Ok, so now it is time to hook up more LEDs so we can do more exciting patterns.
We will turn on the LEDs via the Pin (Port).

Process: Follow the diagram to connect the LEDs and resistors.

Coding:
1. Now, lets try a program that will let us make sure that all of our hardware is made correctly. It is often wise to write a small piece of software to test and make sure your hardware is correct rather than try your full software on the brand new hardware.
Code#1 below, sets up LEDs on pins 2-5 and then cycles through turning each LED on and then off.
const int kPinLed1 = 2;
const int kPinLed2 = 3;
const int kPinLed3 = 4;
const int kPinLed4 = 5;

void setup()
{
pinMode(kPinLed1, OUTPUT);
pinMode(kPinLed2, OUTPUT);
pinMode(kPinLed3, OUTPUT);
pinMode(kPinLed4, OUTPUT);
}

void loop()
{
// turn on each of the LEDs in order
digitalWrite(kPinLed1, HIGH);
delay(100);
digitalWrite(kPinLed2, HIGH);
delay(100);
digitalWrite(kPinLed3, HIGH);
delay(100);
digitalWrite(kPinLed4, HIGH);
delay(100);

// turn off each of the LEDs in order
digitalWrite(kPinLed1, LOW);
}

While this code works just fine, it isn’t very elegant and it seems like there isa lot of writing of very similar things and opportunities for mistakes. Let’s see how we can do a better job.

ARRAYS
An array is a collection of variables that are indexed with an index number. An
example will help us to understand – see Code#2 below.

const int k_numLEDs = 4;
const int kPinLeds[k_numLEDs] = {2,3,4,5}; // LEDs connected to pins 2-5

void setup()
{
for(int i = 0; i < k_numLEDs; i++){
pinMode(kPinLeds[i], OUTPUT);
}
}

void loop()
{
for(int i = 0; i < k_numLEDs; i++){
digitalWrite(kPinLeds[i], HIGH);
delay(100);
}
for(int i = k_numLEDs – 1; i >= 0; i–){
digitalWrite(kPinLeds[i], LOW);
delay(100);
)
)

Can you figure out what this does? If not, don’t panic. We are going to go through this code and look at each part.

1 const int k_numLEDs = 4;
First, we define how many elements are going to be in our array. We use this later to make sure that we don’t try to read (or write) past the end of our array.
(Arduino does not stop you from doing this which can cause all sorts of strange problems.)

2 const int kPinLeds[k_numLEDs] = {2,3,4,5}; // LEDs connected to pins 2-5
Second, we define the array. You’ll notice that we have the number of “elements”in the array in brackets. We could have used the actual number, but it is much better to use a constant. We assign the values to the array here. The values are inside of curly braces and are separated by commas.
Arrays are zero-indexed, which can be confusing. That means the first element in the array k_LEDPins is k_LEDPins[0]. The last element in the array is k_LEDPins[3]. (0-3 is 4 elements.)

4 void setup()
5 {
6 for(int i = 0; i < k_numLEDs; i++){
7 pinMode(kPinLeds[i], OUTPUT);
8 }
9 }
Here we use a for loop to go through each of the elements in our array and set them as OUTPUT. To access each element in the array,we use square brackets with the index inside.

13 for(int i = 0; i < k_numLEDs; i++){
14 digitalWrite(kPinLeds[i], HIGH);
15 delay(100);
16 }
This looks almost exactly the same as what is done in setup. Here we are going through each of the LEDs and turning them on (with a 100 millisecond delay in between them).

17 for(int i = k_numLEDs – 1; i >= 0; i–){
18 digitalWrite(kPinLeds[i], LOW);
19 delay(100);
20 }
Now we are showing that we can use a for loop to go backwards through the loop as well. We start at k_numLEDs – 1 since arrays are zero-indexed. If we started at k_PinLeds[4], that would be past the end of our array. We check >= 0 since we don’t want to miss the first element (the one at index 0.)

Exercises:
1. Make a program using arrays that lights up the LEDs from top to bottom and then goes backwards so only one LED is on at any time. (This is often called a “Cylon”5 or a “Larson”6 light.)
2. Make a program that lights up the LEDs in any pattern that you like.