Basic Output("cout")

    Until now, the example programs of previous sections provided very little interaction with the user, if any at all. Using the standard input and output library, we will be able to interact with the user by printing messages on the screen and getting the user's input from the keyboard
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     C++ uses a convenient abstraction called streams to perform input and output operations in sequential media such as the screen or the keyboard. A stream is an object where a program can either insert or extract character to/from it. We do not really need to care about many specifications about the physical media associated with the stream - we only need to know it will accept or provide characters sequentially.

     The standard C++ library includes the header file iostream, where the standard input and output stream objects are declared.

Standard Output (cout)

    By default, the standard output of a program is the screen, and the C++ stream object defined to access it is cout.

    cout is used in conjunction with the insertion operator, which is written as << (two "less than" signs).

      cout << "Output sentence"; // prints Output sentence on screen
      cout << 120; // prints number 120 on screen
      cout << x; // prints the content of x on screen

    The << operator inserts the data that follows it into the stream preceding it. In the examples above it inserted the constant string Output sentence, the numerical constant 120 and variable x into the standard output stream cout. Notice that the sentence in the first instruction is enclosed between double quotes (") because it is a constant string of characters. Whenever we want to use constant strings of characters we must enclose them between double quotes (") so that they can be clearly distinguished from variable names. For example, these two sentences have very different results:

      cout << "Hello"; // prints Hello
      cout << Hello; // prints the content of Hello variable

The insertion operator (<<) may be used more than once in a single statement:

      cout << "Hello, " << "I am " << "a C++ statement";

    This last statement would print the message Hello, I am a C++ statement on the screen. The utility of repeating the insertion operator (<<) is demonstrated when we want to print out a combination of variables and constants or more than one variable:

       cout << "Hello, I am " << age << " years old and my zipcode is " << zipcode;

    If we assume the age variable to contain the value 24 and the zipcode variable to contain 90064 the output of the previous statement would be:

        Hello, I am 24 years old and my zip code is 90064

    It is important to notice that cout does not add a line break after its output unless we explicitly indicate it,therefore, the following statements:

        cout << "This is a sentence.";
        cout << "This is another sentence.";

will be shown on the screen one following the other without any line break between them:
      
         This is a sentence.This is another sentence.

    even though we had written them in two different insertions into cout. In order to perform a line break on the output we must explicitly insert a new-line character into cout. In C++ a new-line character can be specified as \n  (backslash, n):

      cout << "First sentence.\n ";
      cout << "Second sentence.\nThird sentence.";

This produces the following output:

     First sentence.
     Second sentence.
     Third sentence.

Additionally, to add a new-line, you may also use the endl manipulator. For example:

     cout << "First sentence." << endl;
     cout << "Second sentence." << endl;

would print out:

     First sentence.
    Second sentence
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    The endl manipulator produces a newline character, exactly as the insertion of '\n' does, but it also has an additional behavior when it is used with buffered streams: the buffer is flushed. Anyway, cout will be an unbuffered stream in most cases, so you can generally use both the \n escape character and the endl manipulator in order to specify a new line without any difference in its behavior.