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Learning  How  to  Code  in  Arduino  
Worksheet  3:  Functions    
 
Jason  Krugman  –  Physical  Computing  -­‐  Fall  2012  
 
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What is a function and what’s it good for?
A function is a group of code that we can call upon to do a given task. A function is something
that we create and call on to perform… a function!
We are already using functions in our Arduino coding. This worksheet will explain how to create
customized functions that we can use to achieve a specific task.
void loop
and
void setup
are both functions that we are already using. So are
pinMode, delay,
digitalWrite,
and many others.
The “void” in front of “loop” and “setup” can be a little confusing. It has to do with the fact these
functions do not need to “return” anything. Most functions work in the given manner: you give
them some input variables, called “arguments”, they do calculations with those variables, and
then they “return” the calculations to you. Both loop and setup are functions that are built into the
way Arduino operates. They are actually defined within the Arduino architecture and must be
“called” for every Arduino program to run. For more info, see this paper:
http://web.cecs.pdx.edu/~EAS199/A/notes/06/whats_this_void_loop.pdf
For our purposes, understanding the use of “void” is not terribly important. We just have to follow
these simple guidelines: “void” always goes before setup and loop, and when we are defining one
of our own functions. We do not have to write “void” before a function’s name when we are
“calling” it during loop. “Calling” means calling it into action. We will get there shortly.
As we know,
void setup
runs once when we first start the Arduino, or press the reset button. Then
void loop
will continue to run continuously after that. Lets say we wanted to do something else
when an event happens. For example, blink an LED once when a button is pressed. We can start
off with something very simple, and gradually build up the complexity of the code using different
programming methods. As the tasks you want the Arduino to do for you get more and more
complex, these slightly more complicated programming methods will make things a whole lot
easier in the long run.
Here is some basic code that does that makes an LED blink when a button is pressed using an IF
statement:
Sketch Name: led_blink_simple
Now we will use a FOR loop to blink the LED 5 times every time the button is pressed:
Sketch Name: led_blink_with_for
Now, we can take things a step further by using a function to blink the LED:
Sketch Name: led_blink_function
Notice how the name for our function, “blinkLED” shows up twice in the above program: once
when we “call” the function and once where we define the function. It seems a little weird that we
“call” the function before we actually define it. The loop reaches the line where we say,
“blinkLED(5,1000);” and it goes looking for this function called “blinkLED”. If it finds it, it follows
the instructions we have laid out in
void blinkLED.
Remember, we use the word “void” before the
function’s name when we are first defining it. This is similar to the way we put “int” before a
variable we are defining for the first time.
Besides defining and calling functions, how to use arguments and pass variables. Once we call
the function “blinkLED”, it takes those two numbers we have “passed into it”, the arguments 5 and
1000, and applies them to the function. It says to itself, “Okay 5, you are going to be stored in a
local variable
‘numBlinks’, and 1000, you are going to be stored in a
local variable
‘del’ ”.
“Local variable” means that these variables will only be used within the blinkLED function. If we
say “numBlinks” somewhere else, like in the loop, the Arduino will say “ERROR”. This is what the
function will effectively be doing with those two variables we have passed in:
This is awesome! It means that we can “pass” in numbers to our functions and have the functions
do lots of work for us. We could even use a counter variable or a FOR loop to keep passing in lots
of different numbers! The function will do the same thing to whatever argument variables we pass
in to it. We can put as many or as few arguments into our functions as we like. See below.
Now, we will start to take advantage of our use of functions:
Sketch Name: led_blink_function_with_for
 
All code is available for download at:
http://jasonkrugman.com/classes/physcomp/worksheet3/led_blink_simple/led_blink_simple.ino
http://jasonkrugman.com/classes/physcomp/worksheet3/led_blink_with_for/led_blink_with_for.ino
http://jasonkrugman.com/classes/physcomp/worksheet3/led_blink_function/led_blink_function.ino
http://jasonkrugman.com/classes/physcomp/worksheet3/led_blink_function_with_for/led_blink_fu
nction_with_for.ino
 

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