Re: File reading problem

From:

"BobR" <removeBadBobR@worldnet.att.net>

Newsgroups:

comp.lang.c++

Date:

Sun, 15 Jul 2007 18:12:50 GMT

Message-ID:

<Cstmi.322208$p47.38934@bgtnsc04-news.ops.worldnet.att.net>

Eric Lilja <mindcooler@gmail.com> wrote in message...

On 15 Juli, 16:18, Eric Lilja <mindcoo...@gmail.com> wrote:

On 15 Juli, 15:12, Robert Bauck Hamar <roberth+n...@ifi.uio.no> wrote:

#include <fstream>
#include <iostream>
#include <string>
#include <vector>
using namespace std;

struct s_t{
   s_t() {}
   s_t(const string& si, int xi, int yi) : s(si), x(xi), y(yi) {}
   string s;
   int x, y;
   friend ostream& operator<<(ostream& os, const s_t& rhs);
   friend istream& operator>>(istream& is, s_t& rhs);
};

ostream& operator<<(ostream& os, const s_t& rhs){
   os << rhs.s << endl;
   os << rhs.x << ' ' << rhs.y;
   return os;
}

istream& operator>>(istream& is, s_t& rhs){
   getline(is, rhs.s);
   is >> rhs.x >> rhs.y;
   return is;
}

void readit(){
   int x = 0, y = 0;
   vector<s_t> vec;
   ifstream in("foo.txt");
   in >> x >> y;

After this, the next character in the input stream should be a '\n'.

{
s_t temp;

This will now read to the first newline in the stream and put the

result

into temp.s. Since the first character is a newline, temp.s will be

empty.

Then in will try to put the contents of what's stored as a string into
temp.x. In this case, in finds 'f', and fails. Thus the reading of the

two

ints will become a noop, and the test fails.

I suggest you look up the ignore() member function of istream. You

need to

ignore the rest of the line.

I solved it by adding this to my overloaded operator>>:
if (is.peek() == '\n'){ is.ignore();}

before the getline() call. Now that program works just fine. However,
the *real* program tries to read even though there are no more entries
in the file. Hmm.

And I solved that by checking the return value of the getline() call
and returning the stream directly if it fails.

      while (in >> temp) vec.push_back(temp);
   }
   in.close();
   cout << x << ' ' << y << endl;
   cout << vec.size() << endl;
   for (unsigned int i = 0; i < vec.size(); i++)
      cout << vec.at(i) << endl;
}

int main(){
   int x = 4711, y = 1337;
   vector<s_t> vec;
   vec.push_back(s_t("foo bar", 33, 22));
   vec.push_back(s_t("bar baz", 11, 99));
   ofstream out("foo.txt");
   out << x << ' ' << y << endl;
   for (unsigned int i = 0; i < vec.size(); i++){
      out << vec.at(i);
      if (i < vec.size() - 1)
         out << endl;
   }
   out.flush();
   out.close();
   readit();
}

Your 'vector' in 'main()' is separate from the 'vector' in 'readit()'. If
you want to fill a 'vector' from 'main()', try adding this (no need to
remove your current version):

void readit( std::vector<s_t> &vec ){ // non-const
    ifstream in("foo.txt");
    if( not in.is_open() ){ std::cerr<<"ERROR"; return; }
    int x( 0 ), y( 0 );
    in >> x >> y;
    if( in.peek() == '\n' ){ in.ignore(); }
    s_t temp;
    while( in >> temp ) vec.push_back( temp );
// untested. replace the above two lines with:
// for( s_t temp; in >> temp; /*m_t*/ ){
// vec.push_back( temp );
// } // for(in)
    } // readit( std::vector<s_t>&)

{ // main
    // .....
    std::vector<s_t> vst;
    readit( vst );
    for( std::size_t i(0); i < vst.size(); ++i )
          cout << vst.at(i) << endl;
}

--
Bob R
POVrookie