2. // shift array values
3.   for ( int i  = 0 ; i < xpos.length-1 ; i++){
4.   xpos [i] = xpos[i+1];
5. }

// declare arrays, which have a length of 4
int [] xpos = new int [4];
ok... we make 4 elements in a lis, with an index of 0,1,2 and 3

void setup (){
size (600,600);
smooth ();
// initialize, we give each of them a starting value of 0
for ( int i = 0; i < xpos.length ; i++){
xpos[i] = 0 ;
}}

void draw (){
background (255);
// shift array values
for ( int i = 0 ; i < xpos.length-1 ; i++){
xpos [i] = xpos[i+1];
}

so we are saying, to the whole list minus one ( index 3 in this case ), we would do ''i++ ''

which means: lets add one. So what would that look like ? :

0,1,2 then these plus one ?

1,2... and then what , i cant go further is length minus one... what i'm i  doing wrong ?

2. // declare arrays
3. int [] xpos = new int [50];
4. int [] ypos = new int [50];
5. void setup (){
6. size (600,600);  
7. smooth ();
8. 
   
9. // initialize
10. for ( int i = 0; i < xpos.length ; i++){
11. xpos[i] = 0 ;
12. ypos[i] = 0 ;
13. }
14. }
15. void draw (){
16.   background (255);
17. // shift array values
18.   for ( int i  = 0 ; i < xpos.length-1 ; i++){
19.   xpos [i] = xpos[i+1];
20.   ypos [i] = ypos[i+1];
21. }
22. // new location
23.   xpos[xpos.length-1] = mouseX;
24.   ypos[ypos.length-1] = mouseY;
25. // draw everything
26. for ( int i = 0 ; i < xpos.length ; i++ ){
27.   noStroke ();
28.   fill ( 255-i*5);
29.   ellipse ( xpos[i],ypos[i],i,i);
30. }
31. }