1.Break和Continue
break
中断/跳出
Continue
直接进行循环的下一轮
break和continue
package day0401;
import java.util.Scanner;
public class Test1 {
public static void main(String[] args) {
f1();
f2();
}
private static void f1() {
for(int i=1;;i++)
{
double random = Math.random();
if(random > 0.999)
{
System.out.println("第"+i+"次:"+random);
break;
}
}
}
private static void f2() {
for(int i=1;;i++)
{
System.out.print("输入第"+i+"个文件:");
String s = new Scanner(System.in).nextLine();
if(s.equals("exit")) break;
int index = s.lastIndexOf(".");
if(index == -1) continue;
s = s.substring(index+1);
System.out.println("后缀为:"+s);
}
}
}
2.循环命名
在嵌套循环中,控制外层循环,需要对外层循环命名
Outer:
For()
{
inner:
For()
{
Break outer;
}
}
输入整数n,求n内有多少个质数
package day0402;
import java.util.Scanner;
public class Test1 {
public static void main(String[] args) {
System.out.print("输入一个整数:");
int i = new Scanner(System.in).nextInt();
f(i);
}
private static void f(int i) {
if (i < 2) {
System.out.println("没有质数");
return;
}
if (i == 2) {
System.out.println("一个质数");
return;
}
int counts = 1;
outer:
for (int n = 3; n <= i; n++) {
double max = Math.sqrt(n) + 1;
for (int j = 2; j < max; j++) {
if (n % j == 0) continue outer;
}
++counts;
}
System.out.println(i + "有" + counts + "个质数");
}
}
3.while循环
先判断，再执行
while(执行条件) {
代码
}
package day0403;
import java.util.Scanner;
public class Test1 {
public static void main(String[] args) {
String s = "";
while(!s.equals("exit"))
{
System.out.print("请输入一串数字:");
s = new Scanner(System.in).nextLine();
for(int i=0;i<s.length()-1;i++)
{
int ascll = s.charAt(i);
if(ascll < '0' || ascll > '9')
{
System.out.println("请输入数字");
break;
}
}
}
}
}
4.do-while循环
l 先执行，再判断
l 至少执行一次
do {
代码
} while(执行条件);
l for和while
n 三种循环都可以互相代替
n for
u 固定次数
u 数据规律变化
package day0403;
import java.util.Scanner;
public class Test2 {
public static void main(String[] args) {
String s = "";
do
{
System.out.print("请输入一串数字:");
s = new Scanner(System.in).nextLine();
for(int i=0;i<s.length()-1;i++)
{
int ascll = s.charAt(i);
if(ascll < '0' || ascll > '9')
{
System.out.println("请输入数字");
break;
}
}
}while(!s.equals("exit"));
}
}
猜数字
(0,1000]范围的整数
package day0404;
import java.util.Random;
import java.util.Scanner;
public class Test1 {
public static void main(String[] args) {
int answer = new Random().nextInt(999) + 1;
int i;
do {
System.out.print("请输入一个数字:");
i = new Scanner(System.in).nextInt();
if (i > answer) System.out.println("大了");
else if (i < answer) System.out.println("小了");
} while (i != answer);
}
}