;
        Scanner input = new Scanner(System.in);
        System.out.print("Enter a,b,c,d,e,f : ");
        a = input.nextDouble();
        b = input.nextDouble();
        c = input.nextDouble();
        d = input.nextDouble();
        e = input.nextDouble();
        f = input.nextDouble();
        x = (e * d - b * f) / (a * d - b * c);
        y = (a * f - e * c) / (a * d - b * c);
        if (a*d - b * c ==0){
            System.out.println("The equation has no solution");
        }
        else {
            System.out.println("x is : " + x + " and y is : " + y);
        }
    }
}

3.4 RandomMonth.java

public class RandomMonth {
    public static void main(String[] args) {
        int month = (int) (Math.random() * 12) + 1;
        switch (month) {
            case 1: System.out.println("January");break;
            case 2: System.out.println("February");break;
            case 3: System.out.println("March");break;
            case 4: System.out.println("April");break;
            case 5: System.out.println("May");break;
            case 6: System.out.println("June");break;
            case 7: System.out.println("July");break;
            case 8: System.out.println("August");break;
            case 9: System.out.println("September");break;
            case 10: System.out.println("October");break;
            case 11: System.out.println("November");break;
            case 12: System.out.println("December");break;
            default:
                System.out.println("Error");
        }
    }
}

3.5 FutureDate.java

import java.util.Scanner;
public class FutureDate {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter today's day: ");
        int today = input.nextInt();
        System.out.print("Enter the number of days elapsed since today: ");
        int elapseddays = input.nextInt();
        int nowadays = today + elapseddays;
        System.out.print("Today is " + Week(today) + " and the future day is " + Week(nowadays));
    }
    public static String Week(int day){
        switch (day % 7) {
            case 1:
                return "Monday";
            case 2:
                return "Tuesday";
            case 3:
                return "Wednesday";
            case 4:
                return "Thursday";
            case 5:
                return "Friday";
            case 6:
                return "Saturday";
            case 0:
                return "Sunday";
            default:
                return "error";
        }
    }
}

3.6 HealthBMI.java

import java.util.Scanner;
public class HealthBMI {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        //提示并输入重量
        System.out.print("Enter weight in pounds: ");
        double weight = input.nextDouble();
        //提示并输入英尺
        System.out.print("Enter feet: ");
        double feet = input.nextDouble();
        //提示并输入英寸
        System.out.print("Enter inches: ");
        double inches = input.nextDouble();
        //final定义的变量为常量，其值不发生改变
        final double KILOGRAMS_PER_POUNDS = 0.45359237;
        final double METER_PER_FEET = 0.3048;
        final double METERS_PER_INCH = 0.0254;
        double weightInKilograms = weight * KILOGRAMS_PER_POUNDS;
        double heightInMeters = feet * METER_PER_FEET + inches * METERS_PER_INCH;
        double bmi = weightInKilograms / (heightInMeters * heightInMeters);
        //Display result
        System.out.println("BMI is " + bmi);
        if (bmi < 18.5) {
            System.out.println("Underweight");
        } else if (bmi < 25) {
            System.out.println("Normal");
        } else if (bmi < 30) {
            System.out.println("Overweight");
        } else {
            System.out.println("Obese");
        }
    }
}

3.7 WholeMoneyAgainsttheZero.java

import java.util.Scanner;
//感觉还能进一步精简，冗余度太高了，但是现在太菜了，以后更新
public class WholeMoneyAgainsttheZero {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter an amount in double, for example 11.56: ");
        double amount = input.nextDouble();
        //扩大100倍，便于对数据进行取余或除的操作
        int remainingAmount = (int) (amount * 100);
        //美元数
        int numberOfOneDollars = remainingAmount / 100;
        remainingAmount = remainingAmount % 100;
        //二角伍分
        int numberOfQuarters = remainingAmount / 25;
        remainingAmount = remainingAmount % 25;
        //一角
        int numberOfDimes = remainingAmount / 10;
        remainingAmount = remainingAmount % 10;
        //五美分
        int numberOfNickels = remainingAmount / 5;
        remainingAmount = remainingAmount % 5;
        //一美分
        int numberOfPennies = remainingAmount;
        System.out.print("Your amount " + amount + " consists of ");
        //美元
        if (numberOfOneDollars == 0) {
            System.out.print("");
        } else if (numberOfOneDollars == 1) {
            System.out.print(numberOfOneDollars + " dollar ");
        } else {
            System.out.print(numberOfOneDollars + " dollars ");
        }
        //二角伍分
        if (numberOfQuarters == 0) {
            System.out.print("");
        } else if (numberOfQuarters == 1) {
            System.out.print(numberOfQuarters + " Quarter ");
        } else {
            System.out.print(numberOfQuarters + " Quarters ");
        }
        //一角
        if (numberOfDimes == 0) {
            System.out.print("");
        } else if (numberOfDimes == 1) {
            System.out.print(numberOfDimes + " Dime ");
        } else {
            System.out.print(numberOfDimes + " Dimes ");
        }
        //五美分
        if (numberOfNickels == 0) {
            System.out.print("");
        } else if (numberOfNickels == 1) {
            System.out.print(numberOfNickels + " Nickel ");
        } else {
            System.out.print(numberOfNickels + " Nickels ");
        }
        //一美分
        if (numberOfPennies == 0) {
            System.out.print("");
        } else if (numberOfPennies == 1) {
            System.out.print(numberOfPennies + " Pennie ");
        } else {
            System.out.print(numberOfPennies + " Pennies ");
        }
    }
}

