File I/O

Files

• Store data beyond the run of a program
• Easy way to gather a lot of information together (vs. user input)
• Stored in streams in C++
  • Similar to strings --- sequence of characters
  • To read files, declare an ifstream (input file stream)
  • To write to files, declare an ofstream (output file stream)
    • Similar to cout

Common File I/O Pattern

• Open file

  #include   // standard librark package for files
  #include "filelib.h'  //contains helpful methods
  

  • Example Stanford function

    string promptUserForFile(ifstream& stream, string prompt);
    

    Asks the user for the name of a file (using the string prompt). The file is opened using the reference parameter stream, and the function returns the name of the file. If the requested file cannot be opened, the user is given additional chances to enter a valid file.
• Creating and closing

  ifstream infile;
  promptUserForFile(infile, "File?");
  
  char ch;
  while (infile.get(ch)) {
    //do something with ch
    cout << ch << endl;
  }
  infile.close();
  

  Summary for every file-reading program: (1) Creates ifstream object; (2) closes ifstream object.
• If you already know the name of the file to open (another method for opening a file):

  ifstream infile;
  infile.open("File.txt");
  
  char ch;
  while (infile.get(ch)) {
    //do something with ch
    cout << ch << endl;
  }
  infile.close();
  

• Reading char by char: (1) declare the variable to read data into (ch); (2) while loop continues until read fails. Every iteration of while loop is a new char.
• Reading line by line

  ifstream infile;
  infile.open("File.txt");
  
  string line;
  while (getline(infile, line)) {
    //do something with line
    cout << line << endl;
  }
  infile.close();
  

  Now reads each line (breaks on newline characters). Still declare the line before the while loop. Still continues until getline fails; each while loop iteration has a different line. Notice lowercase l of getline (different from Stanford library's getLine).
• Reading formatted input

  ifstream infile;
  promptUserForFile(infile, "File?");
  
  string word;
  while (infile >> word) {
    //do something word
  }
  
  infile.close();
  

  Now reads each word (removes whitespace). Still declare the variable before the while loop in which the word will be stored. Stll continues until fails to read a new word. Each while loop iteration has a different word. Works with other types (e.g., int or Vector) too. Do not try to mix with getline (recall previous discussion).

Writing output

• Use ofstream

  ofstream ofile;
  promptUserForFile(outfile, "File?");
  string word = "Output";
  int x = 3;
  outfile << word << " " << x << endl;
  outfile.close();
  

  Similar to reading formatted input. Works a lot like cout. Works with (almost) any type. Use ofstream instead of ifstream and << instead of >>.