Java语言程序设计(基础篇)(原书第10版) 练习题答案 第6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

6.1 计算五角数

public static void main(String[] args) {
    for (int i = 1; i <= 100; i++) {
        if (i % 10 == 0) {
            System.out.printf("%-6d\n", getPentagonalNumber(i));
        } else {
            System.out.printf("%-6d", getPentagonalNumber(i));
        }
    }
}

public static int getPentagonalNumber(int n) {
    return n * (3 * n - 1) / 2;
}

6.2 求整数各位数字之和

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入一个整数：");
    Long num = scanner.nextLong();
    System.out.print(num + "的各位数字之和为：" + sumDigits(num));
}

public static int sumDigits(long n) {
    int sumTotal = 0;
    char ch;
    String str = String.valueOf(n).toString();
    for (int i = 0; i < str.length(); i++) {
        ch = str.charAt(i);
        sumTotal = sumTotal + Integer.parseInt(String.valueOf(ch));
    }
    return sumTotal;
}

6.3 回文整数

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.println("请输入一个整数值：");
    int num = scanner.nextInt();
    if (isPalindrome(num) == true) {
        System.out.println("这个数是回数");
    } else {
        System.out.println("这个数不是回数");
    }
}


public static boolean isPalindrome(int number) {
    return (number == reverse(number));
}

public static int reverse(int number) {
    int revNum = 0;
    while (number != 0) {
        revNum = number % 10 + (revNum * 10);
        number /= 10;
    }
    return revNum;
}

6.4 反方向显示一个整数

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入一个整数：");
    int num = scanner.nextInt();
    System.out.print("该数字的倒序输出为：" + reverse(num));
}

public static int reverse(int number) {
    int result = 0;
    while (number != 0) {
        result = (result * 10) + number % 10;
        number /= 10;
    }
    return result;
}

6.5 对三个数排序

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入三个数字：");
    double num1, num2, num3;
    num1 = scanner.nextDouble();
    num2 = scanner.nextDouble();
    num3 = scanner.nextDouble();
    displaySortedNumbers(num1, num2, num3);

}

public static void displaySortedNumbers(double num1, double num2, double num3) {
    double changeNum;
    if (num1 < num2) {
        changeNum = num1;
        num1 = num2;
        num2 = changeNum;
    }
    if (num2 < num3) {
        changeNum = num2;
        num2 = num3;
        num3 = changeNum;
    }
    if (num1 < num2) {
        changeNum = num1;
        num1 = num2;
        num2 = changeNum;
    }
    System.out.print("升序排列后的结果为：" + num3 + "," + num2 + "," + num1);
}

6.6 显示图案

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入一个值：");
    int n = scanner.nextInt();
    displayPattern(n);
}

public static void displayPattern(int n) {
    for (int i = 1; i <= n; i++) {
        for (int j = n - i; j >= 1; j--) {
            System.out.printf("%3s", "");
        }
        for (int j = i; j >= 1; j--) {
            System.out.printf("%3d", j);
        }
        System.out.println();
    }
}

6.7 计算未来投资价值

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入投资额：");
    double investmentAmount = scanner.nextDouble();
    System.out.print("请输入年利率：");
    double monthlyInterestRate = scanner.nextDouble();
    monthlyInterestRate = monthlyInterestRate / 1200;
    System.out.printf("%-8s%15s\n", "Years", "Future Value");
    for (int i = 1; i <= 30; i++) {
        System.out.printf("%-8d%15.2f\n", i, futureInvestmentValue(investmentAmount, monthlyInterestRate, i));
    }
}

public static double futureInvestmentValue(double investmentAmount, double monthlyInterestRate, int years) {
    double futureInvestmentValue = investmentAmount * Math.pow((1 + monthlyInterestRate), (years * 12));
    return futureInvestmentValue;
}