3.8 SortThreeInteger.java

import java.util.Scanner;
//3.8
public class SortThreeInteger {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Please Enter three Integer: ");
        int a = input.nextInt();
        int b = input.nextInt();
        int c = input.nextInt();
        int[] arr = {a, b, c};
        for (int i = 0; i < arr.length; i++){
            //借用程序清单7.2 扑克牌的打乱顺序的一段代码，打乱数组
            int index = (int) (Math.random() * arr.length);
            int temp = arr[i];
            arr[i] = arr[index];
            arr[index] = temp;
        }
        for (int e : arr){
            System.out.print(e + "  ");
        }
    }
}

3.9 Inspection_ISBN_10.java

import java.util.Scanner;
//3.9
public class Inspection_ISBN_10 {
    public static void main(String[] args) {
        //International Standard Book Number(ISBN)
        //提示语句1
        System.out.print("Enter the first 9 digits of an ISBN as integer: ");
        Scanner input = new Scanner(System.in);
        int num = input.nextInt();
        int d1 = num / 100000000 % 10;
        int d2 = num / 10000000 % 10;
        int d3 = num / 1000000 % 10;
        int d4 = num / 100000 % 10;
        int d5 = num / 10000 % 10;
        int d6 = num / 1000 % 10;
        int d7 = num / 100 % 10;
        int d8 = num / 10 % 10;
        int d9 = num % 10;
        int d10 = (d1 * 1 + d2 * 2 + d3 * 3 + d4 * 4 + d5 * 5 + d6 * 6 +
                d7 * 7 + d8 * 8 + d9 * 9) % 11;
        System.out.print("The ISBN-10 number is " + d1 + d2 + d3 + d4
        + d5 + d6 + d7 + d8 + d9);
        if (d10 == 10) {
            System.out.print("X");
        } else {
            System.out.print(d10);
        }
    }
}

3.10 AddTest.java

import java.util.Scanner;
public class AddTest {
    public static void main(String[] args) {
        int number1 = (int) (Math.random() * 90) + 10;
        int number2 = (int) (Math.random() * 90) + 10;
        System.out.print("What is " + number1 + " + " + number2 + " ? ");
        Scanner input = new Scanner(System.in);
        int answer = input.nextInt();
        if (number1 + number2 == answer) {
            System.out.println("You are correct!");
        } else {
            System.out.println("Your answer is wrong.");
            System.out.println(number1 + " + " + number2 +
                    " should be " + (number1 + number2));
        }
    }
}

3.11 TotalDaysInaMonth.java

import java.util.Scanner;
public class TotalDaysInaMonth {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        System.out.print("Enter month and year: ");
        int monthday = 0;
        int month = input.nextInt();
        int year  = input.nextInt();
        if (month == 1 ||month == 3 || month == 5 || month == 7 || month == 8 ||
        month == 10 || month == 12){
            monthday = 31;
        }else if (month == 4 || month == 6 || month == 9 || month == 11){
            monthday = 30;
        }else if (month == 2){
            if (isLeapYear(year) == true) {
                monthday = 29;
            }
            else {
                monthday = 28;
            }
        }else {
            System.out.print("You input a error data!!!");
        }
        //可以用数组来省略
        switch (month){
            case 1: System.out.print("January");break;
            case 2: System.out.print("February");break;
            case 3: System.out.print("March");break;
            case 4: System.out.print("April");break;
            case 5: System.out.print("May");break;
            case 6: System.out.print("June");break;
            case 7: System.out.print("July");break;
            case 8: System.out.print("August");break;
            case 9: System.out.print("September");break;
            case 10: System.out.print("October");break;
            case 11: System.out.print("November");break;
            case 12: System.out.print("December");break;
            default:
                System.out.println("Error");
        }
        System.out.print(" " + year + " has " + monthday + " days ");
    }
    public static Boolean isLeapYear(int year){
        if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0)){
            return true;
        }else {
            return false;
        }
    }
}

关键术语

本章小结