循环小示例，菲波拉契序列，循环解一元二次方程以及switch示例程序

# include <stdio.h>

int main(void)
{
  int n;
  int i;
  int f1, f2, f3;

  f1 = 1;
  f2 = 1;

  printf("请输入您需要求的想的序列：");
  scanf("%d", &n);

  for (i=3; i<n; i++)
  {
    f3 = f1 + f2;
    f1 = f2;
    f2 = f3;
    printf("%d ", f3);
  }

  return 0;
}      

一元二次方程求解：

# include <stdio.h>
      # include <math.h>

      int main(void)
      { 
        double a, b, c;
        double delta;
        double x1, x2;
        char ch;
        
        do {
          printf("请输入一元二次方程的三个系数：\n");
          printf("a = ");
          scanf("%lf", &a);
          
          printf("b = ");
          scanf("%lf", &b);
          
          printf("c = ");
          scanf("%lf", &c);
          
          delta = b*b - 4*a*c;
          
          if (delta > 0)
          {
            x1 = (-b + sqrt(delta))  / (2*a);
            x2 = (-b - sqrt(delta)) / (2*a);
            printf("有两个解，x1 = %f, x2 = %f\n", x1, x2);
          }
          else if (0 == delta)
          {
            x1 = x2 = (-b) / (2*a);
            printf("有唯一解，x1 = x2 = %f\n", x1, x2);
          }
          else
          {
            printf("无实数解！");
          }

          printf("您想继续么(Y/N)：");
          scanf(" %c", &ch);  //%c前面必须得加一个空格 原因略，这是一个空白符的问题

        } while ('y'==ch || 'Y'==ch);
        
        return 0;
      }      

switch 示例：

# include <stdio.h>

int main(void)
{
  int val;
  
  printf("请输入您要进入的楼层：");
  scanf("%d", &val);
  
  switch (val)
  {
  case 1:
    printf("1层开！\n");
    break;
  case 2:
    printf("2层开！\n");
    break;
  case 3:
    printf("3层开！\n");
    break;
  default:
    printf("还没盖到这一层！\n");
    break;
  }
  
  return 0;
}