流程控制

流程控制

1.Scanner

1. next()：以空白为结束符

   Scanner scanner = new Scanner(System.in);
   System.out.print("使用next()输入：");//hello world
   if (scanner.hasNext()) {
       String str = scanner.next();
       System.out.println("输入的是：" + str);//hello
   }
   scanner.close();
   

2. nextLine()：以Enter为结束符

   Scanner scanner = new Scanner(System.in);
   System.out.print("使用nextLine()输入：");//hello world
   if (scanner.hasNextLine()) {
       String str = scanner.nextLine();
       System.out.println("输入的是：" + str);//hello world
   }
   scanner.close();
   

3. 例题：输入多个数字，求其总和与平均数，每输入一个数字用回车确认，通过输入非数字来结束输入并输入出执行结果

   Scanner scanner = new Scanner(System.in);
   System.out.print("请输入：");
   double sum = 0;
   int count = 0;
   while(scanner.hasNextDouble()) {
       double data = scanner.nextDouble();
       System.out.println("你输入的是：" + data);
       sum += data;
       System.out.println("当前的和是：" + sum);
       ++count;
       System.out.println("当前是第" + count + "个数据");
   }
   System.out.println("一共输入了" + count + "个数据");
   System.out.println("总和是：" + sum);
   System.out.println("平均数是：" + sum /count);
   scanner.close();
   

2.选择结构

1. if

   if (布尔表达式) {
       
   }
   

2. switch

   String name = "lohir";
   
   switch (name) {
       case "lohir":
           System.out.println("lohir");
           break;
       case "makun":
           System.out.println("makun");
           break;
       default:
           System.out.println("啥也不是");
   }
   

3.循环结构

1. while

   while (布尔表达式) {
       //循环内容
   }
   

2. do...while

   do {
       //循环内容
   }while(布尔表达式);
   

3. for

   for(初始化; 布尔表达式; 更新){
       //循环内容
   }
   

   1. 计算0到100之间的奇数和、偶数和

      int oddSum = 0;
      int evenSum = 0;
      
      for (int i = 0; i <= 100; i++) {
          if (i % 2 != 0) {
              evenSum += i;
          }else {
              oddSum += i;
          }
      }
      System.out.println("偶数和：" + oddSum);
      System.out.println("奇数和：" + evenSum);
      

   2. 用while或for循环输出1-1000之间能被5整除的数，并且每行输出3个

      1. for

         for (int i = 0; i <= 1000; i++) {
             if (i % 5 == 0) {
                 System.out.print(i + "\t");
                 if (i % (5 * 3) == 0) {
                     System.out.println();
                 }
             }
         }
         

      2. while

         int i = 0;
         while (i++ <= 1000) {
             if (i % 5 == 0) {
                 System.out.print(i + "\t");
                 if (i % (5 * 3) == 0) {
                     System.out.println();
                 }
             }
         }
         

   3. 打印九九乘法表

      for (int i = 1; i < 10; i++) {
          for (int o = 1; o <= i; o++) {
              System.out.print(o + "*" + i + "=" + (o * i) + "\t");
          }
          System.out.println();
      }
      

4. 增强for循环【主要用于数组或集合】

   int[] numbers = {10,20,30,40,50};
   for (int x : numbers) {
       System.out.println(x);
   }
   

4.break和continue

1. break强行退出循环，终止循环
2. continue退出本次循环，后面的内容不执行
3. goto关键字【了解】

5.打印三角形【5行】

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();
}