13、循环结构

循环结构

1、while循环

public class WhileDemo01 {
    public static void main(String[] args) {
        //输出1~100
        int i = 0;
        while ( i < 100 ) {
            i++;
            System.out.println(i);
        }
    }
}

计算1+2+3+...+100=?

public class WhileDemo02 {
    public static void main(String[] args) {
        //计算1+2+3+...+100=?
        int i = 0;
        int sum = 0;
        while ( i < 100 ) {
            i++;
            sum+=i;
        }
        System.out.println(sum);
    }
}

2、do...while循环

public class DoWhileDemo01 {
    public static void main(String[] args) {
        int i = 0;
        int sum = 0;

        do {
            i++;
            sum+=i;
        } while ( i < 100 );
        System.out.println(sum);
    }
}

public class DoWhileDemo02 {
    public static void main(String[] args) {
        int i = 0;
        while ( i < 0 ) {
            i++;
            System.out.println(i);
        }
        System.out.println("=============");

        do {
            i++;
            System.out.println(i);
        } while ( i < 0 );
    }
}

3、for循环

说明

• 最先执行初始化步骤，可以声明一种类型，但可初始化一个或多个循环控制变量，也可以是空语句
• 然后，检测布尔表达式的值，如果为true，循环体被执行，如果为false，循环终止，开始执行循环体后面的语句
• 执行一次循环后，更新循环控制变量（迭代因子控制循环变量的增减）
• 再次检测布尔表达式，循环执行上面的过程

public class ForDemo01 {
    public static void main(String[] args) {
        int i = 1;

        while ( i < 100 ) {
            System.out.println(i);
            i++;
        }
        System.out.println("while循环结束");

        //初始化//条件判断//迭代
        for ( i = 1; i <= 100; i++ ) {
            System.out.println(i);
        }
        System.out.println("for循环结束");

        //死循环
        for ( ; ; ) {
            
        }
    }
}

计算0到100之间的奇数和偶数的和

public class ForDemo05 {
    public static void main(String[] args) {
        //计算0到100之间的奇数和偶数的和
        int i = 0;
        int sum_odd = 0;    //奇数和
        int sum_even = 0;   //偶数和
        for ( i = 0; i <= 100; i++ ) {
            if ( i % 2 == 0 ) {
                //偶数
                sum_even+=i;
            } else {
                //奇数
                sum_odd+=i;
            }
        }
        System.out.println("奇数和为：" + sum_odd);
        System.out.println("偶数和为：" + sum_even);
    }
}

用for循环输出1-1000之间能被5整除的数，并且每行输出3个

public class ForDemo03 {
    public static void main(String[] args) {
        //用for循环输出1-1000之间能被5整除的数，并且每行输出3个
        for ( int i = 1; i <= 1000; i++ ) {
            if ( i % 5 == 0 ) {
                System.out.print(i + "\t");
            }
            if ( i % (5*3) == 0 ) {
                //System.out.println();
                System.out.print("\n");

            }
        }
    }
}

用while循环输出1-1000之间能被5整除的数，并且每行输出3个

public class WhileDemo04 {
    public static void main(String[] args) {
        int i = 1;
        while ( i <= 1000 ) {
            if ( i % 5 == 0 ) {
                System.out.print(i + "\t");
            }
            i++;
            if ( i % (5*3) == 0 ) {
                System.out.println();
            }
        }
    }
}

打印99乘法表

public class ForDemo04 {    public static void main(String[] args) {        //打印99乘法表        for ( int i = 1; i < 10; i++ ) {            for ( int j = 1; j <= i; j++ ) {                System.out.print(j + "*" + i + "=" + (j*i) + "\t");            }            System.out.println();        }    }}

4、在Java5中引入了一种主要用于数组的增强型for循环

public class ForDemo07 {    public static void main(String[] args) {        int[] numbers = {10,20,30,40,50};        for ( int i = 0; i < numbers.length; i++ ) {            System.out.println(numbers[i]);        }        System.out.println("=============");        for ( int i : numbers ) {            System.out.println(i);        }    }}

5、break continue

5.1、break

public class BreakDemo01 {    public static void main(String[] args) {        int i = 0;        while ( i < 5 ) {            i++;            System.out.println(i);            if ( i == 4 ) {                break;            }        }    }}

5.2、continue

public class ContinueDemo01 {    public static void main(String[] args) {        int i = 0;        while ( i < 100 ) {            i++;            if ( i % 10 == 0 ) {                System.out.println();                continue;            }            System.out.print(i + "\t");        }    }}

5.3、goto

public class LabelDemo01 {    public static void main(String[] args) {        //打印101~150之间所有的质数        int count = 0;        outer:for ( int i = 101; i <= 150; i++ ) {            for ( int j = 2; j < i; j++ ) {                if ( i % j == 0 ) {                    continue outer;                }            }            System.out.println(i);        }    }}

5.4、打印一个三角形

public class TestDemo03 {    public static void main(String[] args) {        //打印三角形/*效果图：    *   ***  ***** **************** */        for ( int i = 1; i <= 5; i++ ) {            for ( int j = 5; j > i; j-- ) {                System.out.print(" ");            }            for ( int j = 1; j <= i; j++ ) {                System.out.print("*");            }            for ( int j = 1; j < i; j++ ) {                System.out.print("*");            }            System.out.println();        }    }}

5.5、打印一个菱形

public class TestDemo02 {    public static void main(String[] args) {        //打印菱形/*效果图：  * ******** ***  * */        for ( int i = 1; i <= 3; i++ ) {            for ( int j = 3; j > i; j-- ) {                System.out.print(" ");            }            for ( int j = 1; j <= i; j++ ) {                System.out.print("*");            }            for ( int j = 1; j < i; j++ ) {                System.out.print("*");            }            System.out.println();        }        for ( int i = 1; i <= 2; i++ ) {            for ( int j = 1; j <= i; j++ ) {                System.out.print(" ");            }            for ( int j = 2; j >= i; j-- ) {                System.out.print("*");            }            for ( int j = 2; j > i; j-- ) {                System.out.print("*");            }            System.out.println();        }    }}