One week ago i showed you my project with the 7-segment Display.
Now I want to show you the code and explain how it works.
I dont want to go into detail how "C" works. There are many tutorials out there.

Let's have a look at the C code (Used to programm the arduino):

In the first part we are defining the Pins we are going to use.
Pin 10 and 12 for the two ground connections.
The array out_Pins contains all pins that are used as an output and each correspond to one segment.

#define GND1 10
#define GND2 12

int out_Pins[] = {2,3,4,5,6,7,8,9,14,15,16,17,18,19};

Here I am creating a prototype for a function I am going to use it later:

void segment_on(int , int);

The setup only defines all used pins to the OUTPUT mode. Nothing special here^^

void setup() 
{

  int i = 0;
  for(i;i<=14;i++)
  {
    pinMode(out_Pins[i],OUTPUT);
  }
  //GND1
  pinMode(10,OUTPUT);
  //GND2
  pinMode(12,OUTPUT);
}

Now I am going to show you the function created above:

void segment_on(int gnd, int pin)
{
  digitalWrite(gnd,HIGH);
  digitalWrite(pin,HIGH);
  delay(2);
  digitalWrite(pin,LOW);
  digitalWrite(gnd,LOW);
}

The function takes to integers. The first one saves the ground I want to toggle. The second one the pin -> segment.
All this function does is toggle the specified ground. Turns the segment on. Then waits 2ms. After that it turns off the segment and the ground too. If we now want to display more segments at a time we call this function with different values and because the segments switch so fast we as a human dont notice that there is only one segment lighting at once :D