Java日志第48天 2020.8.24

例3.11 用do-while求1+2+3+...+100的值

public class Demo3_11 {
    public static void main(String[] args) {
        int i = 1;
        int sum = 0;
        do{
            sum += i;
            i++;
        } while (i<=100);
        System.out.println("sum="+sum);
    }
}

 

 

例3.12 用下面公式求π的近似值

 

直到最后一项的绝对值小于为止。

import java.text.DecimalFormat;

public class Demo3_12 {
    public static void main(String[] args) {
        int s =1;
        double n = 1, t=1, pi=0;
        while (Math.abs(t) > 1e-7) {
            pi = pi+t;
            n = n+2;
            s = -s;
            t = s/n;
        }
        pi = pi * 4;
        DecimalFormat df = new DecimalFormat("#.000000");
        System.out.println("pi = "+df.format(pi));
    }
}

 

 

例3.13 求Fibonacci数列前40个数。这个数列有如下特点：第1、2个数为1、1。从第三个数开始，每个数是其前面两个数之和。即

F₁ = 1   (n=1)

F₂ = 1           (n=2)

F_n = F_n-1+F_n-2              (n≥3)

public class Demo3_13 {
    public static void main(String[] args) {
        long f1, f2;
        int i;
        f1 = f2 = 1;
        for (i = 1; i <= 20; i++) {
            System.out.print(f1+" "+f2+" ");

            if(i%2 == 0)
                System.out.println();

            f1 = f1 + f2;
            f2 = f1 + f2;
        }
    }
}

 

 

 

例3.14 找出100～200间的全部素数

 

public class Demo3_14 {
    public static void main(String[] args) {
        int m, k, i, n = 0;
        boolean prime;
        for (m = 101; m <= 200; m=m+2) {
            prime = true;
            k = (int) (Math.sqrt(m));
            for (i = 2; i <= k; i++) {
                if(m%i == 0) {
                    prime =false;
                    break;
                }
            }
            if(prime) {
                System.out.print(m+" ");
                n++;
            }
            if(n%10 == 0){
                System.out.println();
            }
        }

        System.out.println();
        System.out.println("共有"+n+"个素数");
    }
}

 

 

例3.15 译密码。为使电文保密，往往按一定规律将电文转换成密码，收报人再按约定的规律将其译回原文。例如，可以按以下规律将电文变成密码：将字母A变成字母E，a变成e，即变成其后的第四个字母，W变成A，X变成B，Y变成C，Z变成D。字母按上述规律转换，非字母字符不变。如“Wonderful”转换为“Asrhivjyp”。输入一行字符，要求输出其对应的密码。

import java.util.Scanner;

public class Demo3_15 {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        String s;
        s = sc.nextLine();

        char[] c = s.toCharArray();

        for (int i = 0; i < c.length; i++) {
            if ((c[i]>='a'&&c[i]<='z')||(c[i]>='A'&&c[i]<='Z')) {
                c[i] = (char)(c[i]+4);
                if (c[i]>'Z'&&c[i]<=(char)('Z'+4)||c[i]>'z') {
                    c[i] = (char) (c[i]-26);
                }
            }
            System.out.print(c[i]);
        }

    }
}

 

 

疑问：怎样输入字符串的方式输入一个字符数组类型的数据