Programming Arduino Getting Started with Sketches

setup
and set pin 13 to be an output.
2. Run 
loop
and set pin 13 to high (LED on).
3. Delay for half a second.
4. Set pin 13 to low (LED off).
5. Run 
loop
again, going back to step 2, and set pin 13 to high (LED on).
The problem lies between steps 4 and 5. What is happening is that the LED is being
turned off, but the very next thing that happens is that it gets turned on again. This
happens so quickly that it appears that the LED is on all the time.
The microcontroller chip on the Arduino can perform 16 million instructions per
second. That’s not 16 million C language commands, but it is still very fast. So, our LED
will only be off for a few millionths of a second.
To fix the problem, you need to add another delay after you turn the LED off. Your code
should now look like this:

---

Try again and your LED should blink away merrily once per second.
You may have noticed the comment at the top of the listing saying “sketch 3-01.” To
save you some typing, we have uploaded to the book’s website all the sketches with such a
comment at the top. You can download them from 
http://www.arduinobook.com
.
Variables
In this Blink example, you use pin 13 and have to refer to it in three places. If you decided
to use a different pin, then you would have to change the code in three places. Similarly, if
you wanted to change the rate of blinking, controlled by the argument to delay, you would
have to change 500 to some other number in more than one place.
Variables can be thought of as giving a name to a number. Actually, they can be a lot
more powerful than this, but for now, you will use them for this purpose.
When defining a variable in C, you have to specify the type of the variable. We want
our variables to be whole numbers, which in C are called