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Java-02判断语句

tip:[start]学习语言最好的方式就是实践,每当掌握一个新功能时,就要立即将这个功能应用到实践中。——闫学灿tip:[end]

if 语句

基本if-else语句

  • 当条件成立时,执行某些语句;否则执行另一些语句。
import java.util.Scanner;

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

        if (a > 5) {
            System.out.printf("%d is big!\n", a);
            System.out.printf("%d + 1 = %d\n", a, a + 1);
        } else {
            System.out.printf("%d is small!\n", a);
            System.out.printf("%d - 1 = %d\n", a, a - 1);
        }
    }
}
  • else 语句可以省略:
import java.util.Scanner;

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

        if (a > 5) {
            System.out.printf("%d is big!\n", a);
            System.out.printf("%d + 1 = %d\n", a, a + 1);
        }
    }
}
  • 当只有一条语句时,大括号可以省略:
import java.util.Scanner;

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

        if (a > 5)
            System.out.printf("%d is big!\n", a);
        else
            System.out.printf("%d is small!\n", a);
    }
}
  • 练习:输入一个整数,输出这个数的绝对值
import java.util.Scanner;

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

        if (x > 0)
            System.out.println(x);
        else
            System.out.println(-x);
    }
}
  • 练习:输入两个整数,输出两个数中较大的那个
import java.util.Scanner;

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

        if (a > b)
            System.out.println(a);
        else
            System.out.println(b);
    }
}
  • 练习:输入三个整数,输出三个数中最大的那个。
    • if-else语句内部也可以是if-else语句
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt();

        if (a > b) {
            if (a > c)
                System.out.println(a);
            else
                System.out.println(c);
        } else {
            if (b > c)
                System.out.println(b);
            else
                System.out.println(c);
        }
    }
}

常用比较运算符

符号 说明
> 大于
< 小于
>= 大于等于
<= 小于等于
== 等于
!= 不等于
  • 扩展:
    • 六种比较的方式其实用 a < b 这一种方式都可以凑出来
    • a > b 等价于 b < a
    • a>=b 等价于 !(a<b) 即a不小于b
    • a<=b 等价于 !(b<a) 即b不小于a
    • a==b 等价于 !(a<b)&&!(b<a) 类似于高数中的夹逼原理
    • a!=b 等价于 (a<b)||(b<a)
    • 这也是为什么在c++中重载运算符时,只需要重载<小于号就可以了
import java.util.Scanner;

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

        if (a > b) System.out.printf("%d > %d\n", a, b);
        if (a >= b) System.out.printf("%d >= %d\n", a, b);
        if (a < b) System.out.printf("%d < %d\n", a, b);
        if (a <= b) System.out.printf("%d <= %d\n", a, b);
        if (a == b) System.out.printf("%d == %d\n", a, b);
        if (a != b) System.out.printf("%d != %d\n", a, b);
    }
}

if-else连写:

/*
	输入一个0到100之间的分数,
	如果大于等于85,输出A;
	如果大于等于70并且小于85,输出B;
	如果大于等于60并且小于70,输出C;
	如果小于60,输出 D;
*/

import java.util.Scanner;

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

        if (s >= 85) {
            System.out.println("A");
        } else if (s >= 70) {
            System.out.println("B");
        } else if (s >= 60) {
            System.out.println("C");
        } else {
            System.out.println("D");
        }
    }
}
  • 练习:判断闰年
  • 闰年有两种情况:
    • (1) 能被100整除时,必须能被400整除;
    • (2) 不能被100整除时,被4整除即可。
  • 输入一个年份,如果是闰年输出yes,否则输出no。
import java.util.Scanner;

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

        if (year % 100 == 0) {
            if (year % 400 == 0)
                System.out.println("yes");
            else
                System.out.println("no");
        } else {
            if (year % 4 == 0)
                System.out.println("yes");
            else
                System.out.println("no");
        }
    }
}

条件表达式

符号 说明
&&
`
!
  • 例题:输入三个数,输出三个数中的最大值。
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int a = sc.nextInt(), b = sc.nextInt(), c = sc.nextInt();

        if (a >= b && a >= c)
            System.out.println(a);
        else if (b >= a && b >= c)
            System.out.println(b);
        else
            System.out.println(c);
    }
}
  • 练习:用一条if语句,判断闰年。
import java.util.Scanner;

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

        if (year % 4 == 0 && year % 100 != 0 || year % 400 == 0)
            System.out.println("yes");
        else
            System.out.println("no");
    }
}

switch 语句

war:[start]

swtich语句中如果不加break语句,则从上到下匹配到第一个case后,会顺次执行后面每个case中的语句。
switch每个case后最好要加break语句。

war:[end]

import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int day = sc.nextInt();
        String name;

        switch(day) {
            case 1:
                name = "Monday";
                break;
            case 2:
                name = "Tuesday";
                break;
            case 3:
                name = "Wednesday";
                break;
            case 4:
                name = "Thursday";
                break;
            case 5:
                name = "Friday";
                break;
            case 6:
                name = "Saturday";
                break;
            case 7:
                name = "Sunday";
                break;
            default:
                name = "not valid";
        }

        System.out.println(name);
    }
}
posted @ 2023-12-08 17:39  AFEIOOO  阅读(5)  评论(0编辑  收藏  举报