C++ I/O

Contents

• Standard input and output
  • Test Yourself #1
• File I/O
• I/O Parameters
  • Test Yourself #2
• Answers to Self-Study Questions

You can write to the standard output using code like this:

  cout << x << y << endl;

  cout << x;
  cout << y;
  cout << endl;

Similarly, you can read from the standard input using code like this:

  cin >> x >> y;

  cin >> x;
  cin >> y;

Note:

1. Remember that you must #include <iostream> in order to use cin, cout, or endl.

2. To remember which is the input operator and which is the output operator, think of the angle brackets as pointing in the direction the values are flowing; for example, when you write the value of x using "cout << x", the brackets point to cout (so the value is going from x to cout). When you read a value into x you use "cin >> x"; this time, the brackets point to x (so the value is going from cin to x).

3. Both the output operator << and the input operator >> are overloaded; they can be used to write/read any of the primitive types. However, the input operator >> skips all whitespace in the input, so you cannot use it to read a whitespace character (e.g., a space, tab, or newline).

4. It is up to you as the programmer to know what kind of data you are going to read. If the data in the input does not match the type of the variable you are reading into, you will either get a runtime error, or the value you read will be garbage. For example, if you run the following program:

     #include <iostream>
     
     int main() {
         int x;
         
         cout << "enter a number: ";
         cin >> x;
         return(0);
     }
   

   and you type a letter instead of a number, you will either get a runtime error, or the value of x will be garbage.

Write a C++ program that uses a loop to sum ten numbers read from the standard input, and then writes the sum to the standard output.

solution

To read from a file you must use a variable of type ifstream. To write to a file you must use a variable of type ofstream. In both cases, you must open the file before you can read or write.

For example, here's how to open the file named "input.dat" for reading:

  #include <fstream>
  
  ifstream inFile;
  inFile.open("input.dat");
  if (inFile.fail()) {
      cerr << "unable to open file input.dat for reading" << endl;
      exit(1);
  }

Once inFile has successfully been opened for reading, you can use the usual input operator to read values:

  int n, sum = 0;
  while (inFile >> n) {
      sum += n;
  }

If you prefer to read one character at a time (including whitespace characters), you can use the get operation:

  char ch;
  while (inFile.get(c)) {
      ...
  }

It is often useful to write a function that takes an input stream or an output stream as a parameter, and reads from or writes to the given stream (without worrying about whether it is using the standard input/output or a particular file). To do this, you should use parameters of type istream (for input), and ostream (for output). For example:

  void f( istream & input, ostream & output ) {
  int n;
      output << "enter a number: ";
      input >> n;
  }

1. "input" and "output" are the names used in the example for the two parameters, but there is nothing special about those names; as usual, it is up to the programmer to choose parameter names.

2. The ampersands in front of the parameter names mean that the two parameters are passed by reference. The difference between value parameters and reference parameters will be discussed in another set of notes. For now, just remember that input and output streams must always be passed by reference (or you will get a not-very-clear compile-time error).

3. A call to function f can pass either the standard input or a file (an ifstream) as the first parameter, and can pass either the standard output, the standard error, or a file (an ofstream) as the second parameter.

Write a C++ program that includes two functions:

1. function Compare has 3 parameters: 2 input streams and 1 output stream. The function reads the characters from each of the two inputs. If the sequences of characters are exactly the same, it writes "equal" to the output stream; otherwise, it writes "not equal".

2. function main opens the files "Input1" and "Input2" for reading (giving an appropriate error message and quitting if either open fails), then calls Compare, passing the two input files as well as the standard output.

solution

Solutions to Self-Study Questions

Test Yourself #1

Test Yourself #2