I am trying to understand line by line how this code works before I move on.

Question 01. Within draw() it says x = 0;

Isn't this redundant? For at the top it says int x = 0;


Question 02. x = x + spacing;

Which spacing does spacing use? int spacing = 10; at the top or spacing = (mouseX / 2) + 1;

Thanks for advice.

int y = 80;        
int x = 0;        
int spacing = 10;  
int len = 20;      
int endLegs = 150;

void setup() {
 size(200,200);
}

void draw() {
 background(0);
 stroke(255);
 x = 0;

spacing = (mouseX / 2) + 1;
 while (x <= endLegs) {
   line(x,y,x,y + len);

   x = x + spacing;
 }
}

Quote:


"int x = 0;" at the top of the sketch means : before the sketch windows appears, I tell you we're going to use a variable called x, which value is 0 for now. "int x;" would do the same job, since the initial value of an int variable is 0 by default.

"x = 0;" in the draw loop means : each time the window is redrawn, set x to 0. Since x changes later in the draw() method (x = x + ...), it is useful to set it back to 0.

Yes. Be sure you understand what variable scope mean :
http://processing.org/learning/basics/variablescope.html

Lenticular wrote on Aug 18^th, 2009, 4:04pm:


Not at all - you can declare a variable locally within a function, without declaring it at the top, meaning its value can then only be accessed from within the function.  e.g. if you moved "int x = 0" into setup the value of x would only be accessible from within setup and you would get an error.  By declaring it at the top you make it global and therefore accessible from within setup, draw etc...

Lenticular wrote on Aug 18^th, 2009, 4:04pm:


No - the x and spacing variables are global.  I'm not sure you've understood the distinction between:

 int x = 0;

and

 x=0;

The first (when you specify what type of variable it is) declares the variable - where you put this is important as it defines the scope of the variable.  The second simply assigns a value to the variable, can only be done within the scope of the variable and thus can't be done until the variable has been declared.  So when you use that in draw you're simply changing the value, not declaring the variable.

To make matters a little more confusing you can declare variables with the same name in different scopes, so the value returned will obviously depend on the point at which you access that variable name:

Code:

int x = 0;        

void setup() {
 size(200,200);
 int x = 13;
 // prints the local variable x
 println("(setup) x = " + x); 
}

void draw() {
  // prints the global variable x
  println("(draw) x = " + x);
  noLoop();
}  

That might either make things clearer or confuse you more.  There are times when it's perfectly valid to do this, but you might want to avoid it until you've got the idea of scope clear in your head...