Why background() is not updated in a draw() loop

Processing Coding Questions
1

I start with Processing.

I’m studying Daniel Shiffman’s book “Learning Processing”.

In Exercise 6.5 of the book, I can not understand why background(0) which is in draw() is not updated each time a new line is drawn, erasing the previously drawn line, but only when endY returns to 0 at the end, in if(endY > height) …

int endY;

void setup() {
  size(200, 200);
  frameRate(5);
  
  endY = 0;
}

void draw() {
  background(0);
  
  for (int y = 0; y < endY; y += 10) {
    stroke(255);
    line(0, y, width, y);
  }
  endY = endY + 10;
  
  if(endY > height) {
    endY = 0;
  }
}

It’s not very clear to me. Could someone explain to me?

2

in the case you want only one line,
and that is moving down,
the code could be much more simple

int endY=0;

void setup() {
  size(200, 200);
  frameRate(5);
  stroke(255);
}

void draw() {
  background(0);
  //text(nf(frameRate,1,1),10,9);
  //  for (int y = 0; y < endY; y += 10)  line(0, y, width, y);
  line(0, endY, width, endY);
  endY = endY + 10;
  if (endY > height) endY = 0;
}

because now you ( after background(0) == CLEAR )
draw by a FOR loop as many lines as “endY” allows.
hm, where the FOR loop comes from? not from:

3

Well… it’s Exercise 6.5, not Example 6.5
http://learningprocessing.com/exercises/chp06/exercise-06-05-one-line-at-a-time

And thank you for your explanation, it’s a little clearer for me … I hope …

4

// rendering one line at a time using a for loop.

ok, i might have expected same like you.