global vs local variables

Programming Questions

samkhan

in Programming Questions • 2 years ago

Hi,

I'm a novice programmer and I would be really glad if you can help me figure out this problem where I'm trying to use the system variable called width after setting the size().

Purpose of the sketch:

A sketch to explore radial placement of square boxes. Animate a box that seems to move clockwise spiraling inwards.

This code works as intended:

void setup() {

  smooth();

  size(400,400);

  rectMode(CENTER);

}

float i = 180; // PI

float amplitude = 200; // 400/2

void draw() {

  background(200);

  drawSquares(5, 3);

}

void drawSquares(float s, int speed) {

  float x = (width/2)+sin(radians(i))*amplitude;

  float y = (height/2)+cos(radians(i))*amplitude;

  fill(0);

  rect(x,y,s,s);

  i-=speed; //change radians at the rate of speed

  amplitude-=0.5; //decrease amplitude to spiral inwards

  if(i==-180) {

   i=180; // reset radians

  }



  if(amplitude==0) {

   amplitude=width/2; // reset amplitude

  }

}

However when I declare amplitude as

float amplitude = width/2;

the code does not work properly. At the start of the sketch, amplitude is 0 when I use width/2 instead of 200 or some numerical value. I was able to verify this by using text(str(amplitude),20,20) after creating and loading a font. The procedure drawSquares() is able to see the value of i and even width/2 in the line

float x = (width/2)+sin(radians(i))*amplitude;

I don't know what I'm doing wrong. Please let me know if there is a mistake in my programming or if this is a problem your are able to replicate and have found a solution for. All comments are welcome :)

Thanks,

Sam

System:

AMD Phenom II Quadcore 64bit, Windows 7, 8GB DDR3, NVIDIA 9100

Replies(5)

PhiLho

Re: global vs local variables

2 years ago

The global variables are created and initialized before everything is run.
The width variable is 0 until the size() function is called. So your amplitude is set to 0.
Solution: just declare the variable, initialize it in setup() after size().

samkhan

Re: global vs local variables

2 years ago

Thanks phi.lho,

I had already tried that to no avail. It gave me an error, "Nothing named 'amplitude' was found". Things declared in setup() aren't visible to other procedures. I then started experimenting and it looks like the variables width and height are not set till the first time draw() is called.

Try running these codes. I created a font and loaded it. You can use some other font from your system. You will notice that I use noLoop() to stop draw(). I had expected noLoop() to prevent draw() from running at all but draw() is called once before it gets stopped. Also, in the block draw(){ }, the procedure drawSquares() computes before the lines of codes preceding it do. This didn't match my expectations. In the second code I declare a bunch of variables in different place and output them using text(). It showed me that setup() does not set the values of height and width. Which, is quite inconvenient and unexpected.

void setup() {

smooth();

size(400,400);

rectMode(CENTER);

PFont trebuchet24;

trebuchet24 = loadFont("TrebuchetMS-24.vlw");

textFont(trebuchet24,12);

noLoop();

}

float amplitude = width/2;

float i = 180; //PI

void draw() {

background(200);

fill(0);

String strAmplitude = str(amplitude);

String strHalfW = str(width/2);

String strW = str(width);

text(">> "+strW+","+strHalfW+","+strAmplitude,20,100);

drawSquares(5, 3);

}

void drawSquares(float s, int speed) {

float x = (width/2)+sin(radians(i))*amplitude;

float y = (height/2)+cos(radians(i))*amplitude;

fill(0);

rect(x,y,s,s);

i-=speed; //change radians at the rate of speed

amplitude-=0.5; //decrease amplitude to spiral inwards

if(i==-180) {

i=180;

}

if(amplitude==0) {

amplitude=width/2; //reset amplitude

}

String strI = str(i);

String strX = str(sin(radians(i)));

String strY = str(cos(radians(i)));

String strAmp = str(amplitude);

fill(255);

text("i="+strI+"\nx="+strX+"\ny="+strY+"\nA="+strAmp,20,24);

}

This is the second set of code I used for experiment. I tried numerous ways of declaring them at different places:

void setup() {

size(400,600);

PFont trebuchet24;

trebuchet24 = loadFont("TrebuchetMS-24.vlw");

textFont(trebuchet24);

}

float halfWidth = width/2;

float halfHeight = height/2;

String str_W = str(width);

String str_H = str(height);

String str_halfW = str(halfWidth);

String str_halfH = str(halfHeight);

int m1 = width;

float m2 = height;

String strm1 = str(m1);

String strm2 = str(m2);

String str_Pi = str(PI);

void draw() {

float WIDTH = width;

float HEIGHT = height;

String strWIDTH = str(WIDTH);

String strHEIGHT = str(HEIGHT);

text(str_W+"\n"+str_H+"\n"+str_halfW+"\n"+str_halfH+"\n"+str_Pi,20,24);

text(strm1+"\n"+strm2,20,200);

text(strWIDTH+"\n"+strHEIGHT,20,300);

}

I have come to conclude that the problem isn't of global vs local variable since str_Pi is visible to draw() and thereby should be visible to all procedures in the sketch. Other variables declared like str_Pi should work. But it looks like PI is a system constant that is always available even without running setup() and variables width and height are available only after the first call to draw().

bjordal

Re: global vs local variables

2 years ago

Both your programs works perfectly to me, which i might add is rather strange.

When phi.lho said "after size is called" he did not mean "after where the method setup ends in your code" but rather after the method size is executed. So try doing something like this:

int someVariable;
void setup(){
size(200,200);
someVariable = width;
}
void draw(){
println(someVariable);
}

samkhan

Re: global vs local variables

2 years ago

Yep, phi.lho did mean what you said and I did try it. But I tried declaring the variable inside setup() so it wasn't working for me.

So here is a screenshot of what I get when I run my original code. You said they worked for you?

The code you gave however helped me solve my problem. Declaring the variable before setup() makes so much sense. Thank you very much.

PhiLho

Re: global vs local variables

2 years ago

I said: " declare the variable, initialize it in setup() after size()."

// Declare the variable
int amplitude;
void setup()
{
size(...);
// Initialize it in setup() after size()
amplitude = width;
}

I noticed lot of people have difficulty to distinguish declaration of variable with initialization step, perhaps because they are often done in the same line. A common issue is also to re-declare the variable where it is initialized, just making a different volatile local variable...

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