Thread: need help with a basic prog (reading input files)

  1. #1
    Registered User
    Join Date
    Sep 2007

    need help with a basic prog (reading input files)

    Hi, I decided to write a prog that requires me to open an external data file and read and manipulate each line of the file one at a time. I have never taken any classes so I dont have a book, just read a few tutorials online. I cant find seem to find the syntax I need though for this problem. I actually have the data file open, using a tut that used this syntax:
    int main () {
      string line;
      ifstream myfile ("myfile.txt");
      if (myfile.is_open())
        while (! myfile.eof() )
          cout << line << endl;
      else cout << "Unable to open file"; 
      return 0;
    but It doesnt go into any further detail about how to go about reading line by line. Like for instance if the file has a list of names that has to be changed then placed back in the file or output.

    Any suggestions? Help is greatly appreciated.

  2. #2
    The superhaterodyne twomers's Avatar
    Join Date
    Dec 2005
    The best way to read the file is to use getline():

    std::ifstream in( "myfile.txt" );
    if ( in ) {
      std::string line;
      while( std::getline( in, line ) ) {
        std::cout<< line << "\n";
    If you're unfamiliar with using 'std::' just ignore them. I prefer not to use "using namespace std;" globally.

    A 'problem' arises though when you wish to change the contents of a file. You cannot really simply change a line. You have to read in the whole contents of a file manipulate it and save it to another file and delete the original (so you don't loose data on bad luck). (or read line by line or block by block if the file's large).

    You could alternatively look into fstream's rather than ifstreams and ofstreams but maybe get used to them first.

  3. #3

  4. #4
    Registered User
    Join Date
    Sep 2007
    That is what I need twomers. Thank you very much. Thanks robwhit too. The cplusplus site seems really helpful.

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