Comments on: Exercise 6-5: One line at a time

By: Ken Sherwood

Ken Sherwood — Thu, 20 Jan 2011 21:56:12 +0000

Found this initially confusing, until worked through to realize that instead of adding a line each pass through the “for” loop it is actually drawing: line1, then line1 + line2, then line1 + line2 + line3, etc. Here is a variation that helps to visualize this, by changing the colors each time through the for loop:

int endY = 0;
float colorR = 0;
float colorB = 0;
float colorG = 0;

void setup() {
size(200,200);
frameRate(1);

}

void draw() {
background(255);
colorR = random(255); // Introduced to change line color each time through draw
colorB = random(255); //
colorG = random(255); //

// y goes from 0 to whatever endY is
for (int y = 0; y height) {
endY = 0;
}

By: Carlos Lopez

Carlos Lopez — Tue, 30 Nov 2010 02:13:07 +0000

I’ve got the answer! “y” does not meet the condition and 10 is added to “endY”, so the loop over and over… Thank you!

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

By: Carlos Lopez

Carlos Lopez — Tue, 30 Nov 2010 01:59:22 +0000

I don’t understand why this piece of code works if “y” does not meet the condition of being less than “endY”. Both “y” and “endY” are 0. Shouldn’t that make the “for” loop exit straight away?

// y goes from 0 to whatever endY is
for (int y = 0; y < endY; y+=10) {

By: Chris

Chris — Sat, 27 Feb 2010 03:59:50 +0000

Makes perfect sense. Thanks for the correction! By the way, your book is just fantastic.

By: Daniel Shiffman

Daniel Shiffman — Sat, 27 Feb 2010 03:16:07 +0000

The difference in the above is that i (which loops from 50 to 150) is a local variable and x (which loops from 0 to 200) is a global variable. Meaning once x gets above 200, it stays above 200 because it is never reset to 0. The following will make it a more analogous situation:

int y = 80;
int spacing = 10;
int len = 20;

void setup() {
  size(200,200);
  background(255);
}

void draw() {
  int x = 20;
  while (x<200){
    stroke(255,0, 0);
    line(x, y, x, y+len);
    x+=15;
    y++;
  }
  
  for (int i=50; i<=150; i+=spacing) {
    stroke(0);
    line(i,y,i,y+len);
    y++;
  }
}

By: Chris

Chris — Wed, 24 Feb 2010 22:32:53 +0000

I’m not sure if this is the perfect place to post this question, but one of my students ran into this seemingly glitchy problem. Can you explain why ‘y’ continues to increment in the for loop and not in the while loop?

int y = 80;
int x = 20;
int spacing = 10;
int len = 20;

void setup() {
size(200,200);
background(255);
}

void draw() {
while (x<200){
stroke(255,0, 0);
line(x, y, x, y+len);
x+=15;
y++;
}
for (int i=50; i<=150; i+=spacing) {
stroke(0);
line(i,y,i,y+len);
y++;
}
}

By: Daniel Shiffman

Daniel Shiffman — Sun, 31 Jan 2010 18:15:07 +0000

We are choosing to set the initial value of endY to 0. If we did not initialize endY, Processing would give it a default value. It just so happens that the default value is also 0 so either way we get the same result. Still, it’s a good habit to initialize your variables.

By: Dave

Dave — Sun, 31 Jan 2010 18:07:21 +0000

wow, this is one hard piece of code for me to understand.
Like Richard said, endY=0 is not neccessary to be called. How then does Processing know the value of endY in the beginning in order to + 10 every draw? I hope you get my question.

By: admin

admin — Tue, 06 Oct 2009 17:27:09 +0000

You'll need to use a variable to increment / decrement endY, i.e.

  // Increment endY
  endY += dir;

Then you could change the direction of dir depending on certain conditions (like it reaching the bottom or top of the window)

dir *= -1;

By: ww

ww — Fri, 02 Oct 2009 08:00:31 +0000

I rewrote the code to draw lines down to up, it worked. But now I’m confused what should I do if i want to draw lines up to down to up to down…