//给出一个大于等于三的数判断是不是素数
 #include"stdio.h"
 #include"stdio.h"
 int main()
 {
  int i,n,flag;
  printf("please enter a number:\n");
  scanf("%d",&n);
  for(i=2;i<=sqrt(n);i++)
   {
  if(n%i==0)
  flag=1;                                                                                                                                                                                                                                                      
......................