6.8 摄氏度转换

public static void main(String[] args) {
    System.out.printf("%-8s%-8s%8s%8s\n", "摄氏度", "华氏度", "华氏度", "摄氏度");
    System.out.println("----------------------------------------");
    for (int tpr1 = 40, tpr2 = 120; tpr1 >= 31 && tpr2 >= 30; tpr1--, tpr2 -= 10) {
        System.out.printf("%-10.1f%-8.1f", (double) tpr1, celsiusToFahrenheit((double) tpr1));
        System.out.printf("%11.1f%11.2f\n", (double) tpr2, fahrenheitToCelsius((double) tpr2));
    }
}

public static double celsiusToFahrenheit(double celsius) {
    return (9.0 / 5) * celsius + 32;
}

public static double fahrenheitToCelsius(double fahrenheit) {
    return (5.0 / 9) * (fahrenheit - 32);
}

6.9 英尺转换

public static void main(String[] args) {
    System.out.printf("%-9s%-8s%10s%9s\n", "英尺", "米", "米", "英尺");
    System.out.println("----------------------------------------");
    for (int i = 1, j = 20; i <= 10 && j <= 65; i++, j += 5) {
        System.out.printf("%-10.1f%-8.3f", (double) i, footToMeter((double) i));
        System.out.printf("%11.1f%11.3f\n", (double) j, meterToFoot((double) j));
    }
}

public static double footToMeter(double foot) {
    return foot * 0.305;
}

public static double meterToFoot(double meter) {
    return meter * 3.279;
}

6.12 显示字符

public static void main(String[] args) {
    printChars('1', 'Z', 10);
}

public static void printChars(char ch1, char ch2, int numLine) {
    int num = 1;
    for (int i = (int) ch1; i <= (int) ch2; i++) {
        System.out.printf("%2c", (char) i);
        if (num % numLine == 0) {
            System.out.println();
        }
        num++;
    }
}

6.13 数列求和

public static void main(String[] args) {
    System.out.println("i" + "             " + "m(i)");
    System.out.println("--------------------");
    for (int i = 1; i <= 20; i++) {
        System.out.printf("%-2d%17.4f\n", i, subTotal(i));
    }
}

public static double subTotal(int i) {
    double result = 0;
    for (int j = 1; j <= i; j++) {
        result = result + ((double) j / ((double) j + 1));
    }
    return result;
}

6.14 估算π

public static void main(String[] args) {
    System.out.println("i" + "             " + "m(i)");
    System.out.println("-------------------");
    for (int i = 1; i <= 901; i += 100) {
        System.out.printf("%-3d%15.4f\n", i, getResult(i));
    }
}

public static double getResult(int i) {
    double result = 0;
    for (int j = 1; j <= i; j++) {
        result = result + Math.pow(-1, j + 1) / (2 * j - 1);
    }
    result = result * 4;
    return result;
}

6.16 一年的天数

public static void main(String[] args) {
    System.out.println("年份" + "            " + "天数");
    System.out.println("-------------------");
    for (int i = 2000; i <= 2020; i++) {
        System.out.printf("%-4d%14d\n", i, daysOfYear(i));
    }
}

public static int daysOfYear(int year) {
    if (isLeapYear(year)) {
        return 366;
    } else return 365;
}

public static boolean isLeapYear(int year) {
    return ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0));
}

6.17 显示0和1构成的矩阵

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入一个值：");
    int n = scanner.nextInt();
    printMatrix(n);
}

public static void printMatrix(int n) {
    for (int i = 1; i <= n; i++) {
        for (int j = 1; j <= n; j++) {
            System.out.print(Math.round(Math.random()) + " ");
        }
        System.out.println();
    }
}

6.18 检测密码

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入密码：");
    String str = scanner.nextLine();
    switch (checkPassword(str)) {
        case 0:
            System.out.print("密码必须是8位以上！");
            break;
        case 1:
            System.out.print("密码必须只包含字符和数字！");
            break;
        case 2:
            System.out.print("密码必须包含至少两个数字！");
            break;
        default:
            System.out.print("密码检验通过！");
            break;
    }

