AcWing《Java语法基础课》第1章 变量、运算符、表达式、输入与输出

作者： 作者的头像   姜萍Jumping ,  2023-01-15 15:13:08 ,  所有人可见 ,  阅读 634

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第1章 变量、运算符、表达式、输入与输出

笔记

OJ中要求主类必须是public class Main

变量

变量必须先定义，才可以使用，而且不能（在同一个代码块）重名。
变量定义的方式：

public class Main {
    public static void main(String[] args) {
        int a = 5;
        int b, c = a, d = 10 / 2;
    }
}

内置数据类型

类型 字节数 举例
byte 1 123
short 2 12345
int 4 123456789
long 8 1234567891011L
float 4 1.2F
double 8 1.2，1.2D
boolean 8 true，false
char 2 'A'

常量

使用final修饰：

final int N = 110;

类型转化

范围小的类型会自动转换为范围大的范围，例如int会自动转成long。范围大的类型不会自动转换，需要显示转化。

int x = (int) 'A';              // 显示转化
double x = 12, y = 4 * 3.3;     // 隐式转化

运算符

a = 10;         // 赋值运算符
b = 10;         // 赋值运算符
运算符 描述 实例
+ 把两个数相加 a + b将得到30
- 从第一个数中减去第二个数 a - b将得到-10
* 把两个数相乘 a * b 将得到 200
/ 分子除以分母 b / a 将得到2
% 取模运算符，向零整除后的余数，注意余数可能为负数 b % a 将得到 0
++ 自增运算符 a++：先取值后加1；++a：先加1后取值
-- 自减运算符 a--：先取值后减1；--a：先减1后取值
+= 第一个数加上第二个数 a = a + b 可以简写为 a += b
-= 第一个数减去第二个数 a = a - b 可以简写为 a -= b
*= 第一个数乘以第二个数 a = a * b 可以简写为 a *= b
/= 第一个数除以第二个数 a = a / b 可以简写为 a /= b
%= 第一个对第二个数取余数 a = a % b 可以简写为 a %= b

Java和C++都是向下取整，因此-2.5取整后得-3；而python是向零取整，因此-2.5取整后得-2。

在数学中$-30 \mod{20}=10$，余数一定是正数，但在Java中，-30 % 20 = -10，余数的符号与被除数一致。

例

public class Main {
    public static void main(String[] args) {
        double x = 1.5, y = 3.2;

        System.out.println(x * y);  // 4.80000000000001
        System.out.println(x + y);  // 4.7
        System.out.println(x - y);  // -1.7000000000000002
        System.out.println(x / y);  // 0.46875
    }
}

可以发现乘法和减法的计算结果出现误差，在精度要求不高的时候，可以忽略此问题。在比较浮点数时，用绝对值比较，例如Math.abs(x - y) < 1e-8

输入

方式1：Scanner

效率较低，输入规模较小（$10^5$以内）时使用。

import java.util.Scanner;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner scanner = new Scanner(System.in);

        String str = scanner.next();        // 读入下一个字符串
        int x = scanner.nextInt();              // 读入下一个整数
        float y = scanner.nextFloat();          // 读入下一个单精度浮点数
        double z = scanner.nextDouble();    // 读入下一个双精度浮点数
        String line = scanner.nextLine();   // 读入下一行

        scanner.close();
    }
}

方式2：BufferedReader

效率较高，输入规模较大（$10^5$以上）时使用。注意需要在main方法中声明方法会抛出异常throws Exception。

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));

        String str = bufferedReader.readLine();
        int x = Integer.parseInt(str);      // 手动转成整数类型

        String[] strs = bufferedReader.readLine().split(" ");       // 同一行多个输入

        System.out.println(x);

        bufferedReader.close();
    }
}

输出

方式1：println和printf

效率较低，输出规模较小（$10^5$以内）时使用。

print与println的区别在于前者输出后不会换行，后者会。

printf与print的区别在于，前者需要指名输出格式，用于格式化输出字符串，与C++中的printf一样；而后者是直接按原始类型输出。

public class Main {
    public static void main(String[] args) throws Exception {
        System.out.println(123);            // 输出整数 + 换行
        System.out.println("Hello World");  // 输出字符串 + 换行
        System.out.print(123);              // 输出整数
        System.out.print("yxc\n");          // 输出字符串
        System.out.printf("%04d %.2f\n", 4, 123.456D);  // 格式化输出，float与double都用%f输出
    }
}

