java4:选择语句（selection），逻辑运算，循环（loop）
直接用例子来practice
example one： practice first-grader subtraction
Suppose you want to develop a program for a first-grader to practice subtraction. The program
randomly generates two single-digit integers, number1 and number2, with number1 > =
number2 and displays to the student a question such as “What is ” After the student
enters the answer, the program displays a message indicating whether it is correct.
9 - 2?
初级减法运算，利用随机工具产生两个随机数字，num1 ,num2 ,num1>=num2 然后显示 num1-num2=? 让用户输入一个答案，显示是否答对
import java.util.Scanner;
public class Subtraction{
public static void main(String[] args){
int num1=(int)(Math.random()*10);
int num2=(int)(Math.random()*10);
if(num2>num1){
int temp=num1;
num1=num2;
num2=temp;
}
Scanner input=new Scanner(System.in);
System.out.println(num1+" - "+num2+" = ?");
System.out.print("input your answer:");
int in=input.nextInt();
if(in == num1-num2)
{
System.out.println("your are right,"+num1+"-"+num2+"="+in);
}
else
System.out.println("you are wrong,"+num1+"-"+num2+"="+(num1-num2));
}
}
这里注意两个点：1）因为利用Math.random产生了[0,1)之间的一个double数字，所以我们需要使用显式类型转换， 2）注意输入的要求num1>num2 但我们不能决定随机函数先产生大的再产生小的，所以产生后别忘了加一个简单的判断交换。
example two：Lottery
Suppose you want to develop a program to play lottery. The program randomly generates a
lottery of a two-digit number, prompts the user to enter a two-digit number, and determines
whether the user wins according to the following rule:
1. If the user input matches the lottery in exact order, the award is $10,000.
2. If all the digits in the user input match all the digits in the lottery, the award is $3,000.
3. If one digit in the user input matches a digit in the lottery, the award is $1,000
import java.util.Scanner;
public class Lottery{
public static void main(String[] args){
int lot=(int)(Math.random()*100);
Scanner input =new Scanner(System.in);
int gue;
do{
System.out.print("input your guess:");
gue=input.nextInt();
}while(gue<10 || gue>100);
if(lot==gue)
{
System.out.println("you match all,you win $10,000");
System.exit(0);
}
int lot1=lot/10;
int lot2=lot%10;
int gue1=gue/10;
int gue2=gue%10;
if(lot1==gue2 && lot2==gue1)
{
System.out.println("you match the digital but sorry for the order,you win $3,000,the lot is" + lot);
//System.exit(0);
}
else if(lot1==gue1 || lot1==gue1 || lot2==gue1 || lot2 == gue2)
{
System.out.println("you match one digital,you win $1000,the lot is "+lot);
//System.exit(0);
}
else
System.out.println("sorry for that you lose, the lottery is "+lot);
}
}
其中有点奇怪，就是怎么产生的随机数会有个2的呢？？ 我明明ramdon后是乘以100了，那么除了可能产生0（这里其实应该特殊处理才严谨点，下回改进），不应该会有产生2呀？？
example three： 矩阵形式打印九九乘法表
public class MultiTable
{
public static void main(String [] args)
{
int i,j;
System.out.print(" ");
for(i=1;i<10;i++)
{
System.out.printf("%4d",i);
}
System.out.println();
System.out.println("----------------------------------------------------------");
for(i=1;i<10;i++)
{
System.out.printf("%d | ",i);
for(j=1;j<10;j++)
{
System.out.printf("%4d",i*j);
}
System.out.println();
}
}
}
example four: display primenumber
The problem is to display the first 50 prime numbers in five lines, each of which contains
n numbers. The problem can be broken into the following tasks:
■ Determine whether a given number is prime.
■ For number 2, 3, 4, 5, 6, test whether it is prime.
■ Count the prime numbers.
■ Print each prime number, and print ten numbers per line.
public class primeNumber
{
public static void main(String [] args)
{
int num=1;
int count=0;
System.out.printf("%4d",2);
count++;
while(count<50)
{
num+=2;
if(ifPrime(num))
{ System.out.printf("%4d",num);
count++;
if(count%10==0) System.out.println();
}
}
}
public static boolean ifPrime(int n)
{
if(n%2==0) return false;
int h=(int)java.lang.Math.sqrt(n);
for(int i=3;i<=h;i+=2)
{
if(n%i==0) return false;
}
return true;
}
}
这里注意里边多了一个public static boolean ifPrime 用来判断是给定的一个数是否是素数，这里是定义一个类方法，这到后面会见到，这里先知道有这个用法。