A simple C++ program

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Preprocessor directives Program body Comment Output statements Input statements

The following shows a simple C++ program which adds two integers. Click here to see an equivalent Pascal program.

Note that C++ is case-sensitive. For example, SUM, sum and Sum are considered different in C++.

If you are interested, you may also click here to see how C handles input and output (also supported by C++).

Program 1: A C++ program which adds two integers
#include <iostream.h>
 preprocessor directive(s)
main() {

  // A simple C++ program

  int x, y, sum;
  cout << "A program which adds two integers\n";
  cout << "Enter 1st integer: ";
  cin >> x;
  cout << "Enter 2nd integer: ";
  cin >> y;
  sum = x + y;
  cout << "Sum is " << sum << endl;
  return 0;
} 		program body
Sample output (text in red is entered by user):
A program which adds two integers
Enter 1st integer: 3
Enter 2nd integer: 4
Sum is 7

We will use this program to see the basic structure of a C++ program and I/O statements in C++.

Preprocessor directives Top

Before compilation of a C++ program, it is processed by a preprocessor. The preprocessor directives tell the preprocessor to perform some manipulations which must be done before compilation. Almost all C++ programs has at least one preprocessor directive. A preprocessor directive begins with a #.

#include preprocessor directives tell the preprocessor to include the functions provided by the specified file. They are similar to the uses statements in Turbo Pascal.

For C++ programs that output data to the screen and/or input data from the keyboard, the following preprocessor directive must be present:

#include <iostream.h>

This preprocessor directive can be seen almost in every C++ program.

Program body Top

Declarative statements and executable statements

The program body consists of declarative and executable statements. In Program 1,
    int x, y, sum;
is a declarative statement, while the other statements within the braces, {}, are executable statements.

Unlike Pascal, declarative statements need not come before executable statements in a C++ program. Sometimes a declarative statement may be found in the middle of the program body.

The structure of the main program

The line
    main()
is a part of every C++ program. It indicates the beginning of the main program. In addition, braces are required to enclose all the statements of the main program. The left brace ({) and the right brace (}) are equivalent to begin and end in Pascal.

In C++, each statement must be ended with a semicolon (;), even the last statement.

The last statement in the main program is
    return 0;

A return statement is used in a function to exit the function and return a value. Since the main program is considered as a function in C++, return statement is also used. The main program returning 0 indicates successful termination.

Comment Top

In a C++ program, a line that begins with // indicates that the remainder of the line is a comment.

In addition, comments in C++ can also be enclosed by /* and */.

Output statements Top

cout

To print something on the screen, we can use a cout statement. It starts with cout, then the << operator, and finally the items to be displayed. For example, the statement
    cout << "Enter the 1st integer: ";
instructs the computer to display the string "Enter the 1st integer: " onto the screen. Here cout behaves like write in Pascal.

In C++, a string of characters are enclosed by a pair of double quotation marks (").

cout can display multiple items (including strings, numbers and values stored in variables). For example, the C++ statement
    cout << "First value is " << a + b << " and second value is " << c + d;
is equivalent to the Pascal statement
    write('First value is ', a + b, ' and second value is ', c + d);

Special characters

Unlike Pascal, there is no reserved word or standard identifier which behaves like writeln in Pascal. To output a newline character, we use \n within the quotation marks. For example, the statement
    cout << "A program which adds two integers\n";
instructs the computer to display the following on the screen:

A program which adds two integers

The next character to be displayed will be on the next line.

Also, the statement "cout << "Welcome\nto C++!\n";" will causes the computer to display the following on the screen:

Welcome
to C++!

Note that \n inserts a newline character between "Welcome" and "to".

In C++, \ is an escape character. This means that a special character is to be output. The following table introduces some common escape sequences:

Escape sequence Description
\n Newline character
\t Horizontal tab
\\ The backslash \
\" The double quotation mark "

For example, "cout << "1st\t2nd\t3rd\t4th\na\tb\tc\td\n";" instructs the computer to display the following on the screen:

1st     2nd     3rd     4th
a       b       c       d

endl

endl is used with cout. It outputs a newline and then forces any outputs accumulated in the buffer to be printed. For the
purpose of tracing simple programs, you may consider endl to be similar to the newline character "\n".

For example (as seen in Program 1):

  cout << "Sum is " << sum << endl;

Input statements Top

To obtain a value from the keyboard and store it into a variable, we can use a cin statement. It starts with cin, then >> operator, and finally a variable.

In Program 1, the statement "cin >> x;" accepts an integer (x is an integer variable) from the keyboard and stores it in x.

If required, the cin statement can accepts multiple values and store them in the corresponding variables. For example, the statement "cin >> x >> y;" accepts two values from the keyboard and store them in x and y respectively (here x and y can be of different types).

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