%04d表示输出整数，占据4个空格的位置，空白处自动补0。

%.2f表示输出浮点数，保留2位小数。

System.out.printf()中不同类型变量的输出格式：

(1) int：%d
(2) float: %f, 默认保留6位小数
(3) double: %f， 默认保留6位小数
(4) char: %c, 回车也是一个字符，用’\n’表示
(5) String: %s

方式2：BufferedWriter

效率较高，输出规模较大（$10^5$以上）时使用。注意需要抛异常。

import java.io.BufferedWriter;
import java.io.OutputStreamWriter;

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(System.out));

        bufferedWriter.write("Hello World\n");
        bufferedWriter.flush();  // 需要手动刷新缓冲区

        bufferedWriter.close();
    }
}

习题

AcWing 1. A + B

方法1：Scanner和println

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        System.out.println(a + b);
        scanner.close();
    }
}

方法2：BufferedReader和BufferedWriter

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;

public class Main {
    public static void main(String[] args) throws Exception {
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(System.in));
        String[] strs = bufferedReader.readLine().split(" ");
        int a = Integer.parseInt(strs[0]);
        int b = Integer.parseInt(strs[1]);
        bufferedReader.close();

        BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(System.out));
        bufferedWriter.write(String.valueOf(a + b));
        bufferedWriter.flush();
        bufferedWriter.close();
    }
}

AcWing 608. 差

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int a = scanner.nextInt(), b = scanner.nextInt();
        int c = scanner.nextInt(), d = scanner.nextInt();

        scanner.close();

        System.out.printf("DIFERENCA = %d\n", a * b - c * d);
    }
}

AcWing 604. 圆的面积

import java.util.Scanner;

public class Main {

    public static final double PI = 3.14159;

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        double r = scanner.nextDouble();
        double area = r * r * PI;
        System.out.printf("A=%.4f\n", area);

        scanner.close();
    }
}

注意本题指明了PI = 3.14159，因此不能通过import static java.lang.Math.PI获取PI。

AcWing 606. 平均数1

import java.util.Scanner;

public class Main {

    public static final double[] COEF = {3.5, 7.5};

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        double a = scanner.nextDouble(), b = scanner.nextDouble();
        double average = (COEF[0] * a + COEF[1] * b) / (COEF[0] + COEF[1]);
        System.out.printf("MEDIA = %.5f\n", average);

        scanner.close();
    }
}

AcWing 609. 工资

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int number = scanner.nextInt(), n = scanner.nextInt();
        double salary = scanner.nextDouble();

        System.out.println("NUMBER = " + number);
        System.out.printf("SALARY = U$ %.2f\n", n * salary);

        scanner.close();
    }

}

AcWing 615. 油耗

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int total = scanner.nextInt();
        double volumn = scanner.nextDouble();

        System.out.printf("%.3f km/l\n", total / volumn);

        scanner.close();
    }

}

AcWing 616. 两点间的距离

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        double x1 = scanner.nextDouble(), y1 = scanner.nextDouble();
        double x2 = scanner.nextDouble(), y2 = scanner.nextDouble();

        double dx = x1 - x2, dy = y1 - y2;
        double distance = Math.sqrt(dx * dx + dy * dy);

        System.out.printf("%.4f\n", distance);

        scanner.close();
    }

}

AcWing 653. 钞票

import java.util.Scanner;

public class Main {
    public static int[] a = {100, 50, 20, 10, 5, 2, 1};

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);

        int m = scanner.nextInt();

        System.out.println(m);

        for(int i = 0; i < a.length; i++) {
            int nota = a[i];
            System.out.printf("%d nota(s) de R$ %d,00\n", m / nota, nota);
            m -= (m / nota) * nota;
        }

        scanner.close();
    }

}

笔记

  • 贪心策略：从最大面值的钞票开始考虑，每次用最多且不超过当前需要金额的钞票数，直至考虑到最小面值的钞票（类似实际中找钱的策略）
  • 基于贪心策略的方法能保证“所用的钞票数量尽可能少”，可用交换法反证其合理性
  • 可用数组优化编写代码的过程：在数组中按降序存储不同钞票的面值

AcWing 654. 时间转换

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        scanner.close();

        int hour = n / 3600, minute = (n - hour * 3600) / 60, second = n % 60;
        System.out.println(hour + ":" + minute + ":" + second);
    }