}

public static int checkPassword(String pwd) {
    int numDigit = 0;
    int numLetter = 0;
    if (pwd.length() >= 8) {
        for (int i = 0; i < pwd.length(); i++) {
            if (Character.isDigit(pwd.substring(i, i + 1).charAt(0)) ||
                Character.isLetter(pwd.substring(i, i + 1).charAt(0))) {
                if (Character.isDigit(pwd.substring(i, i + 1).charAt(0))) {
                    numDigit++;
                }
                if (Character.isLetter(pwd.substring(i, i + 1).charAt(0))) {
                    numLetter++;
                }
            } else {
                return 1;
            }
        }
        if (numDigit >= 2 && numLetter > 0) {
            return 3;
        } else if (numDigit >= 2 && numLetter == 0) {
            return 1;
        } else if (numDigit == 0 && numLetter > 0) {
            return 1;
        } else return 2;
    } else {
        return 0;
    }
}

6.20 字符串中字符的个数

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入字符串：");
    String s = scanner.nextLine();
    System.out.println("该字符串中一共含有" + countLetters(s) + "个字母");
}

public static int countLetters(String s) {
    int num = 0;
    for (int i = 0; i < s.length(); i++) {
        if (Character.isLetter(s.charAt(i))) {
            num++;
        }
    }
    return num;
}

6.21 电话按键盘

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入字符串：");
    String str = scanner.nextLine();
    System.out.print("对应的数字键盘为：" + changeToNum(str));
}

public static String changeToNum(String str) {
    StringBuffer toNumBuffer = new StringBuffer();
    for (int i = 0; i < str.length(); i++) {
        char ch = Character.toLowerCase(str.charAt(i));
        if (ch == 'a' || ch == 'b' || ch == 'c') {
            toNumBuffer.append("2");
        } else if (ch == 'd' || ch == 'e' || ch == 'f') {
            toNumBuffer.append("3");
        } else if (ch == 'g' || ch == 'h' || ch == 'i') {
            toNumBuffer.append("4");
        } else if (ch == 'j' || ch == 'k' || ch == 'l') {
            toNumBuffer.append("5");
        } else if (ch == 'm' || ch == 'n' || ch == 'o') {
            toNumBuffer.append("6");
        } else if (ch == 'p' || ch == 'q' || ch == 'r' || ch == 's') {
            toNumBuffer.append("7");
        } else if (ch == 't' || ch == 'u' || ch == 'v') {
            toNumBuffer.append("8");
        } else if (ch == 'w' || ch == 'x' || ch == 'y' || ch == 'z') {
            toNumBuffer.append("9");
        } else if (ch == ' ') {
            toNumBuffer.append("0");
        } else if (ch == '+') {
            toNumBuffer.append("*");
        } else {
            toNumBuffer.append(str.charAt(i));
        }
    }
    return toNumBuffer.toString();
}

6.23 指定字符的出现次数

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入一串字符：");
    String str = scanner.nextLine();
    System.out.print("请输入要计算的字符：");
    String strLetter = scanner.nextLine();
    char ch = strLetter.charAt(0);
    System.out.print(ch + "在" + str + "中出现的次数为：" + count(str, ch) + "次");
}

public static int count(String str, char ch) {
    int num = 0;
    for (int i = 0; i < str.length(); i++) {
        if (str.charAt(i) == ch) {
            num++;
        }
    }
    return num;
}

6.25 将毫秒转换成小时数、分钟数和秒数

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入毫秒：");
    long ms = scanner.nextLong();
    System.out.print("转换后的时间为:" + convertMillis(ms));
}


public static String convertMillis(long millis) {
    long totalSecond = millis / 1000;
    long hour = totalSecond / 3600;
    long min = (totalSecond - (hour * 3600)) / 60;
    long second = totalSecond - (hour * 3600) - (min * 60);
    return hour + ":" + min + ":" + second;
}

6.26 回文素数

public static void main(String[] args) {
    int num = 2;
    int count = 0;
    while (count < 100) {
        if (isPalindrome(num) && isPrime(num)) {
            count++;
            System.out.printf("%-7d", num);
            if (count % 10 == 0) {
                System.out.println();
            }
        }
        num++;
    }
}

private static boolean isPalindrome(int num) {
    int reNum = 0;
    int numTmp = num;
    while (numTmp != 0) {
        reNum = numTmp % 10 + (reNum * 10);
        numTmp = numTmp / 10;
    }
    return num == reNum;
}

private static boolean isPrime(int num) {
    for (int i = 2; i < num; i++) {
        if (num % i == 0)
            return false;
    }
    return true;
}

