First C program, Hello World

After talking about the history and compilers it is time to make our first program. We like to make C programs under Linux, so we will use a text editor and the gnu compiler. But all programs we make in these tutorials will also work under Windows. Remember, the examples included in the C and C++ tutorials are all console programs. That means they use text to communicate. All compilers support the compilation of console programs. Check the user’s manual of your compiler for more info on how to compile them. (It is not doable for us to write this down for every compiler).

Open a text editor (vi, emacs, notepad) or make a console project in Visual Studio Express and type the following lines of code (Don’t use cut/paste, type them. This is better for learning purposes):

	#include<stdio.h>

	int main()

	{

		printf(“Hello World\n”);

		return 0;

	}

	

Save the program with the name: hello.c

#include<stdio.h>

With this line of code we include a file called stdio.h. (Standard Input/Output header file). This file lets us use certain commands for input or output which we can use in our program. (Look at it as lines of code commands) that have been written for us by someone else). For instance it has commands for input like reading from the keyboard and output commands like printing things on the screen.

int main()

The int is what is called the return value (in this case of the type integer). Where it used for will be explained further down. Every program must have a main(). It is the starting point of every program. The round brackets are there for a
reason, in a later tutorial it will be explained, but for now it is enough to know that they have to be there.

{}

The two curly brackets (one in the beginning and one at the end) are used to group all commands together. In this case all the commands between the two curly brackets belong to main(). The curly brackets are often used in the C language to group commands together. (To mark the beginning and end of a
group or function.).

printf(“Hello World\n”);

The printf is used for printing things on the screen, in this case the words: Hello World. As you can see the data that is to be printed is put inside round brackets. The words Hello World are inside inverted ommas, because they are what is called a string. (A single letter is called a character and a series of characters is called a string). Strings must always be put between inverted commas. The \n is called an escape sequence. In this case it represents a newline character. After printing something to the screen you usually want to print something on the next line. If there is no \n then a next printf command will print the string on the same line.

Commonly used escape sequences are:

• \n (newline)
• \t (tab)
• \v (vertical tab)
• \f (new page)
• \b (backspace)
• \r (carriage return)

After the last round bracket there must be a semi-colon. The semi-colon shows that it is the end of the command. (So in the future, don’t forget to put a semi-colon if a command ended).

return 0;

When we wrote the first line “int main()”, we declared that main must return an integer int main(). (int is short for integer which is another word for number). With the command return 0; we can return the value null to the operating system. When you return with a zero you tell the operating system that there were no errors while running the program. (Take a look at the C tutorial – variables and constants for more information on integer variables).

Compile

If you have typed everything correct there should be no problem to compile your program. After compilation you now should have a hello.exe (Windows) or hello binary (UNIX/Linux). You can now run this program by typing hello.exe (Windows) or ./hello (UNIX/Linux).

If everything was done correct, you should now see the words “Hello World” printed on the screen.

Congratulations!!!!!
You have just made your first program in C.

Comments in your program

The Hello World program is a small program that is easy to understand. But a program can contain thousands of lines of code and can be so complex that it is hard for us to understand. To make our lives easier it is possible to write an explanation or comment in a program. This makes it much easier to understand the code. (Even if you did not look at the code for years).These comments will be ignored by the compiler at compilation time.

Comments have to be put after // or be placed between /* */ .
Here is an example of how to comment the Hello World source code :

	/*	Description	: Print Hello World on the screen

		Author		: Your name

		Date		: 01/01/2007				*/

	#include<stdio.h>
	int main()

	{

		//Print something and then newline

		printf(“Hello World\n”);

		return 0;

	}

	

Indentation

As you can see the printf and return statements have been indented or moved to the right side. This is
done to make the code more readable. In a program as Hello World, it seems a stupid thing to do. But as the programs become more complex, you will see that it makes the code more readable.

So, always use indentations and comments to make the code more readable. It will make your life much easier if the code becomes more complex.