java-习题学习

1.使用while循环语句计算1+1/2!+1/3!+…+1/20!

package hello;
public class Hello {

    public static void main(String[] args) {
        int num = 2;
        int i_sum = 1;
        float j = 1.0f;
        float sum = 1.0f;
        while(num < 21){
            for(int i = 1;i <= num;i++){
                i_sum = i_sum * i;
            }
            sum = sum + (j / i_sum);

            i_sum = 1;
            num++;

        }   
        System.out.println(sum);

    }

}

2.用java写程序，输出一个实心菱形

package hello;

public class Hello {

    public static void main(String[] args) {
        //打印上半部分
        for(int row = 1;row <= 5;row++){
            for(int i = 1;i <= ( 5 - row );i++){
                System.out.print(" ");    //打印空格
            }
            for(int j = 1;j <= (row * 2 - 1);j++){
                if(row == 1){
                    System.out.print("*");
                    break;
                }
                System.out.print("*");    //打印*号
            }
            System.out.println();
        }
        //打印下半部分
        for(int row = 1;row <= 4;row++){
            for(int i = 1;i <= row;i++){
                System.out.print(" ");   //打印空格
            }
            for(int j = 1;j <= 7 - (row - 1) * 2 ;j++){
                System.out.print("*");   //打印*号
            }
            System.out.println();
        }
    }

}
//以下是输出结果

3.用java程序输出空心菱形

package hello;

public class Hello {

    public static void main(String[] args) {
        //打印上半部分
        for(int row = 1;row <= 5;row++){
            if(row == 1){
                for(int i = 1;i <= 4;i++){
                    System.out.print(" ");
               }
                System.out.print("*");
            }
            else{
                for(int j = 1;j <= 5 - row;j++){
                    System.out.print(" ");    //打印空格
                }
                System.out.print("*");
                for(int k = 1;k <= (row -1) * 2 - 1;k++){
                    System.out.print(" ");
                }
                System.out.print("*");
            }
            System.out.println();
        }
            //打印下半部分
        for(int row = 1;row <= 4;row++){
            if(row == 4){
                for(int i = 1;i <= row;i++){
                    System.out.print(" ");    //打印空格
                }
                System.out.print("*");
            }
            else{
                for(int j = 1;j <= row;j++){
                    System.out.print(" ");   //打印空格
                }
                System.out.print("*");
                for(int k = 1;k <= 5 - (row -1) * 2;k++){
                    System.out.print(" ");   //打印空格
                }
                System.out.print("*");
            }
            System.out.println();
        }

    }

}
//以下是输出结果