6.27 反素数

public static void main(String[] args) {
    int num = 11;
    int count = 0;
    while (count < 100) {
        if (notPalindrome(num) && isPrime(num)) {
            count++;
            System.out.printf("%-7d", num);
            if (count % 10 == 0) {
                System.out.println();
            }
        }
        num++;
    }
}

private static boolean notPalindrome(int num) {
    int reNum = 0;
    int numTmp = num;
    while (numTmp != 0) {
        reNum = numTmp % 10 + (reNum * 10);
        numTmp = numTmp / 10;
    }
    return num != reNum;
}

private static boolean isPrime(int num) {
    for (int i = 2; i < num; i++) {
        if (num % i == 0)
            return false;
    }
    int reNum = 0;
    while (num != 0) {
        reNum = num % 10 + (reNum * 10);
        num = num / 10;
    }
    for (int i = 2; i < reNum; i++) {
        if (reNum % i == 0)
            return false;
    }
    return true;
}

6.28 梅森素数

public static void main(String[] args) {
    System.out.println("p                2^p-1");
    System.out.println("----------------------");
    int num;
    for (int i = 2; i <= 31; i++) {
        num = (int) (Math.pow(2, i) - 1);
        if (isPrime(i) && isPrime(num)) {
            System.out.printf("%-2d%20d\n", i, num);
        }
    }
}

private static boolean isPrime(int num) {
    for (int i = 2; i < num; i++) {
        if (num % i == 0)
            return false;
    }
    return true;
}

6.29 双素数

public static void main(String[] args) {
    for (int i = 2; i <= 998; i++) {
        if (isPrime(i) && isPrime(i + 2)) {
            System.out.println("(" + i + "," + (i + 2) + ")");
        }
    }
}

private static boolean isPrime(int num) {
    for (int i = 2; i < num; i++) {
        if (num % i == 0)
            return false;
    }
    return true;
}

6.30 双骰子**

private static int count = 0;

public static void main(String[] args) {
    int num1 = (int) (Math.random() * 6) + 1;
    int num2 = (int) (Math.random() * 6) + 1;
    System.out.println("你投出了" + num1 + "+" + num2 + "=" + (num1 + num2));
    isWin(num1, num2);
    System.out.print("总共投掷了" + count + "次");
}

private static void isWin(int num1, int num2) {
    int sum = num1 + num2;
    if (sum == 2 || sum == 3 || sum == 12) {
        System.out.println("你输了！");
        count++;
    } else if (sum == 7 || sum == 11) {
        System.out.println("你赢了！");
        count++;
    } else {
        count++;
        int result = 0;
        while (result == 0) {
            System.out.print("请继续！");
            Scanner scanner = new Scanner(System.in);
            scanner.nextLine();
            result = playAgain(num1, num2);
            count++;
        }
    }
}

private static int playAgain(int num1, int num2) {
    int sum = num1 + num2;
    int num1_new = (int) (Math.random() * 6) + 1;
    int num2_new = (int) (Math.random() * 6) + 1;
    System.out.println("你投出了" + num1_new + "+" + num2_new + "=" + (num1_new + num2_new));
    if (num1_new + num2_new == sum) {
        System.out.println("你赢了！");
        return 1;
    } else if (num1_new + num2_new == 7) {
        System.out.println("你输了！");
        return 1;
    } else {
        return 0;
    }
}

6.31 信用卡号的合法性

public static void main(String[] args) {
    Scanner input = new Scanner(System.in);
    System.out.print("请输入16位银行卡号：");
    long cardNum = input.nextLong();
    if (isValidCard(cardNum)) {
        System.out.print("此卡号有效！");
    } else {
        System.out.print("此卡号无效！");
    }
}

private static boolean isValidCard(long cardNum) {
    return (oddCount(cardNum) + evenCount(cardNum)) % 10 == 0;
}

private static int oddCount(long cardNum) {
    int oddCount = 0;
    int odd;
    char oddChar;
    for (int i = 15; i >= 1; i = i - 2) {
        oddChar = String.valueOf(cardNum).charAt(i);
        odd = Integer.parseInt(String.valueOf(oddChar));
        oddCount = oddCount + odd;
    }
    return oddCount;
}

private static int evenCount(long cardNum) {
    int evenCount = 0;
    int even;
    int evenNew;
    char evenChar;
    for (int i = 14; i >= 0; i = i - 2) {
        evenChar = String.valueOf(cardNum).charAt(i);
        even = Integer.parseInt(String.valueOf(evenChar));
        if (even * 2 >= 10) {
            evenNew = even * 2 - 9;
        } else {
            evenNew = even * 2;
        }
        evenCount = evenCount + evenNew;
    }
    return evenCount;
}

