Files

• The keyboard is not the only source of input data.
• Reading from files is a second source of data.
• This is from chapter 4, and we will only highlight file I/O
• You are actually fortunate, you know MOST of what we are going to talk about.
• Headers: #include <fstream>
• New types: ifstream, ofstream
  • ifstream is an input file stream
  • ofstream is an output file stream.
  • Declare as you do anything else

    int x;
    string word;
    ofstream outFile;
    ifstream inFile; 

  • Note: I usually use outFile and inFile, but this is just an identifier, so it can be anything.
  • Operations
    • These are classes, so the operations are member functions
    • .open(c-style string)
    • .close()
    • test for success
    • stream insertion or stream extraction
    • getline, ignore
    • Many more, but these will do for now.
• A simple example: a file, numbers.dat, contains 10 numbers, find the max min and average.
• A closer look at open
  • In the current implementation (we are using) you need a c style string to open a file
  • This changes in c++ 11, but it is not yet fully available.
  • So you need to open a file with a c-string not a c++ string
  • file.open("hello.info") is good

  • ifstream dataFile;
    string fileName = "hello.info"
    
    dataFile.open(fileName) 

  • This will currently not work, but will in future releases
  • But the c++ style string has a member function to return a c-style string
    • string.c_str()

    • ifstream dataFile;
      string fileName = "hello.info"
      
      dataFile.open(fileName.c_str()) 

• Testing the value of a stream
  • A stream can be tested as a boolean
    • If the result is true, the last operation on the stream was successful
    • If the result is false, the last operation on the stream was not successful
  • Usage:
    • After opening a file, test to see if the open was successful

    • ifstream inFile
      
      inFile.open(fileName.c_str())
      if (!inFile) {
          cout << "Failed to open " << fileName << endl;
      } 

    • To read all of the words in from a file

      string word;
      ifstream inFile;
      ...
      inFile >>  word
      while(inFile) {
         // process word
         inFile >>  word
      } 

• Count the number of words in "The Charge of the Light Brigade"
  • A word is anything the stream extraction operator will read for a string.
• Clearing a failed input stream operation
  • probably due to characters when attempting to read a numeric type
  • Or end of file.
  • With the first
    • stream.clear() will clear the error flag
    • Then you need to get rid of the bad data
    • stream.ignore(...) will most likely accomplish this.