Accessing an Array outside a For Loop

Programming Questions

gokpotter

Accessing an Array outside a For Loop

in Programming Questions • 7 months ago

Hey, I'm trying to write a sketch where I join up the points that make up a circle to get a wireframe effect.

However I can't access the array outside the for loop. Is there a better way of doing this?

float radius=50;
int numPoints=20;
float angle=TWO_PI/(float)numPoints;
float x, y;
float[] pointsX, pointsY;
void setup() {
size(300, 300);
for(int i=0;i<numPoints;i++) {
float[] pointsX = new float[numPoints];
float[] pointsY = new float[numPoints];
x = radius*sin(angle*i) + width/2;
pointsX[i] = x;
y = radius*cos(angle*i) + height/2;
pointsY[i] = y;
point(x , y);
}
}

Cheers

Replies(7)

GoToLoop

Re: Accessing an Array outside a For Loop

7 months ago

There are 2 types of scoping a variable -> field & local.

I see that you have declared your 2 arrays -> pointsX & pointsY as field variables.

The bug arises when you re-declare them within the loop!

Local variables have precedence over field ones!

Explaining further, from the place they have been re-declared until the end of their scope ( for loop),
pointsX & pointsY stop being field variables and become local.

After the loop is finished, they return being field variables, but w/o any memory of
what have transpired to them inside the loop!

However, they keep what was stored in them before becoming local!

gokpotter

Re: Accessing an Array outside a For Loop

7 months ago

Huge help, thanks!

I've tried another for loop to incrementally add lines joining the outside points but I get an odd result.

float radius=100;
int numPoints=20;
float angle=TWO_PI/(float)numPoints;
float x, y;
float[] pointsX = new float[numPoints];
float[] pointsY = new float[numPoints];
void setup() {
size(800, 800);
for(int i=0;i<numPoints;i++) {
x = radius*sin(angle*i) + width/2;
pointsX[i] = x;
y = radius*cos(angle*i) + height/2;
pointsY[i] = y;
point(x , y);
}
for(int i=0;i<numPoints;i++) {
line(pointsX[i], pointsY[i], pointsX[i++], pointsY[i++]);
}
}

GoToLoop

Re: Accessing an Array outside a For Loop

7 months ago

Here's a tweaked fixed version

final static byte  numPoints = 20;
final static float angle = TWO_PI/numPoints;
final static float rad = 50;

final static float[] pointsX  = new float[numPoints];
final static float[] pointsY  = new float[numPoints];

void setup() {
  size(300, 300);
  noLoop();

  strokeWeight(8);
  stroke(#0000FF);
  background(0200);

  for (int i=0; i!=numPoints; i++) {
    pointsX[i] = rad*sin(angle*i) + width/2;
    pointsY[i] = rad*cos(angle*i) + height/2;
    
    point(pointsX[i], pointsY[i]);
  }
}

gokpotter

Re: Accessing an Array outside a For Loop

7 months ago

Cheers, I appreciate the help. Looks much cleaner!
What advantage does the final static part have?

Also, do you know why this bit of code to connect the outside dots with lines isn't working correctly?

for(int i=0;i<numPoints;i++) {
line(pointsX[i], pointsY[i], pointsX[i++], pointsY[i++]);
}

GoToLoop

Re: Accessing an Array outside a For Loop

7 months ago

What happened is that you're increasing iterator i 3 times using 3 ++!

Here's a new fixed version! ^_^

P.S.: final can make the compiler to hardcode (inline) the value of a field variable.
That is, the variable behaves and has the performance of a constant literal if it kicks in!

final static byte  numPoints = 20, idxPoints = numPoints-1;
final static float angle = TWO_PI/numPoints;
final static byte  rad = 50;

final static float[] pointsX  = new float[numPoints];
final static float[] pointsY  = new float[numPoints];

void setup() {
  size(300, 300);
  noLoop();
  background(0200);

  strokeWeight(8);
  stroke(#0000FF);

  for (int i=0, w = width>>1, h = height>>1; i!=numPoints;) {
    pointsX[i] = rad*sin(angle*i) + w;
    pointsY[i] = rad*cos(angle*i) + h;

    point(pointsX[i], pointsY[i++]);
  }

  strokeWeight(2);
  stroke(#FFFF00);

  for ( int i=0; i!=idxPoints; 
    line(pointsX[i], pointsY[i], pointsX[i+1], pointsY[++i]) );

  line(pointsX[idxPoints], pointsY[idxPoints], pointsX[0], pointsY[0]);
}

gokpotter

Re: Accessing an Array outside a For Loop

7 months ago

Oh, it all makes so much sense now! aha

Final quick question, why would I use a final static rather than the usual declaration?

Thanks!

GoToLoop

Re: Accessing an Array outside a For Loop

7 months ago

It's a very controversial topic about how it impacts performance.
I guess it's better to look at these links I've found about it for yourself.
As you'll see, there are more advantages to its usage besides a tiny performance increase:

http://stackoverflow.com/questions/8655691/java-compile-time-constants
http://www.javapractices.com/topic/TopicAction.do?Id=23
http://www.ibm.com/developerworks/java/library/j-jtp1029/index.html
http://www.coderanch.com/t/561773/java/java/final-String-compilation

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