6.32 骰子的游戏统计

private static int count = 0;

public static void main(String[] args) {
    for (int i = 1; i <= 1000; i++) {
        startPlay();
    }
    System.out.print("总共投掷了" + count + "次");
}

private static void startPlay() {
    int num1 = (int) (Math.random() * 6) + 1;
    int num2 = (int) (Math.random() * 6) + 1;
    System.out.println("你投出了" + num1 + "+" + num2 + "=" + (num1 + num2));
    isWin(num1, num2);
}

private static void isWin(int num1, int num2) {
    int sum = num1 + num2;
    if (sum == 2 || sum == 3 || sum == 12) {
        System.out.println("你输了！");
        count++;
    } else if (sum == 7 || sum == 11) {
        System.out.println("你赢了！");
        count++;
    } else {
        count++;
        int result = 0;
        while (result == 0) {
            System.out.print("请继续！");
            result = playAgain(num1, num2);
            count++;
        }
    }
}

private static int playAgain(int num1, int num2) {
    int sum = num1 + num2;
    int num1_new = (int) (Math.random() * 6) + 1;
    int num2_new = (int) (Math.random() * 6) + 1;
    System.out.println("你投出了" + num1_new + "+" + num2_new + "=" + (num1_new + num2_new));
    if (num1_new + num2_new == sum) {
        System.out.println("你赢了！");
        return 1;
    } else if (num1_new + num2_new == 7) {
        System.out.println("你输了！");
        return 1;
    } else {
        return 0;
    }
}

6.35 五边形的面积

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入五边形的边长：");
    double side = scanner.nextDouble();
    System.out.print("面积为：" + area(side));
}

private static double area(double side) {
    return Math.round((5 * Math.pow(side, 2)) / (4 * Math.tan(Math.PI / 5)) * 100) / 100d;
}

6.36 正多边形的面积

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入多边形的边数：");
    int n = scanner.nextInt();
    System.out.print("请输入多边形的边长：");
    double side = scanner.nextDouble();
    System.out.print("面积为：" + area(n, side));
}

private static double area(int n, double side) {
    return Math.round((n * Math.pow(side, 2)) / (4 * Math.tan(Math.PI / n)) * 100) / 100d;
}

6.37 格式化整数

public static void main(String[] args) {
    Scanner scanner = new Scanner(System.in);
    System.out.print("请输入一个整数：");
    int number = scanner.nextInt();
    System.out.print("请输入前缀字符串个数：");
    int width = scanner.nextInt();
    System.out.print("格式化后为：" + format(number, width));
}

private static String format(int number, int width) {
    StringBuffer str = new StringBuffer();
    for (int i = 1; i <= width - String.valueOf(number).length(); i++) {
        str = str.append("0");
    }
    str.append(String.valueOf(number));
    return str.toString();
}

6.38 生成随机字符

public static void main(String[] args) {
    for (int i = 1; i <= 100; i++) {
        System.out.print(getRandomUpperCaseLetter() + " ");
        if (i % 10 == 0) {
            System.out.println();
        }
    }
    for (int i = 1; i <= 100; i++) {
        System.out.print(getRandomDigitCharacter() + " ");
        if (i % 10 == 0) {
            System.out.println();
        }
    }
}

private static char getRandomUpperCaseLetter() {
    return getRandomCharacter('A', 'Z');
}

private static char getRandomDigitCharacter() {
    return getRandomCharacter('0', '9');
}

private static char getRandomCharacter(char ch1, char ch2) {
    return (char) (ch1 + Math.random() * (ch2 - ch1 + 1));
}