C :

#include <stdio.h>
int main()
{
  int a[10], i, n;
  for(i = 0; i < 10; i ++)
    scanf("%d", &a[i]);
  scanf("%d", &n);
  for(;;)
  {
    if(n == a[4])
	{
      printf("4\n");
      break;
	}
    if(n > a[4])
	{
      for(i = 5; i < 10; i ++)
    {
      if(n == a[i])
	  {
        printf("%d\n", i);
		break;
	  }
      if(i == 9)
        printf("Not exist!\n");
    }
	}
    if(n < a[4])
	{
      for(i = 3; i >= 0 ; i --)
    {
      if(n == a[i])
	  {
        printf("%d\n", i);
		break;
	  }
      if(i == 0)
        printf("Not exist!\n");
    }
	}
	break;
  }
  return 0;
}

C++ :

#include<iostream>
using namespace std;
int main()
{
	int a[10],key;
	for (int i=0; i<10; i++) cin>>a[i];
	cin>>key;
	int low=0,high=9,mid,ans=-1;
	while (low<=high)
	{
		mid=(low+high)/2;				//折半
		if (key<a[mid]) high=mid-1;		//在前面
		else if (key>a[mid]) low=mid+1;	//在后面
		else							//查到
		{
			ans=mid;
			break;
		}
	}
	if (ans==-1) cout<<"Not exist!"<<endl;
	else cout<<ans<<endl;
	return 0;
}