Section 1

The Hello World Program

In this section you will learn how to write the Hello World program. All you will do is write the words Hello World on the screen. This problem is used universally for all programming languages as the first program to write. This is because it teaches the format a program file must be in to compile and how to output something to the screen. In C, the Hello World program looks something like this:

    1.	//Hello World Program
    2.	#include <iostream.h>
    4.	void main ()
    5.	{
    6.	    cout << "Hello World\n";
    7.	}

Now lets look at each line in detail.

Line 1

This is a comment. A comment is some text in a program which is just used for documentation purposes. A comment starts with the characters '//' then some text. When your program is compiled, the compiler ignores all text from the two slashes up to the end of the line.

You can also use the C programming style of comments where the comment starts with '/*' and ends with '*/'. All text between these are ignored and the text can be over multiple lines. For example:

	/* Hello World Program
	   This is using the C programming style comments
	#include <iostream.h>

	void main ()
	    cout << "Hello World\n";

Line 2

This is a preprocessor directive. Before your program is compiled it is preprocessed and any preprocessor commands are carried out. In this case the preprocessor command is an include command. If you want to use any of the library commands provided then you need to include the header for the library. The header file is a file which contains the commands you can use and the file normally ends in '.h'. In this case you are including the header 'iostream.h' which contains the commands you need to write to the screen and get input from the keyboard.

Line 4

This is the beginning of the main function. It is called the function header. The code that you want to execute must be within a function and you can return values back from a function. Functions will be dealt with in more detail in a later section. Every program must have a main function and it is always the first function executed in a program.

The first part of a function header tells the compiler what type of data should be returned from the function, then the name of a function and then any data you want to pass to a function. In this case the first part of the header is the word 'void', this means that the function does not return any value. After 'main' you will see two parentheses '()'. Inside the parentheses would be any information to pass to the function, but in this case we have no information to pass so we leave it blank.

Line 5

This lines contains the open curly brace character '{'. This tells the compiler that any statements following this are part of the main function. All function must have a start and end brace.

Line 6

This line prints the word Hello World on the screen. The cout command is a data stream which is attached to screen output and is used to print to the screen, it is part of the iostream library. At the end of Hello World there is a special character '\n'. This representes the new line character and the '\n' should be treated as a single character. It tells the computer to move down one line and continue outputting from there. The << symbol is called the stream insertion operator and it is used to insert the following data into the cout stream which then pirnts is to the screen.

Line 7

This line contains the closing curly brace character '}'. This tells the compiler that the end of the function has been reached.

Type in the program, compile and run it. If you have no errors then your should see the words Hello World appear on the screen.