Java_2 判断语句
1 if 语句
1.1 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);
}
}
-
if-else
语句内部也可以是if-else
语句
-
- 练习:输入三个整数,输出三个数中最大的那个。
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(), 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);
}
}
}
1.2 常用比较运算符
-
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
- 扩展:
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);
}
}
1.3 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");
}
}
}
2 条件表达式
- 与
&&
- 或
||
- 非
!
-
- 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");
}
}
3 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);
}
}