1.1.13.5 Input Pattern

• It is bad idea to test the stream on the outside and then read/write to it inside the body of the conditional/loop statement. This is because the act of reading may make the stream bad. It is usually better to do the read as part of the test.
  while(!cin.fail()){  // that is bad programming style 
      cin>>i;  //here may make stream fail(). 
      .....      //then i is not valid value 
  }
• If you just want to input, You don’t want to know eof or deal with failbit. You can use below:
  
  while(scanf("%d",&i) != 1) 
  while((int c = getchar())!=EOF) 
  while( fgets(line, sizeof(line), fp) != NULL ) 
  //in c language, use these to exit loop! 
   
  while(cin>>input) 
  while(cin.get(p, 20) ) 
  while(getline(ifstream, string)) { 
  //in C++ language, use bool operator to exit loop. 
    //do some useful things.  //input is right. 
  }
• If you want to know eof or deal with error. You can use below:
  While(true)  // use break to exit loop; 
  { 
    cin>>i // or getline(ifstream, string); 
    If(cin.eof()){   //If it’s EOF 
       cout<<"EOF␣encountered"<<endl 
       break; 
    } 
    If(cin.fail())          //Invalide input 
    { 
       cin.clear();    //Important. clear state and buffer 
       while(cin.get()!=’\n’)  //get rid rest of line, 
                 continue ; 
         cout<<"please␣input␣a␣number"<<endl; 
         continue; 
    } 
    ...  //do some useful things.  //input is right. 
  }
• In the previous example, Why do we need to distinguish eof and fail? When fail happen, maybe there are invalid character in buffer. After clean the buffer, I can continue to read input from input buffer. Three methods to clean invalid character in the buffer
  cin.clear();    //Important. clear state and buffer 
  while(cin.get()!=’\n’) 
        continue ;      // method 1 
   
  while(!issapce(cin.get())) 
              continue;  //method 2 
   
  basic_istream& ignore(streamsize _Count = 1, \ 
  int_type _Delim = traits_type::eof());  //method 3 
  cin.ignore(5, ’a’); 
  cin.ignore(numeric_limits<streamsize>::max(), ’\n’);
• You can use istream_iterator, It can save you some trouble to judge EOF.
  #include <iostream> 
  #include <vector> 
  #include <algorithm> 
  #include <iterator> 
  using namespace std; 
  class A{ 
  private: 
  int x; 
  int y; 
  friend istream& operator>>(istream& in, A&); 
  friend ostream& operator<<(ostream& in, const A&); 
  }; 
   
  istream& operator>>(istream& in, A& a){ 
  in>>a.x>>a.y; 
  return in; 
  } 
   
   
  ostream& operator<<(ostream& out, const A& a){ 
  out<<a.x<< "␣" <<a.y; 
  return out; 
  } 
   
  int main(){ 
  vector<A> v; 
  copy(istream_iterator<A>(cin), istream_iterator<A>(), 
                                               back_inserter(v)); 
  copy(v.begin(), v.end(),ostream_iterator<A>(cout, "\n")); 
  } 
  }
• In summary, It’s better just use judgment to exit end loop. If you need to different specific action to take to deal with EOF or error. Use while(true), then use eof() of feof() fail() or clear() functions in c++ and c to deal with and break the loop;