Your First Program

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Objectives

Having read this section you should have an understanding
of:

1. a pre-processor directive that must be present in all your C programs.



2. a simple C function used to write information to your
   screen.



3. how to add comments to your programs

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Now that you've seen the compiler in action it's time for you
to write your very own first C program. You can probably
guess what it's going to be - the program that everyone writes
just to check they understand the very, very, very basics of what
is going on in a new language.

Yes - it's the ubiquitous "Hello World" program. All
your first program is going to do is print the message "Hello
World" on the screen.

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The program is a short one, to say the least. Here it is:

#include <stdio.h>
main()
{
    printf("Hello World\n");
}

[program]

The first line is the standard start for all C programs -
main(). After this comes the program's only
instruction enclosed in curly brackets {}. The
curly brackets mark the start and end of the list of instructions
that make up the program - in this case just one instruction.

Notice the semicolon marking the end of the instruction. You
might as well get into the habit of ending every C
instruction with a semicolon - it will save you a lot of trouble!
Also notice that the semicolon marks the end of an instruction -
it isn't a separator as is the custom in other languages.

If you're puzzled about why the curly brackets are on separate
lines I'd better tell you that it's just a layout convention to
help you spot matching brackets. C is very unfussy about
the way you lay it out. For example, you could enter the Hello
World program as:

main(){printf("Hello World\n");}

but this is unusual.

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The printf function does what its name suggest
it does: it prints, on the screen, whatever you tell it to. The
"\n" is a special symbols that forces a new line
on the screen.

OK, that's enough explanation of our first program! Type it in
and save it as Hello.c. Then use the compiler to
compile it, then the linker to link it and finally run it. The
output is as follows:

Hello World

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Add Comments to a Program

A comment is a note to yourself (or others) that you put
into your source code. All comments are ignored by the compiler.
They exist solely for your benefit. Comments are used primarily
to document the meaning and purpose of your source code, so that
you can remember later how it functions and how to use it. You
can also use a comment to temporarily remove a line of code.
Simply surround the line(s) with the comment symbols.

In C, the start of a comment is signalled by the
/* character pair. A comment is ended by
*/. For example, this is a syntactically correct C comment:

/* This is a comment. */

Comments can extend over several lines and can go anywhere
except in the middle of any C keyword, function name or
variable name. In C you can't have one comment within
another comment. That is comments may not be nested. Lets now
look at our first program one last time but this time with
comments:

main() /* main function heading */
{
    printf("\n Hello, World! \n");  /* Display message on */
}                                   /* the screen */ 

This program is not large enough to warrant comment statements
but the principle is still the same.