}

AcWing 605. 简单乘积

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt(), b = scanner.nextInt();
        scanner.close();

        System.out.println("PROD = " + (a * b));
    }
}

AcWing 611. 简单计算

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int id1 = scanner.nextInt(), n1 = scanner.nextInt();
        double price1 = scanner.nextDouble();
        int id2 = scanner.nextInt(), n2 = scanner.nextInt();
        double price2 = scanner.nextDouble();
        scanner.close();

        System.out.printf("VALOR A PAGAR: R$ %.2f\n", n1 * price1 + n2 * price2);
    }

}

AcWing 612. 球的体积

import java.util.Scanner;

public class Main {

    public static final double PI = 3.14159;

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double r = scanner.nextDouble();
        scanner.close();

        double volumn = 4.0 * PI * r * r * r / 3;
        System.out.printf("VOLUME = %.3f\n", volumn);
    }

}

AcWing 613. 面积

import java.util.Scanner;

public class Main {
    public static final double PI = 3.14159;

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double a = scanner.nextDouble();
        double b = scanner.nextDouble();
        double c = scanner.nextDouble();
        scanner.close();

        System.out.printf("TRIANGULO: %.3f\n", a * c / 2.0);
        System.out.printf("CIRCULO: %.3f\n", PI * c * c);
        System.out.printf("TRAPEZIO: %.3f\n", (a + b) * c / 2.0);
        System.out.printf("QUADRADO: %.3f\n", b * b);
        System.out.printf("RETANGULO: %.3f\n", a * b);
    }

}

AcWing 607. 平均数2

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        double a = scanner.nextDouble();
        double b = scanner.nextDouble();
        double c = scanner.nextDouble();
        scanner.close();

        System.out.printf("MEDIA = %.1f\n", (2 * a + 3 * b + 5 * c) / 10.0);
    }

}

AcWing 610. 工资和奖金

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String name = scanner.next();
        double base = scanner.nextDouble();
        double money = scanner.nextDouble();
        scanner.close();

        System.out.printf("TOTAL = R$ %.2f\n", base + 0.15 * money);
    }

}

AcWing 614. 最大值

import java.util.Scanner;

public class Main {
    public static int myMax(int a, int b) {
        return (int)(a + b + Math.abs(a - b)) / 2;
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int a = scanner.nextInt();
        int b = scanner.nextInt();
        int c = scanner.nextInt();
        scanner.close();

        System.out.printf("%d eh o maior\n", myMax(myMax(a, b), c));
    }

}

AcWing 617. 距离

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int distance = scanner.nextInt();
        scanner.close();

        System.out.printf("%d minutos\n", distance * 2);
    }

}

AcWing 618. 燃料消耗

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int t = scanner.nextInt(), v = scanner.nextInt();
        scanner.close();

        System.out.printf("%.3f\n", (double)t * v / 12.0);
    }

}

笔记

计算油耗时，不能用t * v / 12.0，因为t和v都是int类型，计算乘积后可能会超出int范围，因此需要先把它们转成double类型，保证乘积计算正确才行。

AcWing 656. 钞票和硬币

import java.util.Scanner;

public class Main {

    public static final int[] BILLS = {10000, 5000, 2000, 1000, 500, 200};
    public static final int[] COINS = {100, 50, 25, 10, 5, 1};

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int m = (int)(scanner.nextDouble() * 100);
        scanner.close();

        System.out.println("NOTAS:");
        for (int i = 0; i < BILLS.length; i++) {
            System.out.printf("%d nota(s) de R$ %.2f\n", m / BILLS[i], BILLS[i] / 100.0);
            m %= BILLS[i];
        }

        System.out.println("MOEDAS:");
        for (int i = 0; i < COINS.length; i++) {
            System.out.printf("%d moeda(s) de R$ %.2f\n", m / COINS[i], COINS[i] / 100.0);
            m %= COINS[i];
        }
    }

}

笔记

浮点数表示的金额$\times100$转成整数后，就能避免浮点数不精确的问题。

AcWing 655. 天数转换

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int m = scanner.nextInt();
        scanner.close();

        int year = m / 365;
        m %= 365;
        int month = m / 30;
        m %= 30;
        int day = m;

        System.out.println(year + " ano(s)");
        System.out.println(month + " mes(es)");
        System.out.println(day + " dia(s)");
    }

}

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