循环、数组截取、随机数的产生(2015/7/30)

循环练习题

基本知识回顾

public static void main(String[] args) {
int a = 7;
int number = 0;
while(a>0){
int b = 1;
b = a & b;
a >>= 1;//右移相当于除二
if (b == 1) {
number++;
}
}
System.out.println(number);

}

用if进行判断，不同的分支做不同的处理

Scanner input=new Scanner(System.in);
System.out.println("请输入n的值");
int number=input.nextInt();
double sum=0;
for(int i=1;i<number+1;i++){
if(i%2==0){//用if判断，奇数则加，偶数则减。
sum-=(1.0/i);
}else{
sum+=(1.0/i);
}
}
System.out.println(sum);

求奇数和

int sum=0;
for(int i=1;i<21;i++){
int m=1;
m=m<<i;
sum+=m-1;
}

数组截取

public static void main(String[] args) {
int array1[] = { 12, 3, 45, 67, 32, 24, 67, 90, 41 };
int array2[] = Arrays.copyOfRange(array1, 2, 6);// 数组截取
System.out.println(Arrays.toString(array2));

}

用数组存储随机产生十个数

public static void main(String[] args) {
int[] array1=new int[10];
Random rd=new Random();//创建一个新对象
for(int i=0;i<array1.length;i++) {
array1[i]=rd.nextInt(90)+10;//产生随机数
}
Arrays.sort(array1);
//  System.out.println(Arrays.toString(array1));不用for循环输出，而用tostring()输出
for(int i=0;i<array1.length;i++){
System.out.print(array1[i]+"\t");

}

数组按升序排列

public static void main(String[] args) {
int array1[] = { 12, 23, 41, 25, 56, 78, 5, 96 };
int array2[] = new int[10];
Arrays.sort(array1);
//       for(int i=0;i<array1.length;i++){
//       System.out.print(array1[i]+"\t");
//       }
System.out.println(Arrays.toString(array1));//tostring返回指定数组的表示形式
}

求4行4列数组的对角线和副对角线上的乘积

public static void main(String[] args) {
int[][] array1 = new int[4][4];
// Scanner input = new Scanner(System.in);
Random rd = new Random();
int sum1 = 1;
int sum2 = 1;
String str = "";
String str1 = "";
for (int i = 0; i < array1.length; i++) {
for (int j = 0; j < array1[i].length; j++) {
array1[i][j] = rd.nextInt(9);
System.out.print(array1[i][j] + "\t");
}
System.out.println();
}

for (int i = 0; i < array1.length; i++) {
for (int j = 0; j < array1[i].length; j++) {

if (i == j) {
str += "\t" + array1[i][j];

sum1 *= array1[i][j];
}
if (i + j == 3) {
str1 += "\t" + array1[i][j];
// System.out.print(array1[i][j] + "\t");
sum2 *= array1[i][j];

}
}

}
System.out.println("对角线的数：" + str);
System.out.println("对角线的乘积是：" + sum1);
System.out.println("副对角线的数：" + str1);
System.out.println("副对角线上的乘积是：" + sum2);
}
//另一种方法
public static void main(String[] args) {
int[][] array1 = new int[4][4];
Random rd = new Random();
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
array1[i][j] = rd.nextInt(9);

}
System.out.println(Arrays.toString(array1[i]));

}
String str1 = "";
String str2 = "";
for (int i = 0; i < 4; i++) {
for (int j = 0; j < 4; j++) {
if (i == j) {
str1 += "\t" + array1[i][j];

}
if (i + j == 3) {
str2 += "\t" + array1[i][j];
}
}
}
int temp = 1;
int temp2 = 1;
for (int i = 0; i < 4; i++) {
temp = temp * array1[i][i];
temp2 = temp2 * array1[i][3 - i];

}
System.out.println(str1);
System.out.println("对角线上的乘积是:" + temp);
System.out.println(str2);
System.out.println("副对角线上的乘积是：" + temp2);

}

判断邮箱是否正确

public static void main(String[] args) {
Scanner input = new Scanner(System.in);
String email = input.next();
boolean flag = false;
do {
if (email.contains("@")) {
int index = email.indexOf("@");
String start = email.substring(0, index);
boolean isLetterOrNumber = true;
for (int i = 0; i < start.length(); i++) {
char ch = start.charAt(i);
if (!Character.isLetterOrDigit(ch)) {
isLetterOrNumber = false;
break;
}

}
if (isLetterOrNumber) {
System.out.println("开头是以数字或字母开始的");

} else {
System.out.println("不是以数字或字母开头的");
System.out.println("不是正确的邮箱");
break;

}
if (email.contains(".")) {
int index1 = email.lastIndexOf(".");
String middle = email.substring(index + 1, index1);
boolean middleIsLetterOrNumber = true;
for (int i = 0; i < middle.length(); i++) {
char ch = middle.charAt(i);
if (!Character.isLetterOrDigit(ch)) {
middleIsLetterOrNumber = false;
break;
}
}
if (middleIsLetterOrNumber) {
System.out.println("中间是以数字或字母开始的");
} else {
System.out.println("中间不是以数字或字母开始的");
System.out.println("不是正确的邮箱");
break;
}
if (email.endsWith(".com") || email.endsWith(".cn") || email.endsWith(".net")) {
System.out.println("是正确的邮箱");
} else {
System.out.println("但是后缀不正确，不是正确的邮箱");
}

}

} else {
System.out.println("没有@不是正确的邮箱");
}
} while (flag);

}